Chemical Properties of Whey Protein in Protein Powders and Its Impact on Muscle Growth in Athletes: A Review

Abstract

This review examines the chemical properties of whey protein in protein powders and their impact on muscle growth in athletes. We explore the composition of whey protein, including its major fractions and amino acid profile, with a focus on the role of leucine and other branched-chain amino acids. The review discusses the different types of whey protein powders - concentrate, isolate, and hydrolysate - and their distinct characteristics. We analyze the mechanisms by which whey protein stimulates muscle protein synthesis, including its effects on cellular signaling pathways and hormonal responses. The paper evaluates current evidence on the effects of whey protein supplementation on lean body mass, muscle hypertrophy, strength, and athletic performance. Additionally, we consider optimal dosing strategies and timing of intake for maximizing the benefits of whey protein supplementation. This comprehensive review provides insights into the efficacy of whey protein as a dietary supplement for athletes and highlights areas for future research in sports nutrition.

Introduction

Whey protein has emerged as one of the most popular dietary supplements among athletes, fitness enthusiasts, and health-conscious individuals over the past few decades. ¹ This rise in popularity is not without merit, as whey protein offers a unique blend of nutritional and functional properties that make it particularly well-suited for supporting muscle growth and recovery. ² As a byproduct of cheese production, whey was once considered a waste product but has since been recognized as a valuable source of high-quality protein with numerous potential health benefits. ³ The importance of protein in the human diet cannot be overstated, particularly for those engaged in regular physical activity or seeking to build and maintain muscle mass. Proteins serve as the building blocks for muscle tissue, enzymes, hormones, and various other essential components of the human body. ⁴ Among the different types of protein available, whey protein stands out due to its exceptional amino acid profile and rapid digestibility, making it an ideal choice for post-exercise recovery and muscle protein synthesis (MPS).

Whey protein is derived from milk and comprises approximately 20% of the total protein content in cow's milk, with the remaining 80% being casein protein. The production of whey protein involves separating it from casein during the cheese-making process, followed by various purification and processing steps to create different forms of whey protein supplements. ⁵ These forms include whey protein concentrate (WPC), whey protein isolate (WPI), and whey protein hydrolysate (WPH), each with its own unique characteristics and potential applications. ⁶ The chemical composition of whey protein is central to its effectiveness as a supplement for muscle growth. Rich in essential amino acids, particularly branched-chain amino acids (BCAAs) such as leucine, isoleucine, and valine, whey protein provides the necessary building blocks for MPS. ⁷ The high leucine content is of particular interest, as leucine has been identified as a key trigger for initiating MPS through the activation of specific cellular signaling pathways. ⁸ Beyond its amino acid profile, whey protein also contains bioactive compounds such as lactoferrin, immunoglobulins, and glycomacropeptides, which may contribute to its overall health benefits. ⁹ These components have been associated with various physiological effects, including immune system modulation, antioxidant activity, and potential antimicrobial properties. ¹⁰ The presence of these bioactive compounds sets whey protein apart from other protein sources and may contribute to its effectiveness as a supplement for athletes and active individuals. The chemical properties of whey protein also influence its functionality in various applications. Its high solubility, especially at low pH, makes it easy to incorporate into beverages and other food products. ¹¹ The ability of whey proteins to form gels, emulsify fats, and stabilize foams further expands its potential uses in food formulations and may contribute to improved texture and sensory characteristics of protein-fortified products. ¹²

For athletes and individuals engaged in regular resistance training, the impact of whey protein supplementation on muscle growth and performance is of paramount importance. Numerous studies have investigated the effects of whey protein intake on MPS, lean body mass accretion, and strength gains.^13–15 The rapid digestibility and absorption of whey protein, coupled with its rich essential amino acid content, make it particularly effective for stimulating MPS in the post-exercise period. The mechanisms by which whey protein supports muscle growth are multifaceted. In addition to providing the necessary amino acids for protein synthesis, whey protein ingestion has been shown to stimulate anabolic signaling pathways, such as the mammalian target of rapamycin (mTOR) pathway, which plays a crucial role in regulating MPS. ¹⁶ Furthermore, the insulinogenic properties of whey protein may contribute to its anabolic effects by promoting nutrient uptake and inhibiting muscle protein breakdown.

While the benefits of whey protein supplementation for muscle growth and recovery are well-documented, it is important to consider potential safety concerns and adverse effects associated with its use.^1,17 Questions have been raised regarding the long-term effects of high protein intake on kidney and liver function, as well as potential allergic reactions in sensitive individuals. Understanding these potential risks is crucial for developing appropriate recommendations for whey protein use in athletic and general populations. This review aims to provide a comprehensive examination of the chemical properties of whey protein in protein powders and their impact on muscle growth in athletes. By exploring the composition, processing methods, and mechanisms of action of whey protein, this review seeks to elucidate the factors that contribute to its effectiveness as a supplement for muscle growth and recovery. Additionally, this review evaluates the current evidence regarding the effects of whey protein supplementation on muscle hypertrophy, strength gains, and athletic performance, while also addressing safety considerations and potential adverse effects.

Chemical Composition and Properties of Whey Protein

Whey protein, a byproduct of cheese production, has gained significant attention in the nutritional world due to its unique chemical composition and properties. This section delves into the intricate details of whey proteins’ structure, exploring its major protein fractions, amino acid profile, bioavailability, digestibility, and the effects of processing on its chemical properties.

