Short-term postprandial changes in select serum biochemistry analytes in healthy adult cats

Abstract

Objectives

The aim of this study was to evaluate the magnitude, duration and significance of postprandial changes to select serum biochemistry analytes in healthy adult cats in the 12 h period after a meal.

Methods

Nine adult research cats fed commercial food were included. Blood samples were taken after a 12 h fast (hour 0), cats were offered and consumed a meal, and postprandial samples were obtained over a 12 h period starting 2 h after the baseline blood draw (hours 2, 4, 6, 8, 10 and 12). Serum samples were run on a Roche Cobas C501 chemistry analyzer to obtain concentrations of blood urea nitrogen (BUN), creatinine, phosphorus, total calcium, bicarbonate, cholesterol, magnesium, sodium, potassium and chloride. Serum concentrations of each analyte at hours 2, 4, 6, 8, 10 and 12 were compared with concentrations prior to feeding.

Results

Serum concentration for at least one postprandial time point was different from baseline fasted concentration for BUN (hour 2, P = 0.006; hour 4, P <0.0001; hour 6, P = 0.002; hour 8, P = 0.026), phosphorus (hour 2, P = 0.019), bicarbonate (hours 2, 4, 6, 8 and 10; all P <0.01), glucose (hour 12, P = 0.014), magnesium (hour 10, P = 0.029) and chloride (hour 2, P = 0.026; hour 4, P = 0.044; hour 12, P = 0.019). No significant difference was seen at any postprandial sampling point compared with baseline for serum creatinine, total calcium, cholesterol, sodium or potassium concentrations.

Conclusions and relevance

Short-term postprandial serum concentrations of BUN, phosphorus, bicarbonate and chloride differed at multiple time points within a 12 h period compared with the fasted state at baseline, with most values remaining within the reference intervals. Veterinarians should be aware of these alterations, though they are unlikely to be mistaken for pathological disease states in healthy adult cats.

Introduction

A biochemistry panel is part of routine clinical laboratory tests performed in healthy cats during wellness examinations and in sick cats as a part of a diagnostic or follow-up evaluation for disease. Veterinarians rely on the laboratory results to make important decisions about the cat’s health and disease status. There are several preanalytical factors that can influence the variability of the laboratory results, including feeding prior to sample collection and diurnal variation, ¹ which may influence disease diagnosis and the management of some patients. Therefore, an understanding of expected postprandial changes in laboratory analyte concentrations is vital to the interpretation of results.

Although fasting prior to venipuncture for sample collection is often recommended in veterinary medicine, client compliance poses a challenge and, in some cases, veterinarians need to evaluate laboratory results in a cat that has had a meal, usually within the 12 h period prior to sample collection. The short-term postprandial changes in biochemistry analytes is relatively unknown in cats. Previous studies support the fact that a recent meal can influence several blood biochemistry analyte concentrations in healthy cats, including urea, creatinine, phosphate and glucose.^1,2 As the nutrient composition of the diet could impact the findings for some analytes, it is important to confirm repeatability and validate these findings.^{3 –7}

The objective of the study was to assess the short-term postprandial changes in select serum biochemistry analytes (creatinine, blood urea nitrogen [BUN], phosphorus, bicarbonate, glucose, magnesium, chloride, total calcium, cholesterol, sodium, potassium) in healthy adult cats fed a stable controlled commercial diet. We hypothesized that some analytes, particularly BUN and phosphorus, vary over a 12 h period after feeding, compared with a fasted state.

Materials and methods

Cats

This was an opportunistic study utilizing remnant serum samples from a previously published study. ⁸ In this prospective longitudinal study, the serum samples obtained from nine purpose-bred adult research cats were used. The cats were 3-year-old domestic shorthairs (four male castrated; four female intact; one female spayed). The cats were group-housed in an indoor research facility, had free access to food and water, had access to perches and toys, and were socialized daily in addition to routine cleaning of their room and litter boxes. Cats were fed a commercial over-the-counter cat food that was formulated to meet the nutritional levels established by the Association of American Feed Control Officials Cat Food Nutrient Profiles for all life stages (Meow Mix Original Choice; Big Heart Pet) for at least 3 months prior to enrollment. The cat food was analyzed for macronutrient and mineral concentrations using methods validated by the Association of Official Agricultural Chemists International, and the nutrient profile can be found in the supplementary material. All cats had a normal physical examination, complete blood count, serum biochemistry, urinalysis (including a urine specific gravity >1.035) and total serum thyroxine concentration. The median body weight was 4.4 kg (range 3.0–5.8), and the median body condition score (Purina 1–9-point scale) was 5 (range 5–6). Most cats had a normal muscle condition score (World Small Animal Veterinary Association), with one cat determined to have mild paraspinal muscle atrophy on examination. The cats were housed and cared for in accordance with a protocol approved by the Institutional Animal Care and Use Committee at the contract research facility (protocol 170.045).

