Mood disorders (altered thinking, mood or behavior associated with a certain level of distress) are highly prevalent in Canada. In fact, 1 in 7 people suffer from this condition1. Up to 1 in 3 people will experience anxiety at some point in their life2. The implementation of psychotherapy, tools for personal growth and self-management of emotions as well as the prescription of antidepressants of all kinds are part of the first-line therapies to treat a mood disorder3. Surprisingly, it has also been shown that people with a mental disorder such as depression or anxiety are also more likely to have chronic illnesses such as cardiovascular disease, diabetes, high blood pressure, chronic obstructive pulmonary disease, etc4. Knowing that cardio-metabolic diseases of this kind are very often the result of bad lifestyle habits, including bad eating habits; could there be a physiological link between the two? How can diet impact brain function and mood management?
Most of us think of food as the act of ingesting “calories” to burn off energy in different ways every day. To add to this limited conception, food is also proving to be a powerful vector of health by providing the body, with every bite, with thousands of chemical molecules with clear biochemical relevance in the body, without which a ton of enzymatic reactions would not happen properly. Clearly one of those vital functions that are the dynamic and continuous interaction with several essential nutrients from our diet is the production of neurotransmitters that regulate mood and behavior in the brain.
Several studies in humans have looked at the effect of diet on mental health status. It is not surprising to see findings that support the fact that a healthy diet is associated with a lower degree of depression5,6,7,8,9. Indeed, we can definitely conclude that a dietary approach that promotes the consumption of an abundance of vegetables and fruits, unprocessed whole grains, oily fish, olive oil, nuts and seeds is all related to a more balanced psychological health. Conversely, we see an increased risk of depression in heavy eaters of red and/or processed meats (cold cuts), refined grains/cereals (flour, breads, breakfast cereals, pasta, etc.), dairy products rich in fat as well as desserts, candies and confectioneries. Surprisingly, we see that this type of diet is also associated with a significant reduction in suffering from cardiovascular disease, as the biochemical effect induced by these eating habits has an implication in the inflammatory mechanisms that can affect the brain as much as the health of the arteries10. Equally astounding is how the nutrition of a mother carrying her fetus has the power to affect the mental health status of her unborn baby during childhood and adolescence. In fact, the more nutrient-poor this diet is and the higher the number of processed products, the more likely the unborn child is to develop psychiatric problems throughout its development11,12,13,14. But why does diet have such a big impact on mental health? The effect on the microbiota, the balance between inflammation and oxidative stress, as well as the impact on brain plasticity are well described mechanisms.
The microbiota = our second brain
First of all, we cannot ignore the powerful effect of the gut microbiota and its link to mental health which has been the source of thousands of scientific publications in recent years. Considered an “organ” in itself, the intestinal microbiota contains nearly 100 trillion bacteria; more than the number of cells that constitute us15. These bacteria inhabit us in a symbiotic manner by being closely involved in several processes essential to human survival such as the synthesis of vitamins and neurotransmitters, the regulation of systemic immunity and inflammation, the protection of the barrier digestive epithelial for absorption of nutrients, etc.16. It is not surprising that disturbances in the microbiota, particularly loss of diversity, are associated with many chronic cardio-metabolic and inflammatory diseases.17 These bacteria which colonize us react and ensure their growth according to what is given to them to “eat” (i.e., how we eat every day) and interact bidirectionally with the 500 million individual neurons that line the digestive system18. Fiber and several polyphenols (found in nuts, seeds, whole unprocessed grains, legumes, fruits and vegetables) are the nutrients of choice for a good selection, diversity, and growth of the healthiest bacteria19. Conversely, excessive intake of certain medications (antibiotics, antacids, anti-inflammatories, etc.), a state of chronic stress as well as a diet low in fiber and rich in processed foods, sugars of all kinds, sweeteners, and pesticides can greatly affect the quality of the microbiota20,21. When the microbiota is disrupted by the stressors mentioned above, we can witness a phenomenon of loss of integrity of the epithelial barrier lining the lumen of the digestive tract, causing a break between the proteins binding the cells to each other to ensure good permeability between the intestinal contents and the systemic circulation. We call this the “permeable intestine syndrome”, the phenomenon where food molecules, bacterial metabolites and/or bacteria themselves enter the systemic circulation without having been adequately filtered and whose presence triggers the activation of the immune system and a constant state of inflammation. This syndrome has been associated with several systemic pathologies such as inflammatory bowel disease, diabetes, asthma, and psychiatric disorders such as anxiety, depression and autism22,23,24,25,26. The inflammatory molecules generated by the presence of these exogenous molecules, in circulation and neutralized by the immune system, affect the entire body: inability to properly regulate blood sugar levels, chronic pain, fatigue, negative or anxious mood, digestive bloating, etc. In this situation, a sanitation of the diet, the withdrawal of behaviors or substances harmful to the balance of the microbiota, as well as the use of probiotics can prove to be very relevant to find a diversity and an optimal function of the microbiota with its effect expected on the regulation of immunity and systemic inflammation as well as on the metabolism of neurotransmitters and several vitamins and biochemical molecules essential for the function of the body’s systems27.