Major Protein Fractions in Whey

Whey protein is not a single entity but rather a complex mixture of various protein fractions, each with its own distinct characteristics and potential benefits. The major protein fractions found in whey include β-lactoglobulin, α-lactalbumin, bovine serum albumin, immunoglobulins, and lactoferrin. ¹⁸

Figure 1 shows the structure of β-lactoglobulin and α-lactalbumin. β-lactoglobulin is the most abundant protein fraction in whey, accounting for approximately 50–55% of the total whey protein content. This globular protein is rich in BCAAs and plays a crucial role in the overall nutritional value of whey. ¹⁹ β-lactoglobulin's structure allows it to bind to various hydrophobic molecules, potentially enhancing the absorption of fat-soluble vitamins and other bioactive compounds. ²⁰ α-lactalbumin is the second most prevalent protein fraction, comprising about 20–25% of whey protein. ²¹ This protein is particularly rich in tryptophan, an essential amino acid that serves as a precursor to serotonin, a neurotransmitter involved in mood regulation. α-lactalbumin also plays a vital role in lactose synthesis in mammary glands and has been associated with various physiological functions, including immune system modulation and stress reduction.

Figure 1.

Three-dimensional structures of ß-lactoglobulin (left) and α-lactalbumin (right), the two principal protein fractions in whey. ß-lactoglobulin, a globular protein, is depicted with its prominent hydrophobic binding pocket, which facilitates the transport of fat-soluble vitamins and bioactive compounds. α-lactalbumin, shown with its calcium-binding domain, plays a critical role in lactose synthesis and is rich in tryptophan, an essential precursor for serotonin. These structural features underscore the functional and nutritional properties of whey proteins, particularly their solubility, digestibility, and bioactivity in promoting muscle protein synthesis and immune modulation. ²²

Bovine serum albumin (BSA) makes up approximately 5–10% of whey protein. ²³ This large, multi-functional protein is known for its ability to bind and transport various molecules, including fatty acids, hormones, and minerals. BSA's presence in whey contributes to its overall nutritional profile and may enhance the bioavailability of certain nutrients. Immunoglobulins, primarily IgG, IgA, and IgM, constitute about 10–15% of whey protein.^24–26 These antibodies play a crucial role in the immune system, providing passive immunity and potentially contributing to the immunomodulatory effects associated with whey protein consumption. Lactoferrin, although present in smaller quantities (1-2%), is a highly bioactive protein fraction with numerous potential health benefits. This iron-binding glycoprotein exhibits antimicrobial properties through its ability to sequester iron from pathogenic microorganisms, along with antioxidant and anti-inflammatory properties, making it a subject of interest in various fields of nutritional research. ²⁷

The diverse composition of these protein fractions contributes to the overall functionality and nutritional value of whey protein, setting it apart from other protein sources.

Amino Acid Profile and Essential Amino Acid Content

One of the key factors contributing to whey protein's popularity as a nutritional supplement is its exceptional amino acid profile, particularly its high content of essential amino acids (EAAs). Essential amino acids are those that cannot be synthesized by the human body and must be obtained through dietary sources. Whey protein is considered a complete protein, meaning it contains all nine essential amino acids in adequate proportions to support human nutritional needs. ²⁸ The EAAs found in whey protein include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Of particular importance are the BCAAs - leucine, isoleucine, and valine - which play crucial roles in MPS and energy production during exercise. ²⁹

Leucine, the most abundant BCAA in whey protein, has garnered special attention due to its role as a key trigger for MPS. Leucine activates the mTOR signaling pathway, which is central to the regulation of protein synthesis in skeletal muscle (Figure 2). ³⁰ The high leucine content in whey protein, typically around 10–12% of total protein content, is thought to be a major factor in its effectiveness for promoting muscle growth and recovery.

Figure 2.

Role of leucine in the regulation of mTOR signal pathway.

In addition to EAAs, whey protein also contains a balanced profile of non-essential amino acids, including glutamine, which is important for immune function and intestinal health. Whey protein is often considered a high-quality protein source due to its rich essential amino acid profile, particularly leucine, which plays a key role in stimulating MPS and supporting recovery. However, its amino acid composition does not entirely mimic that of skeletal muscle.^31,32

Bioavailability and Digestibility

The nutritional value of a protein source is not solely determined by its amino acid composition but also by its bioavailability and digestibility. Whey protein excels in both these aspects, contributing to its status as a high-quality protein source. Bioavailability refers to the proportion of ingested protein that is actually absorbed and utilized by the body. Whey protein has a high bioavailability, with studies suggesting that it is absorbed more rapidly and completely than other protein sources such as casein or soy. This rapid absorption is particularly beneficial in the post-exercise period when muscle tissue is primed for protein synthesis.^33,34

The digestibility of whey protein is typically measured using the Protein Digestibility Corrected Amino Acid Score (PDCAAS) or the more recent Digestible Indispensable Amino Acid Score (DIAAS). ³⁵ Both methods assess the quality of a protein based on its amino acid composition and digestibility. Whey protein consistently scores at or near the maximum on these scales, indicating excellent digestibility and a well-balanced amino acid profile. For example, WPI and WPC exhibited standardized ileal digestibility (SID) values for indispensable amino acids that were significantly higher than those for plant-based proteins such as pea protein concentrate, soy protein isolate, soy flour, and wheat. The mean SID of indispensable amino acids in WPI and WPC reached 98% and 96%, respectively, while SPI and wheat yielded lower values of 93% and 85%. This highlights the superior amino acid bioavailability of whey proteins. ³⁶ The high digestibility of whey protein is attributed to several factors. Firstly, the protein fractions in whey are relatively small and globular, making them easier for digestive enzymes to break down. ³⁵ Secondly, whey protein lacks anti-nutritional factors that can impair protein digestion, such as trypsin inhibitors found in some plant-based proteins. ³⁷ Lastly, the processing methods used to produce WPC and isolates often result in the removal of lactose and fat, further enhancing digestibility. The combination of rapid absorption and high digestibility means that whey protein can quickly elevate blood amino acid levels, providing a potent stimulus for MPS. This property is particularly advantageous for athletes and individuals engaged in resistance training, as it allows for efficient delivery of amino acids to muscle tissue during the critical post-exercise recovery period.