Sample collection and processing

Cats were sedated (ketamine 20 mg IM; midazolam 10 mg IM) for aseptic placement of a jugular catheter (MILACATH; MILA International). After placement, cats were placed in separate kennels until the end of sample collection with a solid floor, perch and water, and were offered a meal twice daily. The catheters were secured, flushed at least every 6 h with sterile saline and heparin locked (100 U/ml heparin sulfate). Approximately 36 h after jugular catheter placement and 12 h after feeding, a blood sample was collected at 06:00 h (hour 0) and cats were offered 50 g food (195 kcal) for 1 h. All cats ate >75% of the food offered within 30 mins. Additional blood samples were collected at 08:00 h (hour 2), 10:00 h (hour 4), 12:00 h (hour 6), 14:00 h (hour 8), 16:00 h (hour 10) and 18:00 h (hour 12) using a previously described three-syringe technique. ⁸

At each blood draw, cats were collected in the same sequential order by the same author (SS). Blood samples were collected in a glass tube with no anticoagulants, allowed to clot for 30 mins and centrifuged for 10 mins. Serum was immediately harvested, aliquoted and maintained at 20°C until placed in −80°C within 3 h of collection. Serum aliquots were stored at −80°C for 2 years prior to analysis.

Serum biochemistry analysis

Serum samples were thawed at room temperature for 30 mins before analysis. Serum aliquots were batched according to cat and analyzed in duplicate using a chemistry analyzer (Roche Cobas C501; Roche Diagnostics). The chemistry analyzer is calibrated daily and preventative maintenance performed every 6 months by a manufacturer service engineer. BUN, creatinine, phosphorus, total calcium, bicarbonate, cholesterol, magnesium, sodium, potassium and chloride concentrations were measured at each time point.

Statistical analysis

The average of the duplicates taken at each time point for each cat was used in the analysis. The Shapiro–Wilk test and visual inspection of QQ plots were used to determine data normality. Data are presented as mean ± SD. Serum concentrations of each analyte at hours 2, 4, 6, 8, 10 and 12 were compared with concentrations prior to feeding (hour 0) using a mixed-effects analysis with Geisser–Greenhouse correction and a Dunnett’s multiple comparison test. The mean difference between baseline (hour 0) and hours 2, 4, 6, 8, 10 and 12 was calculated for each analyte. Statistical analysis was performed using GraphPad Prism 9.3.1 (GraphPad Software). A P value <0.05 was considered to be statistically significant.

Results

Sera at each time point was available for analysis in nine cats, except for one, at hour 2. Serum concentrations for at least one postprandial time point were significantly increased from baseline fasted BUN (hour 2, P = 0.006; hour 4, P <0.0001; hour 6, P = 0.002; hour 8, P = 0.026), phosphorus (hour 2, P = 0.019), bicarbonate (hours 2, 4, 6, 8, 10, all P <0.01), and chloride (hour 12, P = 0.019), and decreased from baseline fasted concentrations for glucose (hour 12, P = 0.014), magnesium (hour 10, P = 0.029) and chloride (hour 2, P = 0.026; hour 4, P = 0.044) (Figure 1). Serum concentrations of creatinine, total calcium, cholesterol, sodium and potassium were not statistically significantly different at any postprandial time point vs baseline fasted concentrations. The group mean differences in serum analyte concentrations between hour 0 (prior to feeding after a 12 h fast) and hours 2, 4, 6, 8, 10 and 12 are provided in Table 1.

