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Abstract

Climate change is becoming the most significant global challenge and must be addressed on a global scale. At the time that this article is being written, the planetary heat in 2023 was the hottest on record. Similarly, the World Health Organization reports that 99% of the world’s population lives in regions of unhealthy air pollution. Similarly, depression has become one of the leading causes of global mental and physical disabilities, and the impact of depression is predicted to only worsen over the next 25 years. It is interesting to note that climate experts often overlook the adoption of nutrition via a whole plant-based diet as a solution to both mental illness and climate change. In this review, we will touch upon the role of nutrition in gut microbiota and mental health, the impact diet has on greenhouse gases, the role of ultra-processed food, and environmental factors such as air pollution and increasing planetary heat and their growing impacts on mental health. In the end, the promotion of plant-based foods has the potential to improve personal mental and physical health while improving planetary health.

Keywords

climate change plant-based diet mental health gut microbiota depression brain health greenhouse gases

“Several studies have shown that individuals with depression have improved mood while on the Mediterranean Diet (MD), mostly a WFPB diet.”

Key Message

Widespread adoption of a plant-based, whole-food diet can reduce greenhouse gas emissions and improve mental health.

Introduction

Multiple interwoven concepts are discussed in this paper, including the role of nutrition in gut microbiota health, the impact diet has on greenhouse gases, the role of ultra-processed food, and environmental factors such as air pollution and increasing planetary heat. Mental illnesses are an ongoing and significant problem worldwide.^1,2 It is becoming increasingly clear that climate change must be addressed on a global scale. The objectives of this review are: (1) Describe how a plant-based, low ultra-processed food diet may benefit mental health, (2) Discuss how switching to plant-based diets may lower greenhouse gas levels and lower global warming, and (3) Propose that the lowering of global temperatures and decreasing greenhouse gases may further benefit mental health.

Gut Microbiota, Depression, and Climate Change

Nutrition and the complexities of neurochemistry are yet to be fully understood. The role of the gut microbiota, the interrelation of depression, the impact of animal-based diets, and the connection between greenhouse gases (GHG) and diets are important considerations.

Depression

Depression is the leading cause of global mental and physical disabilities.^3,4 Additionally, depressive symptoms have been negatively impacted by COVID-19, the bifurcation of political and social structures, and the worsening of social isolation, and regions that have been most at risk of these social conflicts experience the highest rate of depression.⁴ Historically, depression pathophysiology has been centered on the monoamine hypothesis, which suggests that depression is due to the dysregulation of monoamines (e.g., norepinephrine or serotonin), and the primary treatment has focused on the stabilization of these monoamine pathways via psychotropic medication (e.g., selective serotonin reuptake inhibitors, serotonin-norepinephrine reuptake inhibitors, or tricyclic acids).⁵ There are limited long-term randomized clinical controlled trials that confirm their efficacy⁶ despite the chronicity of depression.⁷ Even with the abundance of medications and therapeutic interventions, the impact of depression is predicted to only worsen over the next 25 years, as current interventions have been unsuccessful in reversing this trend.⁸

The Role of the Gut Microbiota

The human brain and gut are derived from the same neural crest preembryonic material,⁹ and an increasing number of studies have shown that brain development is intrinsically related to gut microbiota.⁹ A healthy microbiota interacts with the brain across the lifespan via the neuroimmune, neuroendocrine, and vagus nerve pathways.¹⁰ Healthy early-life development of the gut microbiota helps prevent later disease development. Nutritional programming is the concept that key cells within the body can absorb or synthesize nutrients de novo in key periods to support early-life development.¹¹ The disruption of these bidirectional patterns has been linked to depression and other neurocognitive disorders. The health industry promotes protein as the optimal food ingredient for better health and overlooks the importance of fiber. As a result, less than 5% of Americans are consuming an adequate dose of fiber,¹² which has been noted to be key in developing and maintaining a healthy brain-gut-microbiota (BGM) system.¹³

