Plant-based diets have been linked to significant improvements in physical health in a multitude of studies. But it seems that mental health can also be positively affected by a healthy diet. Major depression is a mood disorder manifested by feelings of sadness and hopelessness not related to events happening in a person’s life that interfere with the ability to function in daily life activities. 89% of people with depression have mild to moderate symptoms; the remaining 11% experience severe depression (1). In Canada 11.2% of the population suffers from major depression at some time in their lives (2). It has been projected that by the year 2020 depression will be second to heart disease as the leading cause of healthy years of life lost worldwide (3).
Perhaps surprising to many is that the food we eat has a significant effect on our mood state, so much so that dietary alterations may be a better way to both prevent and treat depression.
How well do anti-depressant drugs work?
Apparently not too well, especially in mild to moderate depression. A meta-analysis published in 2012 compiled both published and non-published studies to conclude that indeed, antidepressants do result in a strong therapeutic response in patients with depression, however, this response is only marginally greater than the response to an inactive placebo. In other words, because patients are taking a drug that they think will make them feel better, they do feel better whether or not that medication contains an active ingredient (4). It is not until depression becomes severe that antidepressants actually work markedly better than a placebo (5,6).
This might leave us wondering why these drugs are even available for use in milder forms of depression. Shockingly, the companies that are marketing the drugs are often the sponsors of the studies that look into their effectiveness. The unfortunate consequence is that only studies showing a positive result are published. The literature that is provided to physicians and included in the package inserts of antidepressant medications maintains that the results of nearly all antidepressant trials are positive. However, when FDA-registered trials regarding the efficacy of antidepressants were examined by the US Food and Drug Administration (FDA) the results were eye-opening. Almost half of the studies showed no positive effect at all for control of depressive symptoms by antidepressants. 37 out of 38 studies with positive results for depression were published but, of the 33 studies showing little to no effect, 22 were not published at all and 11 were, as the researchers stated, “published in a way that, in our opinion, conveyed a positive outcome” (7,8).
The concern with these revelations is of course that antidepressants have side effects that can severely impact those that take them. Three quarters of patients on antidepressants suffer sexual dysfunction. Other adverse effects include insomnia, blurred vision, headaches, dizziness, anxiety, suicidal thoughts, weight gain, nausea and diarrhea (9). Additionally it is not easy to stop taking antidepressants. Their dosages must be decreased gradually to avoid unpleasant withdrawal symptoms (1).
Interestingly, exercise is increasingly being used as a first line of treatment for depression. It can effectively decrease symptoms of both depression and anxiety, especially in patients with mild to moderate depression (10).
What is the evidence of the effect of food choices on depression?
Many studies from around the world have illustrated the substantial effect that food choices can have on human mental outlook. Here is a sampling of some of these investigations.
A 2009 study published in the British Journal of Psychiatry analyzed the dietary patterns and incidence of depression in 3,486 participants over a five-year period and found that those eating more whole foods exhibited fewer symptoms of depression than those who ate mostly processed foods (11).
A 2010 cross-sectional study on 138 Seventh Day Adventists illustrated that vegetarian eaters had significantly better scores than omnivores on the Depression Anxiety Stress Scale (DASS), a standardized measurement of mood (12). The researchers noted that vegetarians have lower intake of the long-chain omega-3 fatty acids, DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), which can decrease inflammation. However, the intake of AA (arachidonic acid) by vegetarians is also low and so they avoid the inflammatory by-products of the metabolism of AA (13).
A 2010 study from Japan assessed the dietary patterns and symptoms of depression of 521 municipal employees. Those eating the highest amounts of vegetables, fruit, mushrooms and soy products had 60% fewer depressive symptoms than those eating the lowest quantity of these foods (14).
In 2010 and 2015 volunteers recruited from among employees of Geico, a major insurance corporation, were entered into a study of the effects of a worksite nutrition program for improved health and productivity. More than 400 participants with a BMI of 25 or more or a diagnosis of type-2 diabetes were involved in the two trials of this study with their five months of follow-up. Subjects were divided into two groups, one of which received weekly instruction on a low-fat vegan diet and the other who received no dietary intervention. The vegan group experienced improvements significantly greater than those of the control group. These included an increase in general health, mental health and emotional well-being; less depression, anxiety and fatigue; decreased food costs; and increased overall diet satisfaction. The vegan group reported a 40 to 46% decrease in health-related productivity impairments at work and at home. (15,16)
In 2017 the SMILES Trial in Australia looked at whether improving the diets of people with major depression would improve their mood. This was a randomized controlled trial that provided seven individual nutritional consulting sessions delivered by a clinical dietician to half the participants and a social support session of the same length to the other half of participants. After 12 weeks those who improved their diet with the help of a dietician showed much higher increases in ratings on a depression scale than those in the social support only control group (17).