Effects of Processing on Chemical Structure and Properties

The processing of whey protein can significantly impact its chemical structure and functional properties. Various methods are employed to convert liquid whey into powdered supplements, each with potential effects on the final product's characteristics. WPC is typically produced through a series of filtration and concentration steps. The protein content of WPC can range from 35% to 80%, with the remaining components being primarily lactose, minerals, and fats. ³⁷ WPI undergoes additional processing to achieve a higher protein concentration, typically above 90%. WPH is produced by partially pre-digesting the protein through enzymatic hydrolysis.

Heat treatment, a common step in whey protein processing, can cause denaturation of the protein structure. While this may reduce some functional properties, such as solubility at certain pH levels, it can also expose hidden reactive groups, potentially enhancing digestibility and the protein's ability to form gels. ³⁸ The use of high-pressure homogenization in some processing methods can affect the size and distribution of protein particles, influencing properties such as viscosity and emulsion stability. ³⁹ This technique can be particularly useful in creating whey protein products with improved texture and mouthfeel. Spray drying, the final step in producing whey protein powder, involves rapid dehydration of liquid whey. ⁴⁰ While this process is generally considered to have minimal impact on protein quality, the high temperatures involved can potentially cause some degree of protein denaturation and Maillard reactions between proteins and residual lactose. ⁴¹ Recent advancements in processing technologies, such as membrane filtration and supercritical fluid extraction, aim to minimize the negative impacts of processing on whey protein structure while enhancing desired functional properties.^42,43 These methods offer the potential for creating tailored whey protein products with specific characteristics suited to different applications. In order to provide a clearer comparison of the different types of whey protein and their specific characteristics, Table 1 summarized key properties, including protein content, amino acid profiles, molecular weight ranges, and functional features.

Table 1.

Comparison of Different Types of Whey Protein Based on Protein Content, key Amino Acids, Molecular Weight, and Functional Characteristics.

Type	Protein Content (%)	Key Amino Acids	Molecular Weight (MW)	Characteristics
WPC	35–80	Rich in leucine, isoleucine, and valine	10–100 kDa	Contains lactose, fat, and bioactive compounds like lactoferrin and immunoglobulins. Suitable for general nutrition and muscle recovery but less ideal for lactose-intolerant individuals.
WPI	≥90	High leucine content (∼10-12%)	10–50 kDa	Highly purified with minimal lactose and fat. Rapidly absorbed, making it ideal for post-exercise recovery and low-carbohydrate diets.
WPH	≥90	High in di- and tri-peptides	<10 kDa	Pre-digested for faster absorption. Reduced allergenicity but may have a bitter taste. Commonly used in sports nutrition and medical applications.

Understanding the effects of processing on whey protein's chemical structure and properties is crucial for optimizing its use in various applications, from sports nutrition to functional food ingredients. As processing techniques continue to evolve, so too will our ability to harness the full potential of whey protein's unique chemical composition and functional properties.

Electrochemical Properties of Whey Protein

The electrochemical properties of whey proteins play a crucial role in their functionality and behavior in various systems. These properties arise from the distribution of charged amino acids along the protein chains and their interactions with the surrounding environment. Understanding the electrochemical nature of whey proteins is essential for predicting and controlling their behavior in food systems, as well as for developing novel applications in areas such as biosensors and functional coatings.

One of the key electrochemical properties of whey proteins is their isoelectric point (pI). The pI is the pH at which the net charge of the protein is zero, resulting in minimal electrostatic repulsion between protein molecules. ⁴⁴ For the major whey proteins, the pI values are as follows: β-lactoglobulin (pI 5.2), α-lactalbumin (pI 4.5-4.8), and bovine serum albumin (pI 4.7-4.9). At pH values above their pI, whey proteins carry a net negative charge, while at pH values below the pI, they carry a net positive charge. ⁴⁵ This pH-dependent charge behavior significantly influences the solubility, stability, and interactions of whey proteins in various applications. The charge distribution on whey proteins also affects their electrochemical potential, which can be measured using techniques such as zeta potential analysis. ⁴⁶ The zeta potential provides information about the magnitude of electrostatic repulsion or attraction between particles and is an important factor in determining the stability of protein solutions. Whey proteins typically exhibit negative zeta potentials at neutral pH, contributing to their stability in solution through electrostatic repulsion. ⁴⁷

The electrochemical behavior of whey proteins is also influenced by their ability to bind metal ions. This metal-binding capacity is particularly notable in proteins like lactoferrin, which can strongly bind iron ions. The interaction between whey proteins and metal ions can lead to changes in the proteins’ electrochemical properties, affecting their stability, solubility, and functionality. ⁴⁸ This metal-binding ability has been utilized in applications such as the development of antimicrobial coatings and the removal of heavy metals from contaminated water.^49,50 Furthermore, the electrochemical properties of whey proteins can be modulated through chemical modifications. Techniques such as acylation, phosphorylation, or glycation can alter the charge distribution and electrochemical behavior of whey proteins, enabling the tailoring of their properties for specific applications. ⁵¹ For instance, increasing the negative charge on whey proteins through succinylation can enhance their emulsifying properties and stability at neutral pH. ⁵² The study of whey protein electrochemistry has also led to innovative applications in food packaging. Whey protein-based films and coatings with enhanced antimicrobial properties have been developed by incorporating electrochemically active compounds or through the application of electric fields during film formation. ⁵³ These electrochemically enhanced whey protein materials show promise for extending the shelf life of perishable foods.

Types of Whey Protein Powders

The world of whey protein powders is diverse, with several distinct types available on the market. Each type of whey protein powder undergoes different processing methods, resulting in varying compositions and properties. This section explores the three main types of whey protein powders: WPC, WPI, and WPH.

