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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.

The World Health Organization has declared depression the worldwide, yet . At the same time, more and more data suggests a repeated link between diet quality, gut microbiota, and susceptibility to a variety of mental health ailments, independent from other risk factors. The nascent field of acknowledges the sizable gap between our current treatments for mental health conditions and offers a hopeful other way: adjusting our diet.

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: .

Links Between Inflammation in the Gut and Mental State

The paradigm of mental health disorders as inflammatory disease is still emerging but is . One recent human study showed that a spike in the pro-inflammatory could produce depressive symptoms in men. Another study in patients with major depression found that higher depressive symptoms were associated both with , 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 and .

It may not seem obvious, but the gut is actually the . The surface area of the intestine (called the epithelium)—where our body interfaces with the outside world—. Underneath that surface is a , poised to sample the periphery and maintain order. Above the protective mucus layer that tops the epithelial surface sits the body’s largest collection of friendly (and occasionally unfriendly) microbes. The gut microbiota (as it is called) partners with us for important physiological functions including , , , and even .

The gut microbiota is , as our food is ultimately its food. Our gut microbiota, in turn, 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 and olive oil. People who suffer from diabetes, a strongly diet-influenced disease, are to , and also more likely to have . There is also a 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 (). Moreover, transplanting the gut microbiota 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, , 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, ). 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 or members of the bacterial families . Fiber feeds these bacteria that produce the SCFA by giving them the stable food source they need to grow and persist.

SCFA affect many aspects of health, including the following:

• They serve as the .
• They that keep the intestinal barrier strong.
• They promote the that keeps bacteria a away from the epithelium and underlying immune system.
• They promote pH balance at the epithelial surface by .
  • This produces the .
  • May help prevent the colonization of infectious organisms, which tend to .
• They directly dampen inflammatory response in and .
• .

Higher fiber intake from fruits and vegetables, which is broken down into SCFA by the microbiota, is associated with lower rates of depression . Other concluded that diets rich in nutrients including fiber are linked to a reduced risk for . Both and directly can also protect against the effects of a model of sickness that , 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. 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

Antioxidants are substances that scavenge harmful oxidative substances in our bodies, which can cause cellular damage. The dark of different fruits and vegetables indicate the that may also . Oxidative stress, caused by reactive oxygen species (ROS) or free radicals, can activate , including and , which are both associated with depression. Moreover, oxidative damage has been .

Several antioxidants have been found to either directly dampen depressive symptoms, or are associated with a lower incidence of depression:

• - Yellow and orange-hued compounds found in fruits and veggies such as tomatoes, carrots, oranges, grapefruits, or apricots.
• - A compound found in the spice turmeric that is readily metabolized by the microbiota, acting as a , a and .
• - A vitamin and powerful antioxidant abundant in many fruits and vegetables.
• such as - Micronutrients abundant in tea, citrus, legumes and others.

Flavonoids improved mood in a cohort of young people, and have been shown to and among the elderly. Many studies have observed the association between a lack of peripheral zinc and depressive symptoms, as well as . Similarly, low dietary selenium was predictive of developing de novo depression in a . such as , which are derived by the diet and , are also associated with treatment-resistant depression.

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 , as microbes in the gut help . On the other hand, oxidative stress from a high-fat low-nutrient diet was associated with reduced diversity and . 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

suggests that probiotics may have important mental health benefits, even if their actual passage through the intestine may be transient. Although , that probiotics may be able to central to many mental health disorders. Some probiotic foods promote a stable colonic microenvironment and in the intestine, which may give them an indirect effect on mental health. One meta-analysis concluded that probiotic intervention was associated with in people under 60. One animal study showed that a single probiotic strain (Lactobacillus reuteri 3) , 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 °®¶¹´«Ã½ 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 , , and altered function in in humans, as well as 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 might 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.

Further Information:

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• Podcast: Microbiome Diversity and Structural Variation with Ami Bhatt.

The above represent the views of the author and does not necessarily reflect the opinion of the American Society for °®¶¹´«Ã½.