Figure 1

Serum (a) blood urea nitrogen (BUN), (b) bicarbonate, (c) glucose, (d) phosphorus, (e) chloride, (f) magnesium, (g) creatinine, (h) cholesterol, (i) sodium, (j) potassium and (k) total calcium in nine healthy adult cats over a 12 h period. Serum was collected after food was withheld for a 12 h period (hour 0). Cats were offered a meal for 1 h and then serum was collected every 2 h (hours 2, 4, 6, 8, 10 and 12) within the 12 h period after the initial blood collection. Circles represent the mean and whiskers represent the SD. Horizontal dashed lines represent the lower and upper limit of the reference interval. Asterisks indicate that a statistically significant difference (P <0.05) was found when concentrations were compared with the fasted state (hour 0)

Table 1

The group mean difference in serum analyte concentrations between hour 0 (prior to feeding after a 12 h fast) and hours 2, 4, 6, 8, 10 and 12

	Hour 2	Hour 4	Hour 6	Hour 8	Hour 10	Hour 12
Blood urea nitrogen (mg/dl)	2.0*	3.1*	3.1*	1.9*	1.5	0.1
Creatinine (mg/dl)	0.04	−0.06	−0.05	−0.02	0.08	0.05
Glucose (mg/dl)	−2.9	5.6	2.1	2.8	−4.6	−7.7*
Phosphorus (mg/dl)	0.4*	0.2	0.3	0.08	−0.3	−0.3
Total calcium (mg/dl)	−0.2	−0.3	−0.2	−0.2	−0.2	−0.04
Cholesterol (mg/dl)	−5.3	−3.7	−1.0	−1.8	−5.2	−3.4
Bicarbonate (mEq/l)	3.3*	3.3*	2.5*	2.1*	1.3*	0.7
Magnesium (mEq/l)	−0.03	−0.07	−0.03	−0.09	−0.1*	−0.06
Sodium (mEq/l)	1.1	0	0.5	0.1	0.2	1.2
Potassium (mEq/l)	−0.2	−0.3	−0.1	−0.2	−0.2	−0.07
Chloride (mEq/l)	−1.9*	−2.0*	−1.1	−0.5	0.4	1.2*

*

Indicates a statistically significant difference (P <0.05) was found when concentrations were compared with the fasted state (hour 0)

Discussion

This study objective was to evaluate the 12 h postprandial changes in select biochemistry panel analytes in healthy adult cats to provide a global overview of changes that could occur. Our results support our hypothesis, demonstrating that some analytes were significantly elevated (BUN, phosphorus and bicarbonate) or lower (glucose and magnesium), or exhibited both higher and lower values (chloride) at least at one time point within the 12 h period after feeding. The cause of these postprandial changes is likely multifactorial and could be directly related to recent feeding by intestinal absorption or metabolism of nutrients, secondary to lipemia interference and/or diurnal variation.^1,7,9 In a previous study using feline sera spiked with lipid, lipemia interference was documented for some biochemistry analytes evaluated in our study (carbon dioxide, cholesterol, creatinine, phosphorus, urea) and findings were often more pronounced at higher lipid concentrations. ⁹ The effect of a recent meal and diurnal variation of plasma biochemistry analytes has been previously documented in healthy research cats. ¹ The commercial cat food used in this previous study contained a higher protein concentration (12.1 g/100 kcal) and lower fat concentration (3.1 g/100 kcal) than the food in our study (protein 8.7 g/100 kcal; fat 3.6 g/100 kcal). Despite differences in the macronutrient composition of the foods, the previous study showed similar trends in the plasma concentrations of biochemical analytes compared to this study. ¹

While we found statistical differences in analyte concentrations in the 12 h postprandial period for some analytes, the observed postprandial changes were subtle and unlikely to be misinterpreted as disease in young healthy cats. We found an upward trend in BUN, phosphorus and bicarbonate within 2 h of feeding, and the higher values compared with the fasted state was sustained for up 10 h for BUN and bicarbonate. Despite this upward trend, serum concentrations of BUN and phosphorus remained within the reference interval (RI). On the contrary, two cats had bicarbonate concentrations that rose above the RI at 2 and 4 h postprandially, respectively. These findings were expected, attributable to recent feeding and diurnal variation, and previously documented in healthy cats.^1,3,4,10 A postprandial rise in BUN, phosphorus and bicarbonate could be of importance in both healthy cats and those with disease, particularly in the monitoring and management of kidney disease.

Blood urea concentrations are impacted by overall health status of the individual and dietary protein concentration. We demonstrated that BUN was higher in fed cats vs baseline for 8 h after a meal, with a peak mean increase of 3.1 mg/dl between 4 and 6 h postprandially. This aligns with a similar study in which plasma BUN was significantly higher at every postprandial interval between 2 and 12 h in cats. ¹ However, their observed increase was more robust, with a peak difference of +8.09 mg/dl in fed cats at the 8 h mark. ¹ The more modest increases in BUN postprandially witnessed vs the previous study could be attributed to several factors, including dietary protein concentration, protein source and amount of food consumed.^6,11 Serum BUN concentrations are often interpreted considering the creatinine concentration. Unlike BUN, our data indicate no postprandial effect on creatinine within the 12 h period after a meal. Findings in cats have been highly variable, with a study demonstrating no difference in creatinine concentration after feeding, ² another showed that creatinine decreased after feeding ¹ and one indicated that postprandial elevations in creatinine depend upon the creatinine concentration in the diet consumed. ⁵