Whey Protein Concentrate

WPC is the most common and economical form of whey protein powder. It is produced through a series of filtration and concentration processes that remove water, lactose, and some minerals from liquid whey. The resulting product contains a variable protein content, typically ranging from 35% to 80% by weight, with the remainder comprising lactose, lipids, and minerals. The production of WPC begins with the ultrafiltration of liquid whey, which separates the protein molecules from smaller components like lactose and minerals. ⁵⁴ This process is followed by diafiltration, where water is added to the concentrated protein solution to further wash out residual non-protein components. ⁵⁵ The final step involves evaporation and spray drying to create a powder form.

WPC with lower protein concentrations (35-50%) often retains more of the native whey components, including lactoferrin and immunoglobulins. These bioactive compounds contribute to the potential health benefits associated with whey protein consumption, such as immune system support and antioxidant activity. ⁵⁶ However, the presence of lactose in these lower-concentration WPCs may make them less suitable for individuals with lactose intolerance. ⁵⁷ Higher protein concentration WPCs (70-80%) undergo additional processing to remove more lactose and fat, resulting in a product with enhanced protein content. ⁵⁸ These higher-grade WPCs are often preferred in sports nutrition applications due to their increased protein-to-weight ratio.

The protein fractions in WPC largely retain their native structures, which contributes to its functional properties in food applications. WPC exhibits good solubility, emulsification capacity, and foaming properties, making it versatile for use in various food products beyond just protein supplements.

Whey Protein Isolate

WPI represents a more refined form of whey protein, characterized by its high protein content of 90% or greater. The production of WPI involves more extensive processing than WPC to remove virtually all non-protein components, resulting in a product with minimal lactose and fat content. ⁵⁹ Two main methods are employed to produce WPI: ion exchange chromatography and membrane filtration. Ion exchange chromatography involves passing liquid whey through a charged resin that selectively binds protein molecules. The bound proteins are then eluted using a salt solution, resulting in a highly pure protein fraction. ⁶⁰ This method can potentially alter the ratios of different protein fractions, as some proteins bind more readily to the resin than others. Membrane filtration, specifically microfiltration and ultrafiltration, offers an alternative method for producing WPI. ⁶¹ This process uses specialized membranes with precise pore sizes to separate proteins from other whey components based on molecular size. Membrane filtration tends to preserve the natural proportions of whey protein fractions better than ion exchange, potentially retaining more of the native bioactive compounds.

The high protein content and low lactose levels in WPI make it an excellent choice for individuals seeking to maximize protein intake while minimizing carbohydrate consumption. This characteristic is particularly valuable for athletes following low-carbohydrate diets or those with lactose sensitivities. WPI typically displays excellent solubility across a wide pH range, owing to its high purity and minimal interference from non-protein components. ⁶² This property makes WPI ideal for use in clear beverages and other applications where solubility is crucial. However, the extensive processing required to produce WPI can potentially affect some of the proteins’ native structures, which may impact certain functional properties compared to less processed WPC.

Whey Protein Hydrolysate

WPH represents the most extensively processed form of whey protein. WPH is produced by subjecting whey protein (usually WPC or WPI) to partial enzymatic hydrolysis, breaking down the protein chains into smaller peptides. ⁶³ This process aims to enhance the digestibility and absorption rate of the protein, potentially offering advantages in specific nutritional and functional applications. The production of WPH involves carefully controlled enzymatic reactions using proteolytic enzymes such as pepsin, trypsin, or proprietary enzyme blends. ⁶⁴ The degree of hydrolysis can be adjusted to achieve desired characteristics, with more extensive hydrolysis resulting in smaller peptides and free amino acids. The hydrolysis process is typically followed by heat inactivation of the enzymes, filtration to remove any residual intact proteins, and spray drying to produce the final powder form. ⁶⁵

One of the primary advantages of WPH is its rapid absorption rate. The smaller peptides produced through hydrolysis can be absorbed more quickly in the gastrointestinal tract compared to intact proteins. ⁶⁶ This property has made WPH popular among athletes seeking fast post-exercise recovery and individuals with impaired protein digestion. ⁶⁷ The hydrolysis process can also reduce the allergenic potential of whey protein by breaking down epitopes responsible for allergic reactions. This characteristic makes WPH a potential option for individuals with mild milk protein allergies, although it's important to note that it may not be suitable for those with severe allergies.

However, the hydrolysis process can lead to the development of bitter flavors, which may impact the palatability of WPH products. Manufacturers often employ various strategies, such as using specific enzyme combinations or adding flavoring agents, to mitigate this issue. ⁶⁸ The functional properties of WPH can differ significantly from those of WPC and WPI. The smaller peptides in WPH generally exhibit reduced gelation and foaming capacities but may offer improved solubility and heat stability in certain applications.

Comparison of Chemical Properties Between Types

Understanding the differences among the chemical properties between WPC, WPI, and WPH is crucial for selecting the appropriate type of whey protein for specific applications. These differences stem from variations in composition and processing methods. Figure 3 presents a comparative summary of their processing methods, functional properties, and chemical composition. This table highlights the distinct steps involved in their production—such as ultrafiltration, enzymatic hydrolysis, and ion exchange chromatography—and how these methods influence their protein content, lactose levels, solubility, and other functional characteristics.

Figure 3.

Overview of the processing methods, functional properties, and chemical composition of WPC, WPI, and WPH.

Protein content is one of the most notable differences among the three types. WPC typically contains 35–80% protein, WPI boasts 90% or higher protein content, while WPH's protein content can vary but is often similar to WPI. ⁶⁹ The remaining composition of WPC includes more lactose, fat, and minerals compared to WPI and WPH, which have these components largely removed. Amino acid profiles remain relatively consistent across all three types, with slight variations possible due to processing methods. ⁷⁰ However, the form in which these amino acids are present differs. In WPC and WPI, amino acids exist primarily in intact protein chains, while WPH contains a higher proportion of free amino acids and small peptides.