The SMILES Trial was followed in late 2017 by a larger study. In this trial adults with depression were randomized into two groups; the interventional group received food hampers with Mediterranean diet ingredients, cooking workshops and fish oil supplements every two weeks for 6 months while the control group simply attended a social group every two weeks for three months. At the end of three months the participants following the enhanced diet were eating many more vegetables, legumes, fruits, nuts and whole grains and less red meat, chicken and unhealthy snacks. They showed greater reduction in depression and improved mental health overall compared to the control group. These benefits were sustained at 6 months (18).
In July of 2018 the results of a meta-analysis from the United Kingdom were released. This review looked at eleven studies published between 1999 and 2017 that had examined the health effects for diabetics of eating a plant-based or vegan diet. Results showed that type-2 diabetics eating either vegan or plant-based had much better outcomes than the control subjects eating a more standard diet. Symptoms of depression were considerably lower. Levels of hemoglobin A1c, a marker of lack of diabetes control, was reduced by over 3.5 times and fasting blood glucose levels dropped twice as much as that of the controls. The healthy diet group also reduced both total and LDL cholesterol, lost about twice as much weight as those on a control diet and drastically reduced their requirement for blood pressure lowering medications (19).
In September of 2018 a systematic review of 41 scientific studies that had looked at the link between diet quality and depression was undertaken. Results showed that a diet rich in fruits, vegetables, whole grains and legumes and avoiding processed meat, dairy and trans-fats could reduce risk of depression by 35%. On the other hand, diets that included high amounts of processed meats and trans-fats increased the risk of depression. Researchers suggested that a healthy diet reduces both inflammation and insulin resistance, both of which can have positive effects on mental state. They also stated that the antioxidants found in plants may be helping to regulate emotions (20).
Is depression an inflammatory condition?
Research does indeed reveal a link between inflammation and depression and increasing evidence is indicating that inflammation caused by free radicals such as reactive oxygen species plays an important role in this connection (21). Depressed people have higher levels of inflammatory markers in their bodies (22). Correspondingly those suffering from inflammatory diseases such as asthma and allergies suffer higher rates of major depression (23,44). Additionally, half of patients taking a drug that induces inflammation, for example interferon for cancer or chronic infection, eventually develop major depression (24)
If this is so, where does the inflammation come from? It appears that a range of factors can increase the risk of systemic inflammation and the possibility of development of depression. These include stress, trauma, poor diet, physical inactivity, smoking, obesity, unhealthy gut bacteria and lack of sleep. Fortunately all these factors have the potential to be improved through lifestyle changes (23).
When it comes to diet, numerous population-based studies illustrate the association between diet quality and systemic inflammation. For example, the Nurses’ Health Study concluded that higher intakes of vegetables, fruit, whole grains and legumes is associated with reduced markers of inflammation in the body whereas an unwholesome Western diet pattern high in red and processed meats, refined carbohydrates and other processed foods is associated with increased inflammatory markers (25). Similarly the Health Professionals Follow-up Study found higher levels of the inflammation marker C-reactive protein (CRP) in men eating a Western dietary pattern (26).
What is it about plant-based diets that reduces depression?
The boost in mood from eating plants is thought to be due both to protective effects inherent in plants as well as to the removal of the harmful constituents that are a part of animal-based foods such as meat, fish, poultry and eggs. Certainly the most anti-inflammatory diet is a whole-food plant-based diet (28). Such a dietary pattern will cut C-reactive protein level (a marker of inflammation) by 30% in as little as two weeks (27). Possible mechanisms for this anti-inflammatory effect are many and varied and the major ones are outlined next.