In practice, the magnitude and duration of phosphorus increase after a meal will vary depending on the diet fed. The magnitude of postprandial increases in phosphorus in cats is related to the source of dietary phosphorus (organic vs inorganic), the amount of dietary phosphorus consumed and the calcium-to-phosphorus (Ca:P) ratio of the diet. In cats, inorganic dietary phosphorus increases postprandial concentrations in a dose-dependent manner, while the consumption of diets containing solely organic phosphorus results in either decreased concentrations or unchanged concentrations postprandially.^3,4,10 In an experimental diet trial in cats, the diet with the highest amount of inorganic phosphorus provided as sodium tripolyphosphate (1.5 g/1000 kcal) and the lowest Ca:P ratio (1.0) had the highest peak postprandial plasma phosphorus concentration and the longest duration of concentrations raised above baseline. ⁴ The effect of a meal on serum phosphorus in cats is repeatedly short lived, with peaks observed between 2 and 3 h postprandially and returning to baseline by or before 6 h, depending on diet composition.^3,4 The cats in this study were fed a diet containing inorganic phosphoric acid, 3.1 g/1000 kcal of total phosphorus and a low Ca:P ratio (1.1), which led to a serum phosphorus concentration that was significantly higher at 2 h postprandially (group mean difference +0.4 mg/dl) compared with baseline.

A postprandial increase in bicarbonate was expected and is referred to as an ‘alkaline tide’ secondary to the efflux of bicarbonate into the blood after gastric hydrochloric acid secretion.^{12 –14} Our results demonstrate a significant increase in bicarbonate above baseline consistently seen for 10 h postprandially, with a peak group mean difference of +3.3 mEq/l between 2 and 4 h. Interestingly, 2/9 cats in our study had bicarbonate concentrations above the RI at the same time point that their chloride levels were lower than the RI. To our knowledge, our study is the first to demonstrate a postprandial reduction in chloride in cats. The observed postprandial reduction in chloride appears to be inversely related to bicarbonate levels. Intrinsically, this makes sense, as hydrochloric acid is secreted into the stomach via parietal cells during feeding, leading to a deficit of chloride in the extracellular fluid department. ¹⁵ Our results demonstrate that feeding transiently affects the acid–base status in cats, which may be detected on a biochemical panel in some cats. We do not know whether the alterations in bicarbonate affected the blood pH.

The study had limitations. First, the findings are specific to small number of healthy young adult cats, and the effects of only one diet fed for at least 3 months prior to sample collection were analyzed. Postprandial alterations in analytes could be different based upon diet formulation and length of time the diet is fed. This makes it challenging to apply our results into broad recommendations useful for clinical practice, in which cats are fed a wide variety of diets. Second, the postprandial serum concentrations were all compared with a single preprandial value taken in the morning. A crossover study comparing postprandial concentrations with fasted concentrations, which has been previously carried out in healthy cats, ¹ was not performed. Therefore, it is not possible to discern the effect of diurnal variation from the effect directly related to a recent meal. Diurnal variation has been more extensively studied in human medicine, where it has been shown to affect various hematologic and biochemical values.^{16 –18} Third, cats were offered a standard meal of 50 g, and thus the kcal consumed per kg of body weight differed between cats, and could have affected the results. Fourth, evaluation for the effects of lipemia alone were not undertaken and since lipemia has been shown to influence biochemical measurement in cats and can be machine or methodology dependent, this variable could have influenced the results.^7,9 This study is specific to the chemistry analyzer, patient population (ie, a small number of young healthy cats fed a controlled diet) and sample conditions (ie, freezing and prolonged storage of sera).

Conclusions

This study showed that a few serum biochemical analytes, particularly BUN, phosphorus, bicarbonate and chloride, change within the 12 h period after a morning meal, while many analytes, including creatinine, did not significantly differ from baseline fasted serum concentrations. Veterinarians should be aware of potential postprandial variations in some biochemical analytes and document the time from feeding for accurate interpretation. Further research is warranted to analyze the effects of different diets and underlying disease states on postprandial biochemical analyte concentrations.

Supplemental Material

Supplemental Material

Food composition of the commercial cat food fed to cats enrolled in the study.