When most of us think about mental health, or we find ourselves pinned by its gravity, we generally think more about our brains than about our guts. Maybe we should think again.
This does not suggest that pharmaceutical or psychological interventions should be abandoned. Quite the opposite: mental health is multifactorial, and often requires multimodal therapy. The emergence of adjusting diet as a preventative strategy or possible intervention for depression, anxiety or other mental health disorders should not replace standard therapies. Although our brain creates our emotional and intellectual experience, it can be helpful to remember that it is also an organ with needs for nourishment and care, just like our heart or our liver. The brain, too, is sensitive to its environment.
Scientists are beginning to learn that mental health disorders share an important underlying cause with many other more “somatic” diseases, such as heart disease or diabetes: systemic inflammation.
Links Between Inflammation in the Gut and Mental State
The paradigm of mental health disorders as inflammatory disease is still emerging but is gaining support. One recent human study showed that a spike in the pro-inflammatory cytokine IL-6 in the CSF could produce depressive symptoms in men. Another study in patients with major depression found that higher depressive symptoms were associated both with higher circulating C-reactive protein (CRP), an inflammatory marker, and a lower abundance of the anti-inflammatory cytokine IL-10. Additionally, immune-suppressing medications used to treat autoimmune conditions such as inflammatory bowel disease reduced depressive symptoms both in humans and in animal models.
The gut microbiota is directly affected by the food we eat, as our food is ultimately its food. Our gut microbiota, in turn, affects our inflammatory state by breaking our food down into compounds that modify immune cells. So when we talk about diet, we are really talking about an entire downstream cascade of events which can manifest in poor health status when imbalanced. That imbalance can also affect our minds.
To date, several human studies have observed the reduced risk for depression in diets higher in fruits and vegetables, fish, whole grains and olive oil. People who suffer from diabetes, a strongly diet-influenced disease, are twice as likely to also suffer from depression, and also more likely to have anxiety. There is also a notable comorbidity between people who suffer from heart disease and depression. People treated for major depression using personalized dietary counseling and support were 4 times more likely to experience remission than a social support group alone (32% and 8% remission respectively). Moreover, transplanting the gut microbiota from depressed humans into rats was enough to produce depressive and anxiety-like behavior in the rodents, suggesting that the gut is an important bioreactor for mind-altering microbes.
Several specific nutritional components may have an important impact on the microbiota and the mind. It’s worth noting that a variety of dietary interventions for depression have not shown success, but of those that have, a common thread among many is the emphasis on fruits, vegetables, fiber, and whole grains. While somatic conditions like heart disease and diabetes each have clinically supported diets, it is not yet common practice to prescribe dietary intervention for mental health disorders.
For brevity’s sake, we are not able to entertain a comprehensive list of the nutrients that have been potentially linked to mental health, or to explore their role in a broader variety of mental health conditions. However, we will discuss several dietary components and the evidence that supports a possible role in promoting better mental health, especially with regard to depression and anxiety.
The Role of Fiber (AKA Prebiotics) in the Gut Microbiota
Fiber is composed of the indigestible polysaccharides that serve as roughage in our diet, and can be found in fruits and vegetables, beans, seeds and whole grains. These polysaccharides are a key food source for many healthy bacteria in the intestine. The American diet is notoriously low in fiber on average (only 5% consume the recommended amount, 19-38g per day). Fiber is important; though we can’t break it down ourselves, fiber is broken down by the microbiota into short-chain fatty acids (SCFA), which are arguably the heroes of homeostasis—or balance—in the gut. Not all bacteria are able to metabolize fiber into SCFA, only certain microbes, such as Faecalibacterium prauznitziior members of the bacterial families Lachnospiraceae and Ruminococcaceae. Fiber feeds these bacteria that produce the SCFA by giving them the stable food source they need to grow and persist.
Fiber is broken down by beneficial microorganisms in the gut into short-chain fatty acids (SCFA), such as butyrate. SCFA carry out many important stabilizing functions in the gut beyond. Since humans cannot break down fiber ourselves, we rely on microbes to do this for us.
Source: Reprinted by permission from: Nature Publishing Group, Nature, “Your Microbes at Work: Fiber Fermenters Keep Us Healthy. Copyright 2015
SCFA affect many aspects of health, including the following:
Higher fiber intake from fruits and vegetables, which is broken down into SCFA by the microbiota, is associated with lower rates of depression in a Japanese population. Other literature reviews concluded that diets rich in nutrients including fiber are linked to a reduced risk for inflammation, depression and anxiety. Both fiber and SCFA directly can also protect against the effects of a model of sickness that induces depressive behavior, such as social withdrawal, in animal models.