Antioxidants protect cells by removing free radicals. Oxidative stress and its resulting inflammation occurs when the production of free radicals exceeds natural antioxidant defense mechanisms (21). A study performed in nearly 2000 people looking at body levels of carotenoid-type antioxidants (a bright yellow/orange pigment found in fruits and vegetables) showed that a higher blood carotenoid level was associated with lower risk of depression (29). Lycopene, one of these carotenoids that is found in the red pigment of tomatoes and pink fruits was the focus of a study of about 1,000 seniors which discovered that those who ate the most tomatoes had half the risk of depression compared to those eating the least tomatoes (30). A 2013 study of almost 300,000 Canadians found that higher fruit and vegetable consumption was linked to lower risk of depression and anxiety. The researchers concluded that this was the result of high levels of antioxidants in these foods protecting the participants from oxidative stress (31).
Folate is a B vitamin found in dark, green leafy vegetables that is often low in depressed patients. Studies have found that people with low folate blood levels have a higher risk of developing depression (42,43). Increased intake of folate can also be used to increase the positive effects of conventional treatments on depression (32).
The decrease in inflammation due to antioxidants and other phytochemicals in plant-based foods also helps to balance mood-regulating brain neurotransmitters such as serotonin, dopamine and norepinephrine. An enzyme known as monoamine oxidase (MAO) is involved in breaking down these neurotransmitters. It has been found that many people suffering from depression have high levels of MAO. A phytochemical called quercetin found in plant foods such as kale, grapes, onion, green tea, apples and berries can inhibit MAO and result in subsequent increases and balancing of the amounts of these neurotransmitters in the brain (33,34).
Tryptophan is an essential amino acid (meaning that is must be derived from food). It is converted in the body by an enzyme into serotonin, a brain neurotransmitter also known as the “happy hormone”, responsible for feelings of wellness and happiness. Plants such as leafy greens, soybeans, mushrooms, broccoli, sunflower seeds, pumpkin seeds and peas supply tryptophan to the body. Tryptophan can also be derived from some animal-sourced foods such as turkey and milk although the absorption into the brain of tryptophan from animal sources is considerably lower that that from plant-based sources. This is because amino acids must compete among themselves to be transported from the bloodstream into the brain. A meal high in protein contains much higher amounts of amino acids than a meal rich in carbohydrates. This excess of amino acids overwhelms the transport system, reducing the amount of tryptophan absorbed into the brain with consequent low serotonin production and depressed mood (35,36,37). High carbohydrate foods also increase insulin production which encourages muscles to take up higher amounts of amino acids to be used as fuel, further reducing competition for tryptophan transport into the brain (37).
Arachidonic acid is a fat found only in animal meats. It is not “essential” that it be obtained from the diet because the human body can convert linoleic acid into arachidonic acid to make the amount needed for body processes. Linoleic acid is ubiquitous in the human diet. Excess arachidonic acid is metabolized into inflammatory chemicals in the human body which increase general inflammation and can cause an over-reactive immune system (38). This type of inflammation in the brain can manifest in depressive symptoms, stress and anxiety (39). In the Seventh Day Adventist study mentioned above (12) it was noted that omnivores ate nine times more arachidonic acid than vegetarians (40). People who eat a diet low in arachidonic acid generally feel happier and more positive (12).
Essential omega-3 fatty acids have anti-inflammatory effects and patients with depression are often advised to increase the amount of fatty fish in their diet. However, a randomized controlled trial completed in 2009 illustrated that this practice is likely not helpful. In this study participants were divided into three groups – one ate their regular amounts of animal products, one consumed three to four servings of fish and shellfish a week but restricted other meat and poultry and the third group ate a vegetarian diet. After a short two weeks the vegetarian group was the only one to show statistically significant improvements in mood and depression symptoms. Though the fish-eating group did indeed increase their intake of omega-3 fatty acids, arachidonic acid levels were also high (41). It appears that the healthy omega-3 fats could not compensate for effects of the unhealthy fats also present in fish (arachidonic acid, cholesterol and saturated fat). Vegetarian diets can also take advantage of the alpha-linolenic acid (ALA) found in plants that can be converted in the body to EPA and DHA. Good sources of ALA include flax seeds, chia seed, leafy green vegetables and walnuts.
The take-home message….
Eating a diet based on plants is becoming increasingly recognized as an effective method for protecting against development of depression as well as treating depression that already exists. This is due not only to the beneficial nutrients found in plants but also to the crowding out of animal-based foods that contain substances harmful to the brain. This realization can bring hope and a wholesome therapeutic alternative to the many people suffering from this debilitating disease.
Watch for the next blog examining the new research revealing the surprising connection in our bodies that may be the real reason why food has such power over our mood.
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