First Person

Food and mental health: Happy gut, happy mind

I remember the first time I read a study suggesting that diet could influence mental health. At the time, I was completing my PhD in nutritional sciences, the mental-health epidemic was still in its infancy and the idea that diet could influence one’s state of mind was unfounded. Suddenly, here was a new world of possibility on the importance of healthy eating, igniting an insatiable thirst to understand the complexity of human biology and the sustenance that maintains it.

What had originally drawn me to the field of nutrition was diet’s well-established role in issues such as diabetes, heart disease, certain forms of cancer and, of course, obesity. This new connection not only paved the road toward a deeper understanding of nutrition’s impact on mental health, but much, much more.

My initial, naïve, assumption was … if diet affects mental health, then mental health must be, to some degree, a product of nutritional deficiencies. This can be true in certain instances, since deficiencies of key nutrients prevent the optimal functioning of many body systems, including the brain. However, the old-school view of nutrition as being entirely related to the presence or absence of vitamins and minerals was not nearly sophisticated enough to explain the complex connection between diet and one’s state of mind.

Making sense of the link between diet and mental health requires new ways of thinking about nutrition. It involves the integration of gastroenterology, microbiology, immunology, neurology and psychology. It took years of PubMed searches to clarify, combine and distill these disparate lines of research into a working, evidence-based model that explains not only how food influences mood, but more importantly, a host of increasingly prevalent health issues.

Understanding the diet-brain connection starts with the realization that we all carry a half-pound of bacteria around inside us, also known as the microbiome. These bacteria reside largely in our colon (large intestines) and every day, they eat whatever we eat. The next-door neighbours to these bacteria are our immune cells. Just like a fence separates the properties between two neighbours, there is a single layer of cells (the gut epithelial lining) that divides our microbiome from our immune system. Like good neighbours, these microbes and immune cells talk back and forth through the fence. One of the things they talk about is what you eat. When you eat certain foods like fiber-rich fruits, vegetables, beans and whole grains, these bacteria tell your immune cells how happy they are. Those immune cells then proceed to share that message with the rest of the body. They do so by mailing postcards that are delivered via the bloodstream to the far reaches of the body, including the brain.

When you eat junk food these gut bacteria convey their disappointment and worry to the immune cells that live next door.

However, when you eat junk food, too much meat, fat and sugar, these bacteria convey their disappointment and worry to the immune cells that live next door. This once again prompts the immune cells to amplify that message via the aforementioned proverbial postcards, called cytokines. Cytokines are messengers that can convey signals of peace and tranquility (also known as anti-inflammatory cytokines) or chaos and threat (known as pro-inflammatory cytokines). Cytokines are unique because they can cross the blood-brain barrier (another fence that divides regions of the human body) and enter the brain.

People who suffer from depression tend to have higher levels of certain pro-inflammatory cytokines compared to people who don’t have depression. Furthermore, in children and adolescence, pro-inflammatory cytokine levels have been shown to predict future depression.

Therefore, part of the biological explanation behind the increasing number of randomized controlled trials that link diet and depression involves these cytokines that are ultimately a reflection of what we eat.

That being said, let’s not oversimplify things here. Mental-health issues are chronic illnesses, and like all chronic illnesses, their etiology is multi-factorial. Chronic disease is never the product of one thing, it’s the product of many factors, diet among them. Nutrition isn’t the sole solution to such problems but it is a crucial component of a larger prevention and treatment strategy.

After completing my PhD, I went to medical school with a goal of translating nutrition science into clinical settings. By this time, the list of potential health issues that I thought could be influenced by diet had grown to not only include mild anxiety and depression, but also acne, cognitive decline, polycystic ovarian syndrome and even infertility.

Once again, I came to learn that this list was still too narrow.

One group of conditions that I increasingly see in my clinical practice is the full spectrum of autoimmune diseases ranging from Lupus to Hashimoto’s Thyroiditis to Inflammatory Bowel Disease. There is increasing evidence, and even testimonials from my patients, that implicates diet in the process of autoimmunity. Our immune cells are programmed by our gut bacteria according to what we eat and the signals sent by those bacteria help determine which types of immune cells are dominant, be it the rogue ones that can fuel an attack on the self or the regulatory ones that prevent such attacks.

Most recently, I’ve been amazed to hear staff mention diet when seeing patients with obscure presentations like Postural Orthostatic Tachycardia Syndrome (POTS), an immune-related disorder of the autonomic nervous system that, because it can be triggered by a viral infection, has become increasingly prevalent post-pandemic. This should come as no surprise since our diets and our immune systems are intertwined.

The food we eat every day is the subject of an ongoing conversation between the bacteria in our gut and our immune cells. This conversation dictates whether we are taking one step closer to health or one step closer to disease. I now realize that the list of potentially diet-related conditions is vaster than I had ever imagined. Pushing through the confusion toward a new model has fostered an understanding that has fundamentally changed my approach to nutrition and to medicine.

Now, I operate under the assumption that, if you think about it long enough, you’ll likely find a way to implicate diet in nearly everything.

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  • Dona Tiberio says:

    Mary’s analogies are always so clear and understandable for the lay person. I’m always learning so much from her!
    The field of nutrition and brain health is so exciting and I’m very pleased to see doctors like Mary taking us in this new direction. She may never know the full impact of her studies on the improving care and lives of patients. Looking forward to more from her.

  • milena forte says:

    Fantastic piece. It’s a fascinating area of study. (the microbiome and mode of birth is also a really interesting and emerging area of research)


Mary Sco.


Dr. Mary Sco. is a family physician with a PhD in nutrition. She completed her degrees at the University of Toronto and her residency at Women’s College Hospital. She is currently pursuing additional training at the Eastern Virginia Medical School, in Norfolk, Va. She is passionate about translating nutrition science into clinical settings and shares her evidence-based educational nutrition guides at:

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