Emerging research has demonstrated the vital role of the (BGM) system in mental health. The brain and gut establish a vital bidirectional connection via the enteric nervous system.¹⁴ Essential gut microbiota produces short-chain fatty acids (SCFA) (as acetate, propionate, and butyrate), which are the building blocks for signaling molecules within the body.¹³ These essential gut microbiota must maintain a delicate balance and often can be replaced by other bacterial colonies that do not produce SCFA if their preferred food source, non-digestible fiber, is not in the diet,^13,15 a process known as dysbiosis. Disruption of this bidirectional pathway is linked to depression and other neurocognitive disorders.¹⁶

A key component of the whole-food, plant-based (WFPB) diet is fiber, which allows for the healthy production of SCFAs. Tryptophan is the only amino acid that the human body is unable to produce on its own and must be consumed. A healthy gut microbiota, including Bifidobacterium, Lactobacillus, Lachnospiraceae, Blautia, Coprococcus, Roseburia, and Faecalibacterium organisms,^16-18 is able to convert tryptophan to serotonin, kynurenine, and indoles.¹⁹ Overall, emerging literature in this new field continues to show connections between the roles of the gut, brain, and source of nutrients in the diet.²⁰

Depression and Whole-Food, Plant-Based Diet

Several studies have shown that individuals with depression have improved mood while on the Mediterranean Diet (MD), mostly a WFPB diet. The HELFIMED study was a randomized study that showed that individuals with depressive symptoms could follow an MD, make dietary changes, and see improvements in depressive symptoms.²¹ Next, the SMILES study was the first randomized control study to examine the benefits of diet in depression. Those in the study sample avoided ultra-processed food (UPF) and fast food, and, following the study, they had a greater rate of remission of depressive symptoms. The number needed to treat (NNT) was 4.1.²² Additionally, the SMILES study was more cost-effective than a typical diet for participants.²³

Men in the AMMEND study showed improvement in the Beck Inventory Scales after 12 weeks of MD compared to the control group.²⁴ Finally, a 2019 meta-analysis of epidemiological studies showed that every increase in 100 g of whole fruits or vegetables corresponded with a 5% reduction risk of depression.²⁵ Given these findings, the Royal Australian and New Zealand College of Psychiatrists recommend lifestyle changes, including diet, exercise, and therapy, be included as primary treatment modalities for mild and moderate forms of depression and an adjunct for severe forms of depression.²⁶ Similarly, the World Federation of Society for Biological Psychiatry has adopted a WFBP within its treatment recommendations for depression.²⁷

Impact of Animal-Based Diets

In contrast, individuals who consume diets with high animal content are often found to have gut microbiota with Bacteroides²⁸ and Enterobacteriaceae, especially Escherichia, Shigella, Proteus, and Klebsiella.¹⁶ As a result, the altered microbiota produces higher levels of enterotoxins¹⁶ and decreased levels of SCFAs.²⁹ Diets high in animal content also contain elevated amounts of L-carnitine, chlorine, and betaine, which are metabolized in the gut to a trimethylamine precursor and then to trimethylamine N-oxide (TMAO).²⁹ Although TMAO has historically been linked to atherosclerosis and cardiac complications, emerging literature has linked the concentration of TMAO with depression³⁰ and other neuropsychiatric disorders,²⁹ independent of the oxidative stress and neuroinflammation changes that TMAO causes within the gut microbiota.²⁹

Connecting the Dots of Greenhouse Gases and Diet

Recent studies point to food supply chains creating approximately 13.7 billion metric tons of carbon dioxide equivalent (CO₂eq) annually (roughly 26% of the world’s global GHG emissions).³¹ An additional 2.8 billion metric tons of CO₂eq is caused annually by deforestation.³¹ When the distribution and production of food products are taken into consideration, the total of the world’s annual GHG is closer to 34%. Recent findings by Lawerence et al suggest that tropical deforestation significantly contributes to global warming and increases CO₂ concentration between the 50^o N to 50^o S latitude zone (central part of the earth), while deforestation contributes to global cooling north of the 50^o N and south of 50^o S latitude (polar regions).³²