Bioavailability and absorption rates vary among the types. WPH generally exhibits the fastest absorption rate due to its pre-digested nature, followed by WPI, and then WPC. ⁷¹ The presence of other nutrients in WPC, particularly fat, can slow its digestion rate slightly compared to the more purified forms. Solubility is another key differentiating factor. WPI typically demonstrates the best solubility across a wide pH range due to its high purity. WPC's solubility can be affected by its higher content of non-protein components, particularly at lower pH levels. ⁷² WPH often shows good solubility, but this can vary depending on the degree of hydrolysis. Thermal stability differs among the types, with implications for processing and application in food systems. WPC generally retains more of the proteins’ native structures and thus may be more susceptible to heat-induced denaturation. WPI, ⁷³ having undergone more processing, may show altered thermal behavior. WPH, with its shorter peptide chains, often exhibits improved heat stability compared to intact proteins.

Functional properties such as emulsification, foaming, and gelation also vary. WPC often performs well in these areas due to the intact nature of its proteins and the presence of phospholipids. ⁷⁴ WPI, while excellent in many functional applications, may show slight differences due to the removal of certain components during processing. ⁷⁵ WPH typically exhibits reduced functionality in these areas due to the breakdown of protein structures, although this can be advantageous in specific applications requiring high solubility or low viscosity. ⁷⁶ Flavor profiles differ significantly among the types. WPC often retains more of the native milk flavor, which can be desirable in certain applications. WPI, being more purified, typically has a milder flavor. ⁷⁷ WPH can develop bitter notes due to the exposure of hydrophobic amino acid residues during hydrolysis, which can limit its use in some applications without additional flavoring. ⁷⁸

Mechanisms of Action for Muscle Growth

The process of muscle growth, or hypertrophy, is a complex physiological phenomenon that involves multiple interconnected mechanisms. Whey protein, with its unique composition and properties, plays a crucial role in stimulating and supporting these mechanisms. This section delves into the various ways in which whey protein contributes to muscle growth, exploring the stimulation of MPS, the specific role of leucine and other branched-chain amino acids, hormonal responses, and the cellular signaling pathways involved in this process.

Stimulation of MPS

MPS is the biological process by which new proteins are generated within muscle cells, leading to muscle growth and repair. This process is in constant balance with muscle protein breakdown, and net muscle growth occurs when synthesis exceeds breakdown over time. ⁷⁹ Whey protein has been shown to be particularly effective at stimulating MPS, which is a key factor in its popularity as a supplement for muscle growth.^14,80 The efficacy of whey protein in stimulating MPS can be attributed to several factors. Firstly, whey protein is rapidly digested and absorbed, leading to a quick and significant increase in blood amino acid levels. This rapid availability of amino acids creates an anabolic environment conducive to protein synthesis. Secondly, whey protein contains all the essential amino acids in proportions that closely match the requirements for human MPS. This complete and balanced amino acid profile ensures that the body has all the necessary building blocks readily available for constructing new muscle proteins. Furthermore, the timing of whey protein ingestion can significantly impact its effectiveness in stimulating MPS. Recent studies have indicated that different types of whey protein, such as WPI and WPC, exhibit varying effects on MPS. For instance, whey protein isolate, which contains a higher concentration of protein and lower levels of fat and lactose, has been shown to stimulate MPS more effectively than whey protein concentrate due to its rapid digestion and absorption characteristics. Research by Witard et al ¹⁴ demonstrated that post-exercise ingestion of whey protein isolate resulted in significantly higher rates of myofibrillar MPS compared to concentrate, underscoring the importance of protein quality in dietary supplementation for muscle growth.

Consumption of whey protein immediately following resistance exercise has been shown to be particularly effective, as this is when muscle tissue is most receptive to nutrient uptake and protein synthesis. For example, Tang et al ¹³ investigated the effects of consuming WPI combined with carbohydrates on MPS following resistance exercise in trained young men. Eight participants, aged 21 ± 1.0 years with a BMI of 26.8 ± 0.9 kg/m², engaged in a double-blind randomized crossover trial. They performed unilateral leg resistance exercises and then consumed either a whey protein plus carbohydrate beverage (10 g protein and 21 g fructose) or a carbohydrate-only beverage (21 g fructose and 10 g maltodextrin). MPS was measured using pulse-tracer injections. Results indicated that exercise stimulated MPS in both the whey protein and carbohydrate groups, with the whey protein group showing a significantly higher rate of MPS in the exercised leg compared to the carbohydrate group (p < .001). Specifically, MPS in the whey protein-exercised leg was greater than in the carbohydrate-exercised leg. Macnaughton et al ⁸¹ also conducted a similar work. Participants were divided into two groups based on their lean body mass (LBM): lower LBM (≤65 kg) and higher LBM (≥70 kg). They ingested either 20 g or 40 g of whey protein (WPI provided by GlaxoSmithKline as a 500 mL drink) post-exercise in a randomized, double-blind, crossover design (Figure 4). Results showed that MPS was significantly higher with the ingestion of 40 g of whey protein (0.059 ± 0.020%·h^-1) compared to 20 g (0.049 ± 0.020%·h^-1, p = .005). This response was consistent across both LBM groups. Plasma leucine concentrations peaked at 45 min with 20 g and at 60 min with 40 g, with higher overall concentrations observed with 40 g. Phenylalanine oxidation rates and plasma urea concentrations were also greater with 40 g of protein. These findings indicated that 40 g of whey protein stimulated a greater MPS response than 20 g, regardless of total LBM, suggesting that a higher protein dose may be more effective for maximizing MPS following whole-body resistance exercise. In another study, MPS in middle-aged women and men revealed significant differences in muscle protein handling after whey protein ingestion. ⁸² They investigated 12 women (46 ± 2 years) and 12 men (43 ± 3 years) who consumed 25 g of whey protein while receiving amino acid infusions. Postabsorptive myofibrillar protein synthesis rates were similar between women (0.035 ± 0.004%/h) and men (0.030 ± 0.002%/h). However, after whey protein ingestion, women demonstrated a substantially greater myofibrillar protein synthetic response compared to men. During the early postprandial period (0-2 h), protein synthesis rates increased to 0.061 ± 0.004%/h in women versus 0.031 ± 0.002%/h in men. Over the entire 5-h postprandial period, synthesis rates remained 30% higher in women (0.045 ± 0.002%/h) compared to men (0.034 ± 0.002%/h). Importantly, protein digestion and absorption kinetics showed no significant differences between sexes, with approximately 55% of protein-derived amino acids appearing in circulation (14.0 ± 0.3 g in women vs 13.3 ± 0.3 g in men). These findings demonstrated that while whey protein was equally well digested and absorbed in both sexes, women exhibited greater muscle protein synthetic responses to the same protein dose, suggesting they may require less dietary protein to achieve similar anabolic effects compared to men.

