Your gut-brain axis can tell if you're at risk for depression

People don’t generally connect diet with mental health, and it’s certainly not on the radar of most health professionals when treating depression. But as global rates of depression and other non-communicable diseases have risen, the average diet quality has changed drastically.

Recent studies have shown that not only are diet and depression related, but people with unhealthy diets are more likely to become depressed. (That’s even after controlling for factors like smoking, alcohol, physical activity, marital status, education, and income.) Conversely, healthy diets are protective.

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After years of exploring the effects of nutritional supplements on mental health as a researcher at the University of South Australia, I became frustrated. After all, people eat food, not nutrients. Food delivers a virtual orchestra of vitamins, minerals, proteins, fats, carbohydrates, fiber, and phytonutrients, all of which contribute to the symphony that is our living body and brain.

It’s no accident I say body and brain — they are intimately connected. Confirming Hippocrates’ observation circa 400 BC, scientists have identified multiple connections between the brain and the rest of your body. Most revealing are pathways between the brain and gut. This “gut-brain axis,” or the biochemical signaling between your gastrointestinal tract and nervous system, can affect everything from mood to libido. (And vice versa. Think of the queasy, fluttering feeling in your stomach before a speech or job interview.)

So what does the gut-brain axis tell us about depression? This debilitating condition afflicts over 300 million people and carries the largest burden of disease worldwide. Not the least of that burden is a higher rate of heart disease and other protracted ailments. Biological factors, like inflammation, poor nutrition, and dysbiosis (imbalanced gut bacteria), are related to both physical illness and depression, so they might help explain this connection. But the best way to show cause and effect – to answer the question of whether diet improves mood, or if depression causes poor diet – is with a randomized controlled trial.

So in 2014 and 2015, my team recruited people with depression. We eventually signed up 152 people, ages of 18 to 65, with self-reported or diagnosed depression. These volunteers visited our clinic for blood tests and completed questionnaires about their diet and mental health. Then we randomly allocated them to a diet intervention or control group. To keep everything else constant, participants were instructed not to change any existing treatments or start any new ones.

Running this kind of study is no easy task, and our group was one of the first to do it in people with depression. In some clinical trials, it’s easy to design a controlled experiment: participants take an active supplement or placebo and everyone finds out afterward which one they took. But changing people’s diets is a different matter. First, we researched strategies that nudge people towards healthier eating. Then we tested the program. Our multi-pronged approach included interactive nutrition education, individual goal setting, food hampers, recipes and – yes – cooking workshops. Eat your heart out, MasterChef. My post-doc, Dorota Zarnowiecki, and I even made some cooking videos, bloopers and all. (Turns out, you can’t put an onion back together if you messed up while chopping it.)

Depressed people often feel better just by engaging in research or activities that generate peer support. So volunteers allocated to the control group not only continued their normal diets, they attended social groups instead of cooking workshops. In these they shared holiday tales, played games, amused themselves with personality tests, and enjoyed tea and snacks. Cooking workshops and social groups were held once a fortnight for three months. Our team was so wonderful that everyone enjoyed themselves – I even started worrying the social group was having too much fun.

Cook like a nonna

Married to an Italian whose parents cultivated their own produce and pressed home-grown olives into oil, I am partial to a Mediterranean diet. It turns out abundant research strongly supports this traditional diet’s health benefits for chronic disease, and its bountiful nutrients sustain healthy brain function. So this is what we recommended. 

Over years of cooking for a family while studying and working, I had developed a portfolio of simple, quick, tasty Mediterranean-style meals that could be reheated as leftovers or frozen. Popular recipes included chili beans with avocado, eggplant parmigiana, and Greek lentil soup. They sound simple – and they are. One participant summed it up nicely: “I like the fact that you turn up to a cooking class and you’ve got half a dozen ingredients sitting on a bench, and you sort of look at them and go, ‘Oh, that’s not gonna add up to much.’ But then you cook it up and go, ‘Wow, that small number of ingredients cooked up to make such delicious food.’”

The recipes embodied Mediterranean diet principles: abundant plant foods like vegetables, fruit, legumes, nuts, seeds, and olives, as well as generous dollops of extra virgin olive oil, fish, and moderate portions of dairy. Traditional Mediterraneans ate virtually no processed food like refined grains, sweets, or red meat. Because people with mental illness have particularly low omega-3 levels, we also supplemented the diet group with fish oil to boost their omega-3s.

One of the challenges we faced was that human studies are often plagued by drop-outs. Several people pulled out before they even got to baseline assessment — understandable if you suffer depression. Others left once they discovered their group allocation. (Many wanted to do the dietary intervention.) So we ended up testing 95 people after three months, and 85 after six months. Fortunately, clever statistical analyses enabled us to factor in all cases; in other words, to estimate the missing data for those who had left the program.

"One participant went so far as to say she was ‘born again’ by good food.

As suspected, the results showed all mental health parameters improved in both groups. But mood improved significantly more in the Mediterranean diet group: the severity of depressive symptoms was 45 percent lower in the diet group, compared to 26.8 percent in the social group. At the beginning of the study, 80 percent of all participants reported ‘extremely severe’ depression; this dropped by 60 percent overall. Some people made life changes and got new jobs. One participant went so far as to describe herself as “born again,” saying, “It’s really changed me – it’s changed my life.” She now uses her deep-fryer basket to blanch bulk vegetables. 

Our budding chefs rejoiced in shunning low-fat diets and embracing extra virgin olive oil  – and some even lost weight. Many reported a new-found love of legumes. But others found it hard; for instance, when going out, or if partners or children were reluctant to depart from old favorites. Our team reported that encouraging participants to plan ahead helped some of them mitigate these barriers.

How we can be confident these changes in diet played a role in the participants’ recovery? The diet group increased their Mediterranean diet score on a 14-item questionnaire, and reported eating more fruit, vegetables, nuts, legumes, and greater diversity of fruit and vegetables. They also ate less red meat and unhealthy snacks. Better yet, improved diet and the correlated mental health changes were still apparent at six-month follow up assessments. Blood omega-3 levels increased in the treatment group, and were associated with some mental health outcomes, but surprisingly not with reduced depression.

This study was limited, since it wasn’t a double blind trial for obvious reasons (people were clearly assigned to groups that either cooked or just socialized). This could influence participants’ perceptions or reports of their diet and mental health. Future research would also benefit from measuring blood indicators of improved diet, like carotenoid – the pigments responsible for bright red, yellow, and orange hues in plants – as a proxy for increased fruit and vegetable consumption. It would also be useful to measure inflammatory markers, like C-reactive protein, interleukins, or tumor necrosis factor, to investigate physiological changes that could help explain enhanced mood. Longer term follow-ups would show if improved diet and mental health can be sustained.

This research and another recent pioneering study with similar findings tell us the benefits of wholesome diets extend beyond better physical health. My dream is for children to grow up enjoying real food. Imagine the difference it would make if health professionals embraced diet as an essential tool in their clinical kits: doctors could prescribe food vouchers and cooking workshops, or subsidize healthy food for people with chronic health issues, including depression. It may seem strange now, but research is catching up to common sense: eating well makes you feel better.