Since the effects of fiber are mediated almost entirely by our gut microbiota, it’s important that we consume fiber consistently to support the populations of microorganisms that survive by breaking it down. Low or inconsistent consumption of fiber may not have reliable effects, since those microbes may not persist in a gut ecosystem that does not favor them.
The Role of Antioxidants & Phytonutrients in the Gut Microbiota
Eating fruits and vegetables with darkly pigmented colors helps ensure a healthy antioxidant intake. The good news is, they are also a great source of fiber. This combo is a win-win for the gut microbiota and your mental health.
Fresh fruits and vegetables are bursting with many of these antioxidants and phytonutrients, which makes their health benefits manifold, considering their coincident content of fiber as well. Many antioxidants are also metabolized by the gut microbiota, as microbes in the gut help modulate their bioavailability. On the other hand, oxidative stress from a high-fat low-nutrient diet was associated with reduced diversity and greater numbers of potential pathogens in the microbiota. If this is true, then a low-antioxidant diet is a double loss, both from the loss of the phytonutrients themselves and also from the suppression of a microbial population able to make them bioavailable. Note: it is recommended to get vitamins and antioxidants from food rather than supplementation, since fresh foods deliver a broad diversity of nutrients and fiber at once, and in the biological packaging your body is adapted to receive.
The Role of Probiotics in the Gut Microbiota
Evidence suggests that probiotics may have important mental health benefits, even if their actual passage through the intestine may be transient. Although results have been mixed, some argue that probiotics may be able to mitigate the neurological inflammation central to many mental health disorders. Some probiotic foods promote a stable colonic microenvironment and protect against opportunistic infection in the intestine, which may give them an indirect effect on mental health. One meta-analysis concluded that probiotic intervention was associated with reduced depressive score in people under 60. One animal study showed that a single probiotic strain (Lactobacillus reuteri 3) protected mice from depression associated with social defeat, which may have implications for the far-reaching effect of our guts over our felt experiences.
One important note about probiotics is their method of delivery. In the United States, the Food and Drug Administration (FDA) currently does not regulate the production of commercial probiotic supplements, leaving them vulnerable to variability and inefficacy. For this reason, many recommend fermented foods that contain live cultures in their place. Although fermented foods are more ambiguous with regard to species representation and actual dosage, they contain micronutrients absent in supplements. Consumption of fermented foods has been associated with decreased social anxiety, lower risk of depression, and altered function in brain regions that process emotion in humans, as well as reduced stress in animal models. Admittedly, many more studies are merited to flesh out these claims and to establish meaningful treatment guidelines.
How Can My Diet Promote Mental Health
Although there may one day be an elegantly developed “depression diet” with precise clinical recommendations, a good research-supported shorthand for now may be:
Eat your fiber, and eat colorful fruits and vegetables. If nothing else changes in your diet, bolstering your intake of colorful plant-based foods that are high in fiber and antioxidants may be a boost for your gut microbes and your brain.
Try to reach the recommended 19-38g fiber per day. You may be surprised how much intentionality this takes! Eating with this goal in mind may naturally crowd out some of the less-healthy temptations with satiating foods.
Small changes matter. Simple additions like throwing chia seeds and some berries on your yogurt or snacking on dates instead of a pastry can have real impacts on your daily fiber intake. No change is too small. The handful of nutrient-rich berries or nuts you eat is better than the home-cooked organic meal you don’t.
Remember: you’re building an ecosystem. And ecosystems take time. Think of strengthening your gut like reforesting an area. Consistency and time are key ingredients.
Microbes and Mental Health: Concluding Thoughts
When you walk into the doctor’s office with cardiovascular disease or diabetes, it's normal to discuss what you eat. That clinical precedent has not yet been established with mental health, but it may become part of mental health check-ups of the future. The more we understand pathologies as expressions of inflammation that is out of equilibrium, the more logical it becomes to intervene not just with the pathology, but with the entire physiological system. One of the primary ways we have within our power to do that is through the diet.
The manifold connections between the gut microbiota and the brain have emerged in recent years, leading to an exciting area of research both in the prevention and the treatment of disease. Harnessing our diets for the cultivation of our microbes, and in turn our mental and physical health, may well be the next medical frontier.
Christy Harrison, Ph.D., is a writer and scientific communicator whose research work focused on gut immunity and the microbiota, along with public health disparities in nutrition.