Additionally, agriculture also comprises 50% of the world’s habitable land, 70% of the world’s freshwater, and contributes to 78% of the world’s water pollution.^31,32 For example, harvesting 1 kilogram of beef requires 60 kg of CO₂eq (including land use, farming, processing, and transportation).³¹ According to Poore & Nemeck in 2018, moving away from an animal diet “has transformative potential, reducing food’s land use by 3.1 (2.8 to 3.3) billion hectare (a 76% reduction), including a 19% reduction in arable land; food’s GHG emissions by 6.6 (5.5 to 7.4) billion metric tons of CO₂eq (a 49% reduction); acidification by 50% (45 to 54%); eutrophication by 49% (37 to 56%); and scarcity-weighted freshwater withdrawals by 19% (−5 to 32%) for a 2010 reference year.”³¹ Additionally livestock and poultry farming are the most significant contributors to global ammonia emissions,³³ which in turn contributes to overall PM2.5 levels.³⁴

Farchi et al evaluated the impact of GHG reduction in Italy if all Italians adopted a full Mediterranean diet. They calculated that the per capita reduction would be 263 KgCO₂eq/year/person, with higher reductions in Northwestern and Central areas of Italy.³⁵ Based on assessing over 520 different landmass usage patterns, Theurl et al. found that diets are one of the main determinants of GHG emissions.³⁶ In 2023 Springman et al., through modeling, posit that land use for dietary choices significantly contributed to air pollution and that the adoption of plant-based diets is “a valuable mitigation strategy for reducing ambient air pollution.”³⁴

Impact of Ultra-processed Food on Mental Health

Ultra-processed Foods in Our Diet

With the end of World War II and the new Liberal Order, the U.S. War on Poverty began. In 1961, the new Secretary of Agriculture, Orville Freeman, committed the USDA to “raise the nutritional standards and improve dietary standards of all Americans.”³⁷ In a means to address childhood poverty within the U.S., the Child Nutrition Act of 1965 highlighted that school lunches were one of the only consistent meals that individuals would receive. As such, meals were fortified to maximize their nutritional content.³⁷ Although UPF was first seen following World War II, the events of the 1960s mainstreamed UPFs within U.S. culture.³⁸ By definition, UPF contains excessively high levels of refined sugars, saturated fat, trans-fat, caffeine, and sodium with regard to both overall macro- and micronutrient content and energy density, as well as very low levels of dietary fiber. Multiple attempts have been made to create a classification for UPF, and currently, NOVA is the most used classification in the field.³⁹ It categorizes food into 4 subgroups and provides examples for each category. The subgroups were divided as follows: (1) unprocessed and minimally processed foods, (2) processed culinary ingredients, (3) processed foods, and (4) UPF, which are food substances with no or rare culinary use.

Historically, the processing of food has allowed for more efficient distribution of products throughout the U.S. and eventually the world by increasing its shelf life and distribution capability.^16,18,38,40 Recently, negative health consequences of processed foods, especially ultra-processed foods, have emerged. Even prior to the COVID-19 pandemic, sadly, the typical U.S. diet consisted of approximately 60% UPF, with consumption patterns consistent between gender and race.⁴¹ Data show that global consumption and reliance on UPF have continued this negative upward trajectory in the post-pandemic era.⁴²