Inflammation, oxidative stress, and the need for antioxidants
Inflammation is a normal phenomenon essential to survival that allows the healing of an injured structure following damage of any kind to the structures of the body. It is important in acute and occasional situations when the damage is accidentally created, but becomes a problem when it is chronically present due to regular and persistent damage. For example, daily exposure to toxins (such as cigarettes or mold in the environment), stress and chronic sleep deprivation (involving persistent pro-inflammatory hormonal disturbances), and a diet filled with sugar and processed foods harmful to the digestive system are situations that contribute to causing chronic inflammation. Chronic low-grade inflammation has been shown to be associated with depression, schizophrenia and bipolar disease28,29,30. Oxidative stress is the result of chronic inflammation and results in the production of free radicals which attack healthy structures in the body (including neurons!) and is the cause of the loss of organ function and accelerated aging. One of the ways to counter oxidative stress is to give the body a significant amount of “antioxidants” which are actually the various phytonutrients, vitamins and minerals found in a healthy, plant-rich, unprocessed and diverse diet. It has indeed been observed in some studies that several antioxidant markers were reduced in individuals with acute depressive episodes31,32.
Brain plasticity, essential for emotional regulation
An essential region of the brain, the hippocampus, is involved in the phenomena of learning, the creation of memory and emotional regulation. Neurons in this region of the brain appear to be able to form and grow under the influence of a substance called BDNF (Brain-derived neurotrophic factor)33,34. Without having a well-developed and functioning hippocampus, one can face memory problems, difficulty learning new tasks and poor management of emotions; symptoms that are very often seen in depression and anxiety. Stress is one of the most powerful factors that can negatively affect BDNF levels35, but there is a growing body of evidence that a high nutrient density diet favorably affects BDNF levels which can boost neurogenesis, while diets high in sugar and poor-quality fat found in processed foods have a completely opposite effect36,37,38.
For all the physiological reasons discussed above, good mental health relies on an optimal supply of essential nutrients to ensure all the biochemical reactions of the body involved in the selection of a diverse microbiota, in the synthesis of neurotransmitters, in the neurogenesis and the attenuation of oxidative stress generated by the environment and/or lifestyle that can cause cellular damage to neurons in the brain. Each bite you eat should be as nutritious as possible:
- No food processed by the food industry: Food consumed should come as close as possible to what could be picked from a garden or removed from a free-range animal with an optimal environment.
- An abundance of plants of all kinds. For example, fetching the rainbow of colors through fruit and vegetable choices over the course of a week. It would be wise to prioritize vegetables over fruits to avoid excess sugar that may be less well tolerated in metabolically susceptible individuals.
- A significant amount of fiber to add daily in the form of nuts, seeds, vegetables and fruits, whole unprocessed grains and legumes.
- A good quality fat intake such as olive oil, nuts, oily fish, etc., while being careful not to overheat the oil as its favorable properties may be lost.
- Providing probiotics through fermented foods (sauerkraut, kimchi, plain yogurt, kefir, tempeh, etc.).
- No refined or concentrated sugars from whole foods.
Diet should always be optimized before offering supplementation to ensure a sufficient intake of any nutrient since a whole food provides much more biochemically than what can be found in a tablet. However, it is sometimes necessary to have to recourse towards supplementation for various reasons (food intolerance to certain foods, inability to ingest the recommended nutritional intake of a certain nutrient, a lifestyle that requires a greater than recommended intake of a certain substance that the diet cannot supply in sufficient quantity, etc.). Personalization of the approach is essential in order to target the risks of deficiencies and the levels of nutrient intake and above all to assess the quality and diversity of the microbiota that inhabits us while seeking strategies to optimize it, knowing its well-demonstrated positive impact on mental health. All in all, it is clear that psychotherapy and the use of antidepressants are not the only options in the management of mood disorders. Turning to healthy eating and lifestyle strategies can sometimes be much more powerful in the quest to improve mental health!
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