The connection between diet and mental health
As coaches and fitness enthusiasts, we often think of nutrition as a variable to manipulate to achieve weight loss, strength gain, a specific body composition, or to enhance performance in sports and recreational activities. However, the thought of nutrition choices significantly impacting our mental health is not something we often consider when creating our meal plans.
There is a complex interplay between the human gastrointestinal, hormonal, and neurological systems that are heavily influenced by what types, quantities, and variety of foods we consume.
LET’S TALK ABOUT THE GUT-BRAIN AXIS (GUT MICROBIOME)
There are several ways that food can affect your mood, but any discussion on mental health and diet should pay special attention to emerging research on the gut-brain axis, or rather, the connection between the gut microbiome and its link to neurological health.
The gut microbiome refers to the fragile ecosystem of microorganisms (bacteria, protozoa, viruses, etc) that occupy the gastrointestinal tract. Although we commonly associate these microorganisms with infectious diseases, some microorganisms are necessary for optimal health.
Previously, it was thought that it was impossible for these microorganisms occupying our gastrointestinal systems to communicate with our brains due to the blood-brain barrier, however, recent research has begun to not only contradict this line of thinking but demonstrate how the correct balance of microorganisms in the GI tract directly affect our mental health and well-being (Butler et al., 2019).
It is hypothesized that much of this communication occurs through the vagus nerve which provides information from the gut to the brain inducing neurological responses. Additionally, much serotonin (an important neurotransmitter, too little of which is a potential mechanism for clinical depression) production occurs in the gut which has a direct influence on mood. Microorganisms living in the gut are also capable of producing mood-altering substances such as GABA, melatonin, and histamine which can directly impact neurological function (Carabotti et al., 2015).
The gut microbiome is a very fragile ecosystem that can be altered by dietary and lifestyle habits. Consumption of foods high in trans fats and simple sugars can, in a sense, feed one certain type of bacteria giving it a competitive advantage over other types of necessary bacteria.
Likewise, overuse of antibiotics and high-stress lifestyles can kill off many necessary microorganisms, also giving too much of one type a competitive advantage. This can cause adverse effects in the human body including but not limited to weight gain, irritability, depression, and potentially leading to more serious neurological disorders such as Parkinson’s disease, autism spectrum disorders, and Alzheimer’s disease (Ljungberg et al., 2020).
Li et al. (2019) examined the effects of the administration of specific pre/probiotics (Bifidobacterium longum, L. rhamnosus) on the behavior of lab rats. Fifty rats were randomized into groups, some of which were given the pre/probiotics and some not. A group of treated and untreated rats was exposed to stressful conditions and markers of depression were measured (preference for sucrose, serotonin levels, weight, etc). The researchers determined that the treated rats showed decreased body weight, reduction in depression symptoms and anti-social behavior, and significant improvement in serotonin metabolism as compared to the controls.
In an earlier landmark study, Sudo et al. (2004) determined that mice born and raised in a completely sterile environment with no exposure to microorganisms did not develop proper activation of the hypothalamic-pituitary-adrenal axis when exposed to stressful conditions. However, of greater interest, these effects seemed to be reversed by introducing microorganisms into the systems of the “germ-free” mice.
Overall, it is becoming clear that the gut and brain communicate with one another, and we need to pay attention to the state of our gut health for physical and mental well-being. How do we accomplish this with diet?
FOODS TO BOOST YOUR MOOD
Improving your gut microbiota is not as simple as just taking a pro-biotic. Our understanding of what bacteria are necessary and in which quantities is emerging at best. However, it is well accepted that having a diverse gut microbiome is highly associated with general and mental well-being. Certain foods are very rich in prebiotics, meaning that they provide the fuel necessary to nourish a variety of helpful microorganisms. Fermented foods such as sauerkraut and kefur and other foods such as bananas, leeks, lentils, asparagus, broccoli, and wheat flour are included in this group (Butler et al., 2019).
There seems to be some good evidence that adherence to a Mediterranean diet is linked to the prevention or reduction of depressive symptoms. The Mediterranean diet is rich in fruits, vegetables, legumes, fish, whole grains, nuts and seeds, and foods rich in omega-3 fatty acids. These are all foods that can support a healthy gut microbiome. Likewise, the Mediterranean diet tends to be low in foods mostly composed of simple sugars and trans fats which can be harmful to the gut microbiota (Oddo et al., 2022).