Impact of Ultra-processed Foods on the Gut

As part of the 2017 systematic analysis of the Global Burden of Disease Study, dietary patterns of individuals from 195 countries during 1990-2017 were analyzed.⁴³ The study showed that diet-related risk factors were responsible for approximately one-fourth of all deaths among adults, as well as almost one-fifth of all disability-adjusted life-years among adults. Notably, this investigation also reported suboptimal worldwide consumption of almost all healthy foods and nutrients.⁴³ Diets high in UPF promote low-grade inflammation, which affects the gut microbiota,⁴⁴ leading to a shift in overall gut microbial diversity, a condition known as dysbiosis. It is significant to note that dysbiosis can lead to dysregulated neuroimmune responses,^17,44 neuroinflammation,⁴⁵ and altercations within the neuroendocrine system.⁴⁵ Advancing theories of the etiology of depression have expanded to include neuroimmune, neuroinflammatory, and neuroendocrine mechanisms,^5,46 which have a common link to the BGM system.^10,46

Ultra-processed Food and Depression

A 2021 meta-analysis by Lane et al, compiling data from over 345 000 individuals, noted that higher rates of UPF consumption predicted an increased risk of subsequent mental health symptoms.⁴⁷ In a cross-sectional study of 10 359 participants, Hecht et al showed that individuals who consumed primarily NOVA4 had an odds ratio for developing depression (OR: 1.81; 95% CI 1.09, 3.02). They had a risk ratio for being more mentally unhealthy (RR: 1.22; 95% CI 1.18, 1.25). Finally, they were also significantly less likely to report zero mentally unhealthy days (OR, 0.60; 95% CI 0.41, 0.88).⁴⁸ Although some authors question the merits of the NOVA system, as it does not allow food to be defined unequivocally as UPF, most research literature to date on UPF uses the NOVA classification. The challenges to the NOVA system should not overshadow the importance of Hecht’s cross-sectional design; rather, it highlights the need for additional classification criteria.³⁹

Overall, the production of UPF negatively impacts the environment through over-utilization of monoculture crops, high energy demands to process and manufacture food products, long transportation, and complex packaging.⁴⁹ UPF significantly impacts land use, water use, and the release of GHG,⁵⁰ and its impact is significantly greater than that of WFPB diets.⁵⁰ Therefore, UPF is detrimental to mental health and the environment.

Putting It all Together

Impact of Climate Disasters

Evidence has grown exponentially, linking extreme changes in climate and natural disasters.⁵¹ Additionally, the frequency and intensity of these disasters have escalated in the U.S. and throughout the world. Damage is now being measured and tracked by events causing orders of $20 billion (U.S.) in damage instead of $1 billion (i.e., 2021 had a record 5 events resulting in at least $20 billion (U.S.) damage per event. This figure only reflects property damage and not medical-related incidents).⁵² Although much of the focus of climate change has been on non-mental health illnesses, extensive reviews have linked climate change to an increase in the incidence of depression and post-traumatic stress disorder (PTSD) with hurricanes,^53,54 wildfires,^55,56 droughts,⁵⁷ and floods.^58,59

Air Pollution and Depression

Although air pollution has historically been thought of as a contributor to pulmonary, cardiac, renal, and obstetric complications, there is a growing body of literature linking it with worsening mental health, particularly depression. Air pollution is commonly thought to be a combination of multiple agents, including carbon dioxide (CO₂), carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide, ozone, and particulate matter smaller than 10 μm or smaller than 2.5 μm (PM 10, or PM 2.5).⁶⁰ The World Health Organization (WHO) estimates that 99% of the world’s inhabitants breathe air that affects their health,⁶¹ with most of the affected individuals living in low-to-middle-income countries.⁶⁰

Recent literature from China has shown that individuals are at risk of developing mental health symptoms when the Air Quality Index (AQI) is greater than 92.79.⁶² For PM 2.5 less than 50 μg/m³, the conversion to AQI is approximately 1:3, so a PM 2.5 of 32 μg/m³ is converted to an AQI of 93.⁶³ Molitor et al. noted that suicide risk increases by 0.27 deaths per million when an individual has been exposed to increased PM 2.5 of 7.8 μg/m³ (around 30 AQI points).⁶⁴ In a review article, Zundel et al showed that chronic exposure to PM 2.5 leads to decreased volume in the prefrontal cortex, hippocampus, and amygdala compared to controls. These volumetric changes may account for the worsening depressive symptoms previously mentioned.^65,66