Figure 4.

Schematic diagram of infusion trial protocol. ⁸¹

Role of Leucine and Branched-Chain Amino Acids

Among the amino acids found in whey protein, leucine and the other BCAAs - isoleucine and valine - play a particularly important role in muscle growth. ⁸³ Leucine, in particular, has been identified as a key trigger for initiating MPS, acting as a signaling molecule in addition to its role as a building block for new proteins. ⁸⁴ Leucine's unique ability to stimulate MPS is largely due to its role in activating a key regulatory protein called the mTOR. ¹⁶ mTOR acts as a master switch for protein synthesis, integrating signals from nutrients, growth factors, and mechanical stress to regulate cell growth and proliferation. When leucine levels in the cell increase, it triggers the activation of mTOR, which in turn initiates a cascade of events leading to increased protein synthesis. ⁸⁵

The other BCAAs, isoleucine and valine, while not as potent as leucine in directly stimulating MPS, play supportive roles in the process of muscle growth. They contribute to the overall amino acid pool available for protein synthesis and may help to spare leucine from oxidation, thereby prolonging its anabolic effects. ⁸⁶ Additionally, BCAAs can be metabolized directly in muscle tissue, providing a readily available energy source during exercise and potentially reducing muscle protein breakdown. ⁸⁷ Whey protein is particularly rich in leucine compared to other protein sources, containing approximately 10–12% leucine by weight. This high leucine content is a significant factor in whey protein's effectiveness for stimulating MPS and supporting muscle growth. Some research suggests that a leucine threshold of around 2–3 grams per meal is necessary to maximally stimulate MPS, a level easily achieved with a typical serving of whey protein.^28,88

Hormonal Responses (eg Insulin)

The consumption of whey protein also elicits various hormonal responses that contribute to its muscle-building effects. One of the most significant of these is the stimulation of insulin secretion.^89–91 Insulin is an anabolic hormone that plays a crucial role in nutrient uptake and utilization by cells, including muscle cells. When whey protein is ingested, it causes a rapid increase in blood amino acid levels, which in turn stimulates the pancreas to release insulin. This insulin response serves several functions that support muscle growth.^92,93 Firstly, insulin enhances the uptake of amino acids into muscle cells, ensuring that the building blocks for new protein synthesis are readily available. Secondly, insulin activates various anabolic pathways within the cell, including the mTOR pathway, further promoting protein synthesis. Lastly, insulin has an anti-catabolic effect, suppressing muscle protein breakdown and thus shifting the overall balance towards muscle growth. However, recent findings suggest that the role of insulin in MPS is more nuanced than previously understood. Everman et al ⁹⁴ demonstrated that while insulin suppresses whole-body proteolysis, it does not stimulate MPS when only BCAA are elevated in plasma. Their study showed that insulin infusion decreased whole-body phenylalanine appearance rates (a marker of proteolysis) by approximately 25% in both control and BCAA-supplemented conditions. However, fractional synthesis rates of muscle proteins remained unchanged, even with increased plasma BCAA concentrations. These results highlight the importance of non-BCAA EAAs in enabling insulin to stimulate MPS. Without sufficient levels of non-BCAA EAAs, insulin's anabolic effects on MPS appear limited. The insulinogenic effect of whey protein is particularly pronounced compared to other protein sources, likely due to its rapid digestion and high content of certain amino acids, such as leucine and isoleucine, which are known to be potent insulin secretagogues. This robust insulin response contributes to whey protein's effectiveness as a post-workout supplement, as it helps to quickly replenish muscle glycogen stores in addition to supporting protein synthesis. It's worth noting that while the insulin response to whey protein is generally considered beneficial for muscle growth, excessive or chronic elevation of insulin levels can have negative health consequences. Therefore, the timing and quantity of whey protein consumption should be considered in the context of overall diet and health goals.

Cellular Signaling Pathways Involved

The process of muscle growth involves a complex network of cellular signaling pathways that coordinate the various aspects of protein synthesis, cell growth, and adaptation to exercise. Whey protein interacts with and influences several of these pathways, contributing to its muscle-building effects. The pathway, as mentioned earlier, is a central regulator of protein synthesis and cell growth.^16,29,30 Whey protein, particularly through its high leucine content, activates mTOR complex 1 (mTORC1), which then phosphorylates downstream targets such as p70S6 kinase (p70S6K) and 4E-binding protein 1 (4E-BP1). These phosphorylation events lead to increased translation initiation and elongation, key steps in protein synthesis. Another important signaling pathway influenced by whey protein is the AMPK (AMP-activated protein kinase) pathway. ⁹⁵ AMPK is typically activated during energy-depleted states, such as during exercise, and acts to inhibit anabolic processes while promoting catabolic ones. The rapid provision of amino acids from whey protein can help to suppress AMPK activation, thus removing this inhibitory signal on protein synthesis and further promoting an anabolic state. The MAPK (mitogen-activated protein kinase) pathway is also involved in the muscle growth response to whey protein and exercise.^96,97 This pathway, which includes ERK1/2, p38 MAPK, and JNK, responds to various cellular stresses and growth signals. Activation of MAPK pathways can lead to increased expression of genes involved in muscle growth and physiological adaptations to exercise, such as enhanced hypertrophy, improved metabolic efficiency, and resistance to fatigue. Whey protein consumption also influences pathways involved in muscle protein breakdown, such as the ubiquitin-proteasome pathway. ⁹⁸ By promoting an anabolic state and increasing insulin levels, whey protein helps to suppress this protein degradation pathway, further shifting the balance towards muscle growth.

