I’ve written extensively about the link, documented in research published in peer-reviewed medical journals, between what we eat and how we feel (see my articles Beat the holiday season blues – with broccoli and berries!, Good mood food, Want to feel happier? Change what’s on your plate! and Eat your way to better mental health?, for starters).
However, up until quite recently, most research on the food-mood link has been epidemiological rather than clinical. That is, researchers have looked for associations between people’s dietary patterns and their risk of having become depressed now or becoming depressed in the future, rather than investigating whether changing the diet of depressed people can improve their mood.
But that is all beginning to change. A meta-analysis (that is, a study of studies) published in February 2019 pulled together data from 16 randomised controlled trials on a total of nearly 46 000 participants, and found that dietary interventions that aim to improve nutritional quality, reduce dietary fat intake and/or produce weight loss do indeed improve mood, both in clinically depressed people and those with non-clinical depression.
The article, The effects of dietary improvement on symptoms of depression and anxiety: a meta-analysis of randomized controlled trials, also found that dietary interventions aimed at reducing fat intake also led to a significant reduction in anxiety in women, although not in men.
The 16 trials used a variety of dietary interventions. 9 were primarily aimed at improving nutrient intake, 4 at decreasing fat intake and 4 at reducing bodyweight.
While only one dietary intervention was completely plant-based, all counselled participants to decrease their intake of ultraprocessed foods such as packaged snacks, pre-prepared meals, instant noodles, soft drinks and ice cream, as well as most animal products, and to eat more fruits, vegetables and whole grains. In some studies, participants were also instructed to eat more nuts and seeds.
The authors of the meta-analysis acknowledge that scientists don’t fully understand why depression and anxiety decrease when people improve their eating habits. However, there are several mechanisms that could explain it:
- Oxidative stress: People with psychological disorders have been found to have higher levels of oxidative stress, which is quenched by the antioxidant compounds found in abundance in whole and minimally processed plant foods.
- Inflammation: Depression in particular has been linked to elevated levels of inflammation throughout the whole body, including the brain; whole plant foods have an anti-inflammatory effect whereas meats (especially processed meats) and refined carbohydrates promote inflammation.
- Mitochrondial dysfunction: Mitochondria are the ‘power plants’ that generate energy within each cell, including brain cells. Mitochondrial dysfunction has been implicated in some mental disorders, including bipolar. Plant-based foods provide an abundance of the nutrients required for optimal mitochondrial function.
- Gut microbiota dysbiosis: The bacteria inhabiting our gut, and the compounds they produce from metabolising the undigested remnants of our food, as well as our digestive secretions such as bile, are heavily involved in modulation of our response to stress, immune function, neurotransmission (communication between brain and other nerve cells), and neurogenesis (the formation of new brain and nerve cells). The indigestible carbohydrates and polyphenols (pigments) found in plant foods promote the growth of beneficial gut bacteria and provide the raw materials needed for these bacteria to produce an array of biologically active compounds that improve mood. On the other hand, high fat intake leads to excessive bile secretion which leads to an overgrowth of bile-eating bacteria that produce inflammatory compounds. Artificial sweeteners and emulsifiers found in ultraprocessed foods also adversely alter the gut microbiome, activating inflammatory pathways. Saturated fat has a particularly devastating effect on the balance of gut bacteria.
- Neurotransmitter balance: Unrefined plant foods are rich in nutrients (including B vitamins and minerals such as magnesium) required for synthesis of neurotransmitters known to be involved in mood, such as serotonin.
- Weight loss: Individuals who lose weight have been shown to enjoy better mental health; the more weight lost, the greater the improvement in depression. Weight loss results in decreased inflammation levels which probably accounts for the majority of its mood-related benefits, but the improved self-esteem that ‘successful losers’ experience may also play a role. A diet rich in unprocessed plant foods promotes sustainable weight loss without the need to reduce portion size or count kilojoules.
While we wait for those detailed-oriented scientists to fill in all the knowledge gaps about how dietary change can lift our mood and relieve anxiety, we certainly don’t need to wait to implement the type of diet that has been consistently shown to produce these effects.
A wholefood plant-based diet ticks all the boxes:
- High in micronutrients such as vitamins and minerals
- High in antioxidants
- High in polyphenols, fibre and other indigestible carbohydrates to feed our friendly gut bacteria
- Low in or devoid of inflammation-promoting saturated and trans fats, artificial sweeteners, emulsifiers and other food additives which cause gut dysbiosis and drive inflammation
- Promotes sustainable weight loss without ‘deprivation dieting’.
One final note: only studies in which the dietary intervention was delivered by a trained nutrition professional – as opposed to research staff – found beneficial effects on participants’ mood and anxiety levels.
Exactly why this is has not yet been explained, but my sense is that the level of trust and commitment required for participants to make significant changes in their diet is fostered by the knowledge that the way of eating they’re adopting is nutritionally sound.
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