Xue et al. showed a correlation between worsening air pollution and elevated temperatures and mental health in a survey of 20 000 individuals.⁶⁷ Molitor et al. noted that rates of suicide were higher in non-urban communities within the United States due to increased rates of wildfire smoke exposure. More specifically, they noted a higher correlation between working-aged white men without a college education and successful suicide completion.⁶⁴ In addition to impacting depression and other mental illnesses, air pollution may be a contributing factor to global food insecurity.⁶⁸ Recent studies have shown correlations between decreased crop harvests⁶⁹ as air pollution impacts photosynthesis and overall plant growth.⁶⁸ Equally significant, intense air pollution may reduce the quality and quantity of nutrients present in plants,⁷⁰ which may further contribute to the growing impact of depression.

Heat and Mental Health

At the time that this article is being written, 2023 was the hottest on record (1.32°C above pre-industrial baseline), with 2022 being the sixth hottest on record.⁷¹ Projections have been calculated that demonstrate the impact of worsening climate disasters, worsening mental and physical health, and loss of resources when the earth surpasses 1.5°C above the pre-industrial baseline and beyond.⁷¹ When analyzed on a granular level, 156 cities (with a population of at least 1 million) in 37 countries experienced extreme heat (temperatures exceeding 32°C and 90% humidity) for at least 5 days consecutively, roughly 1.9 billion worldwide.^67,72 A recent meta-analysis by Liu et al examined the relationship between mental health, mortality, and morbidity. They showed an RR of 1.022 (95%CI: 1.015-1.029) mortality and 1.009 (95% CI: 1.007-1.015) morbidity within individuals with depression, and each increased 1°C greater than 37°C.^72,73

Nori-Sarma et al looked at emergency department visits across the U.S. between 2010-2019 and cataloged all psychiatric encounters, and compared them to episodes of extreme heat (temperatures exceeding 32°C and 90% humidity). Psychiatric encounters were stratified across extreme heat exposure, and in addition to a correlation with depression, the incidence rate ratio (IRR) for all mental health encounters was an IRR of 1.08 (95% CI, 1.07-1.09).^73,74 The individual IRR was specific for all disorders measured: substance use disorders (IRR, 1.08; 95% CI, 1.07-1.10); anxiety and/or stress-related disorders (IRR, 1.07; 95% CI, 1.05-1.09); mood disorders (IRR, 1.07; 95% CI, 1.05-1.09); psychotic disorders (IRR, 1.05; 95% CI, 1.03-1.07), self-harm (IRR, 1.06; 95% CI, 1.01-1.12); and childhood-onset behavioral disorders (IRR, 1.11; 95% CI, 1.05-1.18).^73,74

In addition to impacting depression and other mental illnesses, global temperature also increases global food insecurity.^75,76 Recent studies have shown correlations between decreased crop harvests in India, China, Europe, and the United States and climate change.^77-79 Equally significant, intense heat also reduces the quality and quantity of nutrients present in plants,^76,80 which may further contribute to the growing impact of depression.

Limitations

Our review had multiple limitations. Both authors (SS and GM) are biased toward a whole-food, plant-based diet and subscribe to this nutritional diet for chronic disease management. SS and GM are board-certified by the American College of Lifestyle Medicine, and one is a fellow (GM). Furthermore, research on dietary interventions is difficult to conduct as attempts to assess a patient’s lifestyle behaviors and cannot control for additional food content that may be consumed or to the degree to which participants accurately record their meals. Similarly, design studies and randomizing adequate controls may lead to misinterpretation of results, given the difficulty of randomizing interventions. Likewise, the study data do not consider the social determinants of health, which may confound the ultra-processed food data. Finally, mental health monitoring within heat and air pollution represents correlative data, not causative data.