Skarupski et al. (2013) conducted a longitudinal study looking at the correlation between adherence to a Mediterranean diet and symptoms of depression in older adults. The researchers surveyed a group of 3,502 adults over the age of 65 with no diagnosis or current symptoms of depression. The participants were also given a MedDietScore questionnaire to evaluate their dietary patterns.
Most participants were re-evaluated after 7 years. The rate of depressive symptoms was 98.6 percent lower in the group whose dietary pattern resembled the Mediterranean diet. The study authors concluded that adherence to a diet rich in fruits, vegetables, legumes, fish, and whole grains had a protective effect against depression in older adults.
FOODS TO AVOID
Food High in Simple Sugars
It has been well established that diets high in simple sugars promote inflammation throughout the body, disrupt endocrine processes, and cause gastrointestinal distress. Similarly, there has been much recent research conducted linking these diets to symptoms of depression (Knüppel et al., 2017). These foods are generally considered to have a high glycemic index and are often stripped of nutrients (vitamins, minerals, fiber, etc), and digested quickly. Although these types of foods may be helpful to athletes during competitions to quickly refill glycogen stores or provide easy energy, most people do not use them for that purpose. These foods cause rapid spikes in blood glucose levels which result in a subsequent spike in insulin levels followed by a cascade of hormonal disruptions.
This is not the only mechanism for how diets high in refined carbohydrates affect mood. Much attention has been given to the link between diets high in refined carbohydrates and reductions in brain-derived neurotrophic factor (BDNF). BDNF is a neurotransmitter modulator found in tissues in the central nervous system and gut. It is thought to play a role in neuroplasticity and the facilitation of learning and memory (Bathina & Das, 2015).
Low levels of BDNF have also been linked to symptoms of depression and other mental health conditions. There is a not insubstantial correlation between diets high in refined sugars and low levels of BDNF. It is hypothesized that this may be one mechanism for how high sugar diets are linked to depressed moods or a diagnosis of clinical depression as well as Parkinson’s Disease and Alzheimer’s Disease (Knüppel et al., 2017).
It is also important to note that consumption of foods with a high content of simple sugars triggers a dopamine surge which stimulates the pleasure/reward centers in the brain leading to cravings for more of these foods. This can lead to a cycle of overconsumption resulting in symptoms of depression and other mood disturbances (Fajstova et al., 2020).
Foods High in Saturated and Transfats
We have all heard that excessive consumption of trans fats can be unhealthy, but we now know that these types of fats can destroy the diversity in our gut microbiomes. It is hypothesized that this effect on the microbiome may be the link between the consumption of trans fats and the development of associated disease processes (Ge et al., 2018).
However, it is not solely the consumption of trans fats that can disrupt the gut’s ecosystem. High-fat diets, especially those high in saturated fats, can greatly increase the population of bile-tolerant bacteria which will then out-compete other types of necessary bacteria. There is some thought in the scientific community that diets high in saturated and trans fats may contribute to the development of autism spectrum disorders due to their devastating effects on the gut microbiota.
It is probably wise to consume fats in moderation, and when possible, consume fat sources that are rich in Omega-3 fatty acids (which reduce inflammation) rather than other types that promote this disruption in the gut’s ecosystem. Foods such as butter, fatty meats, cheese, cured meats, and lards should be consumed in moderation (Kittana et al., 2021).
EFFECTIVE TIPS TO STAY MENTALLY HEALTHY THROUGH NUTRITION
• Reduce your intake of simple sugars and trans-fats. This means limiting things like cookies, cakes, hot dogs, and french fries. These are foods generally considered to be junk foods (Knüppel et al., 2017).
• Increase intake of fermented foods such as yogurts, kefur, sauerkraut, pickles, miso, tempeh, and kombucha (Leeuwendaal et al., 2022).
• Increase your intake of a variety of fruits, vegetables, and whole grains which can help feed a diverse population of bacteria (Oddo et al., 2022).
• Ensure you are consuming foods rich in Omega-3 fatty acids or consider a supplement. These foods are fish, walnuts, flaxseed, chia seeds, and soybeans (Costantini et al., 2017).
• Consider targeted pro-biotic therapy if warranted.
The longstanding advice to limit sugars and trans fats in conjunction with consuming a diet rich in fruits, vegetables, whole grains, healthy fats, and lean meats seems to be not only important for our waistlines but also important for the function of our gut and brain.
Although more research is needed to determine the exact mechanisms as to how the gut is linked to the brain, it is very clear that there is a strong link. Remember that your body is an ecosystem and not just a singular being where one system (i.e., the gut) affects other systems (i.e., the brain).
Feed your ecosystem well!
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