The integration of these various signaling pathways results in a coordinated cellular response that promotes muscle hypertrophy. This response includes increased protein synthesis, decreased protein breakdown, enhanced nutrient uptake, and activation of satellite cells (muscle stem cells) that contribute to muscle growth and repair. ⁹⁹ It's important to note that the activation of these pathways by whey protein is most effective when combined with resistance exercise. The mechanical stress of exercise creates a permissive environment for muscle growth, priming the muscle cells to respond more robustly to the anabolic signals provided by whey protein. ¹⁰⁰

Impact on Muscle Growth in Athletes

The use of whey protein supplements has become ubiquitous in the world of athletics and bodybuilding, with proponents claiming significant benefits for muscle growth, strength, and overall performance. For the purposes of this discussion, an athlete is defined as an individual who engages in regular physical training and competition in sports, whether at a recreational or professional level. This section examines the empirical evidence surrounding these claims, exploring the effects of whey protein supplementation on LBM and muscle hypertrophy, its influence on strength and athletic performance, considerations for optimal dosing and timing of intake.

Skeletal Muscle Types and Their Role in Athletic Training

Skeletal muscle is composed of different fiber types, each with distinct structural and functional properties, which play critical roles in various forms of athletic training. ¹⁰¹ These muscle fibers are broadly categorized into two main types: Type I (slow-twitch) and Type II (fast-twitch). Type I fibers are highly oxidative and fatigue-resistant, making them well-suited for endurance activities such as long-distance running or cycling. ¹⁰² These fibers rely primarily on aerobic metabolism and are characterized by a high density of mitochondria and capillaries, which support sustained, low-intensity efforts. In contrast, Type II fibers are subdivided into Type IIa and Type IIx, both of which are fast-twitch fibers. ¹⁰³ Type IIa fibers exhibit a blend of oxidative and glycolytic properties, allowing them to support moderate-intensity, repetitive activities. Type IIx fibers, however, are highly glycolytic and are specialized for short bursts of high-intensity, explosive efforts, such as sprinting or resistance training. ¹⁰⁴ These fibers are more prone to hypertrophy in response to resistance exercise, as they have a greater capacity for rapid force production and adapt more significantly to mechanical overload. The hypertrophic response to resistance training, as discussed in subsequent sections, is predominantly observed in Type II fibers, particularly Type IIx. ¹⁰⁵ This selective adaptation is crucial for athletes engaged in strength and power-based sports. On the other hand, endurance training tends to enhance the oxidative capacity of Type I fibers without significantly increasing their size. Understanding the distinct roles of these muscle fiber types provides a framework for interpreting the effects of whey protein supplementation and resistance exercise on muscle growth and performance.

Effects on LBM and Muscle Hypertrophy

The primary goal for many athletes incorporating whey protein into their dietary regimen is to increase LBM and promote muscle hypertrophy, which are critical factors in enhancing overall muscle strength and athletic performance. Research has shown that increasing LBM is foundational for achieving improvements in muscle strength.^106,107 Therefore, while muscle strength is a key objective, the processes of increasing LBM and promoting muscle hypertrophy play an essential role in realizing that goal. Numerous studies have investigated the effects of whey protein supplementation on these parameters, with results generally supporting its efficacy, particularly when combined with resistance training programs. Research has consistently shown that whey protein supplementation, when paired with regular resistance exercise, can lead to greater increases in LBM compared to placebo or carbohydrate supplements. In a meta-analysis conducted by Miller et al, ¹⁰⁸ the effects of whey protein and resistance exercise on body composition were examined across fourteen randomized controlled trials (RCTs) involving 626 adult participants (246 males and 380 females). The study aimed to determine the impact of WP on body weight, BMI, body fat, LBM, fat-free mass (FFM), and waist circumference. Results indicated significant reductions in body weight and body fat within groups where whey protein was used as a replacement for other calorie sources, with an average weight loss of 4.20 kg and body fat reduction of 3.74 kg. Comparisons between whey protein and carbohydrate controls showed more favorable outcomes for whey protein, although these findings did not reach statistical significance. Additionally, studies incorporating resistance exercise alongside whey protein supplementation reported a significant increase in LBM, averaging 2.24 kg. The meta-analysis concluded that WP, whether as a dietary supplement combined with resistance exercise or as part of a weight loss or maintenance diet, contributed positively to body composition improvements. These findings support the use of whey protein for enhancing body composition, particularly when integrated with resistance training.

Muscle hypertrophy, the increase in muscle fiber size, is closely related to gains in lean body mass. ¹⁰⁹ Whey protein supplementation has been shown to enhance muscle fiber cross-sectional area, particularly in type II (fast-twitch) muscle fibers, which are most responsive to hypertrophic stimuli.^110,111 This selective hypertrophy of type II fibers is particularly beneficial for athletes engaged in power and strength-based sports. It's important to note that the effects of whey protein on LBM and muscle hypertrophy are most pronounced in individuals engaging in regular resistance training. ¹¹² The synergistic effect of protein supplementation and resistance exercise creates an optimal environment for muscle growth, with the exercise providing the necessary stimulus and the whey protein supplying the building blocks for new muscle tissue. The impact of whey protein on LBM and muscle hypertrophy appears to be more significant in novice or recreational athletes compared to highly trained individuals. ¹¹³ This observation suggests that as athletes become more advanced in their training, the relative contribution of protein supplementation to further muscle gains may diminish. However, it is essential to consider the role of dosing and protein distribution throughout the day. Advanced athletes may require higher supplementation levels, with recommended intakes ranging from 1.4 to 2.0 g/kg/day, to support continued muscle growth effectively. Furthermore, distributing protein intake across multiple servings throughout the day, particularly around training sessions, can enhance MPS and optimize the anabolic response. This highlights the importance of progressive training and comprehensive nutritional strategies for sustained improvement in muscle mass and strength.