Next Steps

The implications of sustainability on health should include the realm of mental health. Emerging data show the adjunctive role of plant-based diets in multiple mental health disorders, with the majority focusing on depressive disorders. The global impact of mental health disorders continues to be of concern to individuals and society at large. Therefore, the potential impact of addressing both crises with similar interventions is a compelling next step. In an all-hands-on-deck approach,⁸¹ it is interesting to note how climate experts often overlook adopting nutrition via a whole-food, plant-based diet (WFPB). Not only can WFPB improve mental health, but it can help reduce greenhouse gasses (GHG) as much, if not more, than any other commonly used sustainability intervention.^31,34,36

For individuals, the first step is to educate healthcare providers on the merits of plant-based diets and their potential impact on sustainability and mental health. To achieve this aim, efforts are underway at the American Psychiatric Association to develop curricula in the emerging field of Lifestyle Psychiatry that address these gaps in education at the undergraduate, graduate medical, and continuing medical education levels.^82-84

At the population level, current climate models point to the importance of addressing mental health issues since previous climate disasters have shown that those with a history of mental health are at a higher risk of experiencing comorbid death.⁸⁵ One example, the Shared Socioeconomic Pathway (SSP), is a computer model that calculates the change in global temperature and carbon dioxide through the end of the 21st century based on socioeconomic behaviors.⁸⁶ There were 5 scenarios, SSP1-SSP5. SSP1 is based on the population making significant carbon-sustainable use changes. If that model of behavior is adapted, it is estimated that CO₂ levels will only rise to 430-480 ppm with an estimated temperature increase of 0.9-2.3°C above the pre-industrial baseline. The SSP5 is based on the population, making no substantial changes to current behaviors. In this trajectory, CO₂ levels will rise to more than 1000 ppm with an estimated temperature increase of 3.2-5.4°C above the pre-industrial baseline. Under this model, there will be an increase of 10.1-12.0 deaths per 100 000 related to these climate changes.^87,88

Experts recommend a 67% reduction in agricultural emissions in alignment with the Paris Accord Agreement⁸⁹ to help counter the SSP5 model. Arguably, this reduction is possible if a predominant whole-food, plant-based diet is universally adopted.^31,34,36 Many forward-thinking organizations are taking steps to enact these recommendations. For example, the planners of the 28th Conference of the Parties to the UN Framework Convention on Climate Change (COP 28) served as the predominant whole-food, plant-based menu for its participants.⁹⁰ The promotion of plant-based foods has the potential to improve personal mental and physical health while improving planetary health.^91-93

Footnotes

Author Contributions

Both authors (SS and GM) contributed to the conception, design, drafting of the paper, research, analysis, and interpretation of the data, revising it critically for intellectual content, and final approval of the version to be published. SS and GM agree to be accountable for all aspects of the work.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Steven G. Sugden

Gia Merlo

Data Availability Statement

Data sharing is not applicable to this article as no new data were created or analyzed in this study.

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What do Climate Change,Nutrition,and the Environment Have to do With Mental Health?

Abstract

Keywords

Key Message

Introduction

Gut Microbiota, Depression, and Climate Change

Depression

The Role of the Gut Microbiota

Depression and Whole-Food, Plant-Based Diet

Impact of Animal-Based Diets

Connecting the Dots of Greenhouse Gases and Diet

Impact of Ultra-processed Food on Mental Health

Ultra-processed Foods in Our Diet

Impact of Ultra-processed Foods on the Gut

Ultra-processed Food and Depression

Putting It all Together

Impact of Climate Disasters

Air Pollution and Depression

Heat and Mental Health

Limitations

Next Steps

Footnotes

Author Contributions

Declaration of conflicting interests

Funding

ORCID iDs

Data Availability Statement

References