The longer I practice medicine, the more I appreciate firsthand how mental, emotional, and physical health overlap far more than we commonly acknowledge in our society. And unfortunately, our mental and emotional health are not in good places right now. My impression is that in this era of digital “connectedness,” binge-watching on streaming platforms, and around-the-clock smartphone use, loneliness and depression have become pervasive epidemics.
My impression reflects what surveys have been telling us for years. According to a 2020 data brief from the National Center for Health Statistics[1]:
Rates of mental health disorders in the US might be higher than average—it is estimated that around four percent of the global population experience depression—but these are by no means exclusively American problems.[2] Approximately 280 million worldwide suffer from depression, and more than 700,000 annual deaths result from suicide, a leading cause of death in adolescents and young adults.
As you might expect, given the COVID-19 pandemic after the publication of these data, we have been generally unsuccessful in addressing these trends. Authors of a 2023 article analyzing the lasting effects of the pandemic write that “According to the World Health Organization, anxiety and depression prevalence increased by 25% globally.”[3] They highlight three groups who have tended to experience heightened mental health difficulties: individuals with higher education, women, and young adults. “The long-term effects on adolescents are yet to be seen,” they conclude, “while effects of pain, fear, and isolation on the general population are already presenting themselves.”
But to return to my original point, this epidemic is not just feelings. It is also physical. The evidence of a bidirectional relationship between emotional/psychological health and physical health is plentiful. For example, we know that those who are depressed are more likely to be obese, and the prevalence of obesity increases as depression becomes more severe.[4]
How does this work? We can intuitively understand that if we feel depressed, we may not make the best choices. To medicate our anger, loneliness, stress, and depression, we may turn to hyper-palatable foods. These foods, high in processed sugars and fats, trigger pathways involved in feelings of happiness, including our dopamine and serotonin pathways (something I talk about in more detail in The Campbell Plan). Being low on serotonin, as we would be during depressive episodes, it makes sense that we would try to fill the serotonin deficit with a tasty jelly donut or, better yet, three or four.
This response is common and understandable. If we are honest, we can all relate to this behavior. We even have a phrase for it—eating your feelings.
But think about causality in the other direction. What I mean is this: what if depression not only causes unhealthy food and lifestyle choices but also results from unhealthy food and lifestyle choices?
Research shows that people who are obese have suppressed dopamine pathways. The more obese they are, the more limited are their dopamine receptors.[5] This limitation means that they need more stimulus (e.g., more hyper-palatable food) to get the dopamine surge required to achieve what we might call satisfaction. We now know from animal experiments that sugar diets can alter chemical receptor activity in the brain, a process sometimes called neuroadaptation.[6] It turns out that hyper-palatable foods may well be altering the chemical pathways involved in happiness.
In one study of older adults without depression followed for over seven years in Chicago, those who adhered to a more plant-based, Mediterranean diet (more fruits, vegetables, whole grains, legumes, fish, and olive oil and less meats and high-fat dairy) had lower rates of new depressive symptoms.[7] This finding was similar to that of a study in Spain.[8]
We know that metabolic syndrome and various psychological disorders often occur together and that they share features like increased levels of chronic inflammation and dysregulated hormone systems.[9] Both of these features are affected by diet and lifestyle.
In one of the intriguing experiments from my father’s research, we know that simply changing the amount of protein in the diet will affect physical activity in rats. And it is not how you might expect—the rats consuming lower amounts of protein voluntarily exercised more.[10]
One study put all this information to the test in a small two-week pilot study; they found that omnivores who avoided fish and other meat (including poultry) for two weeks improved in some measurements of mood, including stress.[11]
Principle 8 in The China Study is that all things are connected: physical, mental, and emotional health; nutrition and physical activity; spirituality and social connection; animal welfare, environmental sustainability, and more. These are interconnected issues that researchers and consumers have recently grown more accepting of. This acceptance is especially evident in the interest in gut health, a major hot topic, and the mounting research on the microbiota-gut-brain axis.[12] We have known that dietary choices affect the gut for a long time. What is fascinating is how a healthy gut may, in turn, affect so much else, including major depressive disorder, anxiety, sleep disorders, attention-deficit hyperactivity disorder (ADHD), and Parkinson’s disease.[13][14][15] (Learn more about the growing interest in gut health.)
So remember, although mental illness may drive unhealthy lifestyle choices, it is also likely that harmful food choices drive mental illness. In addition to eating our feelings, we often feel what we eat. This subtle but powerful shift in how we think about mental illness opens the door to a more substantial role for diet in treating depression than we currently acknowledge. Besides, I don’t know about you, but I would rather not feel like a ground-up cow, a dead bird, or a spongy micronutrient-deficient gas station pastry.
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