Influence on Strength and Performance

While increases in muscle mass are often a primary goal, athletes are ultimately concerned with how these changes translate to improvements in strength and performance. The influence of whey protein supplementation on these parameters has been the subject of numerous studies, with results generally indicating positive effects, albeit with some variability. Strength gains associated with whey protein supplementation are often correlated with increases in muscle mass. As muscle cross-sectional area increases, so does the potential for force production. Studies have shown that individuals supplementing with whey protein during resistance training programs often experience greater increases in measures of muscular strength, such as one-repetition maximum (1RM) lifts, compared to those consuming placebo or carbohydrate supplements. ¹¹⁴ The magnitude of strength gains can vary depending on the specific exercises tested, the duration of the study, and the training status of the participants. Some studies have reported increases in strength measures of 5–10% beyond what is achieved with training alone, although results can be highly individual. ¹¹⁵

In terms of athletic performance, the effects of whey protein supplementation can be more nuanced.^116,117 While strength improvements can directly benefit performance in power-based sports, the impact on endurance performance is less clear. Some studies have suggested that whey protein supplementation may improve recovery between training sessions, potentially leading to improved performance over time.^118–120 However, acute effects on endurance performance are generally not observed. For sports requiring a combination of strength and endurance, such as team sports or combat sports, the benefits of whey protein supplementation may be more pronounced. ¹²¹ The potential for improved recovery, maintenance of LBM during intensive training periods, and enhanced muscle repair could all contribute to improved overall performance in these contexts.

Optimal Dosing and Timing of Intake

Determining the optimal dosing and timing of whey protein intake is crucial for maximizing its benefits for muscle growth and performance. While individual needs can vary based on factors such as body size, training volume, and overall energy intake, research has provided some general guidelines for effective supplementation strategies. In terms of dosing, studies have identified a dose-response relationship between protein intake and MPS, with a plateau effect occurring at higher doses. ¹²² For most individuals, a single dose of 20–25 grams of whey protein appears to be sufficient to maximally stimulate MPS. This amount typically provides about 2–3 grams of leucine, which is considered the threshold for triggering significant increases in MPS. ¹²³ However, recent research by Trommelen et al ¹²⁴ suggests that the anabolic response to protein ingestion may not have an upper limit. Their findings indicate that larger individuals or those engaged in very high-volume training may benefit from slightly higher doses, up to 30–40 grams per serving, as a dose of 100 grams resulted in a greater and more prolonged MPS response compared to 25 grams. While traditional views have suggested that consuming more than 25 grams in a single dose is unlikely to provide additional benefits for MPS and may lead to increased oxidation of excess amino acids, Trommelen et al demonstrated that protein ingestion has a negligible impact on whole-body amino acid oxidation rates, particularly at higher doses. This indicates that larger doses can be effectively utilized for MPS without significant oxidative losses. To meet daily protein requirements, athletes are advised to consume approximately 1.2 to 1.8 grams of protein per kilogram of body weight. Therefore, it is recommended that individuals consume 2–3 servings of whey protein throughout the day to achieve optimal protein intake and support muscle recovery. Given the evidence from recent studies, it may also be beneficial for athletes to consider larger single doses of protein, particularly following intense training sessions, to maximize their anabolic response.

The timing of whey protein intake has been a subject of much debate in the scientific and athletic communities. The concept of an “anabolic window” immediately post-exercise has been popular, suggesting that protein should be consumed within 30 min of completing a workout for optimal results. ¹²⁵ While consuming protein soon after exercise is certainly beneficial, recent research suggests that this window may be wider than previously thought. Current evidence indicates that consuming whey protein within a few hours before or after resistance exercise can be effective for stimulating MPS and promoting muscle growth.^14,80,126 Some studies have even shown benefits from pre-sleep protein intake, particularly for overnight recovery and MPS. ¹²⁷ For athletes engaged in multiple training sessions per day, spacing protein intake evenly throughout the day in 3–4 servings may be more beneficial than consuming larger amounts less frequently. ¹²⁸ This approach helps to maintain elevated MPS rates throughout the day and may be particularly important for preserving LBM during periods of energy restriction. It's also worth considering the co-ingestion of whey protein with other nutrients. Consuming whey protein alongside carbohydrates can enhance the insulin response, potentially improving nutrient uptake and glycogen resynthesis. However, the addition of large amounts of fat to a whey protein shake may slow its digestion, potentially blunting the rapid amino acid spike that is characteristic of whey protein. ¹²⁹

Conclusions

Based on the comprehensive review presented in this paper, it is clear that whey protein plays a significant role in muscle growth and athletic performance. The unique chemical properties of whey protein, including its high content of essential amino acids, particularly leucine, contribute to its effectiveness in stimulating MPS. The rapid digestibility and absorption of whey protein, especially in its isolate and hydrolysate forms, make it an ideal supplement for post-exercise recovery. The mechanisms by which whey protein promotes muscle growth involve multiple pathways, including the activation of mTOR signaling and the stimulation of insulin secretion. Studies have consistently shown that whey protein supplementation, when combined with resistance training, can lead to significant increases in lean body mass, muscle hypertrophy, and strength gains in athletes. The optimal dosing appears to be around 20–25 grams per serving for most individuals, with timing being flexible within a few hours before or after exercise. While whey protein has demonstrated superiority in many aspects compared to other protein sources, it is important to note that its effects may be more pronounced in novice or recreational athletes compared to highly trained individuals. Future research should focus on long-term effects, potential synergies with other nutrients, and individualized supplementation strategies to maximize the benefits of whey protein for muscle growth and athletic performance.