What Research Tells Us about Diet and Depression

What Research Tells Us about Diet and Depression

Amid a sea of single ingredients marketed as “super,” it’s easy to lose track of a simple truth: Research shows that entire diets comprised of a variety of whole foods—mostly vegetables, fruits, healthy fats, and whole grains—are healthy. And a big part of that picture of health extends to the mind. “Diets that consist of foods we know to be healthy are consistently linked with a reduced risk for depression,” says Felice Jacka, PhD, the director of Deakin University’s Food and Mood Centre in Australia. “We’ve seen this across countries, across cultures, and most importantly, across age groups.”

Jacka’s research career is all about this: trying to understand how individuals’ diets interact with mental health risk and how we can use that information to prevent mental health disorders. The nuances of her research are fascinating.

A Q&A with Felice Jacka, PhD

Why do you study diet and nutrition in relation to depression?

An unhealthy diet is one of the main contributing factors to early death across middle and high-income countries. At the same time, mental disorders—primarily depression—are the leading cause of illness and disability.

Depression is a multifactorial disorder, but many of those factors, such as genetic history and early-life trauma, cannot be easily changed. In order to prevent depression, we need to think about the factors we can modify.

We have shown, and many others have shown, that raising the quality of young people’s diets can reduce their risk of mental health problems. Even more importantly, we have shown that the diets of mothers during pregnancy are very closely linked to both the cognitive and mental health of their children’s first few years of life.

How do you measure the impact of diet on mental health?

We ran the first randomized, controlled trial to assess this. Half of a group suffering from clinical depression received social support, while the other half received dietary support from a clinical dietitian. What we found was that at the end of the three-month study, those who had received dietary support had a far more profound improvement in their depression scores. This study was replicated a few months later in a group-based setting with a larger study size. It showed that those who were taught how to cook a Mediterranean-type diet, prepare food, and shop—compared with a social support group where people went to the cinema and participated in other social activities—had substantially improved their symptoms.

Two important things came out of that trial: First, we performed a cost analysis showing that the diet we were recommending was cheaper than what people had been eating before coming in to the study. Second, we performed a detailed health economic evaluation that showed our approach to be highly cost-effective. This suggests that taking a dietary approach can improve the whole person—not just bits of their brain or their depression, but their overall health and cognitive functioning, as well.

Diet appears to be the most important factor affecting the gut and microbiota, and the gut and the microbiota are really important to the health of the brain as well as the immune system, body weight, metabolism—virtually every aspect of our functioning. You can change the microbiota within days just by changing your diet.

Therefore, diets that consist of foods we know to be healthy (such as fruits and vegetables, whole-grain cereals, fish, lean red meats, nuts, seeds, legumes, and olive oil) are consistently linked with a reduced risk for depression. Whereas people whose diets are higher in junk and processed foods are at higher risk. We’ve seen this across countries, across cultures, and most importantly, across age groups.

Was the change in diet more effective for people who had poor diets to begin with?

Less than 10 percent of the population eats anywhere in line with the dietary guidelines, and less than 5 percent of adults in Australia eat the recommended vegetables and legumes. People’s diets are very poor in general, and this cuts right across social class and education levels. It’s because unhealthy processed foods are the most easily accessible, heavily marketed, and socially acceptable options for eating.

We recruited people who already had poor-quality diets. What we saw in both studies was that the degree of dietary improvement was very closely associated with the degree of improvement in people’s depression. In other words, those who improved their diets received the most benefits. A lot of the people who participated in the study also said that they were able to keep up with the dietary changes and practices. As a result, they continued to experience benefits to both their mental and physical health.

What does the ideal diet look like?

Whole-foods diets in Japan, Norway, Spain, or Australia will look very different from one another, but they are all equally beneficial and protective for mental health. As long as your diet mainly consists of whole foods, you don’t need to be particular about the components of food you are consuming.

The Mediterranean-style diet has lots of different types of plant foods. The more diverse your plant-food intake, the more diverse the bacteria that live in your gut, and that will make a healthy gut.

The gut microbiota ferments dietary fiber, which are the components of plant foods that are not easily broken down. These are the things you get in high-fiber foods such as vegetables and fruits, lentils, legumes, chickpeas, beans, and whole grains. All of these foods have dietary fibers, and dietary fibers are essential because they provide food for the bacteria. When the fiber is broken down, the microbiota produces metabolites, also known as short-chain fatty acids. These short-chain fatty acids affect gene activity, metabolism, and body weight; they also profoundly affect our immune system, which in turn affects our risk for depression. All of these things are dependent on a supply of dietary fiber. In most Western countries, we do not consume anywhere near the amount of dietary fiber that we should.

Another aspect of diet is polyphenols. These are the things that you find in colorful fruits and vegetables, dark chocolate, green tea, and red wine. These polyphenols are very important in the gut, and they may also prevent weight gain.

There are also fats—the polyunsaturated fats you get from fish are beneficial for your gut. The saturated fats you get from processed foods and animal meats seem to promote the growth of less-healthy bacteria.

There are also fermented foods, in the form of yogurts or kefir. I make my own kombucha and fermented vegetables; all of these are very valuable sources of both bacteria and metabolites, which the bacteria produce when fermenting foods.

All of these are essential, not only for the health of our body but also for our brain health.

And of course, at the other end of the spectrum, artificial sweeteners and emulsifiers, very common in junk food, appear to have a negative impact on the gut. New research also suggests that elevated blood sugar may induce leaky gut. This underscores the real importance to avoid foods high in refined carbohydrates, added fats, and sugars. Diets high in these types of foods are consistently linked to a higher risk of depression and reduced brain health.

Is diet complementary to other interventions, like therapy and antidepressants?

Yes. It’s important to understand that most of the people in our study were taking antidepressants, going to therapy, or both. This was designed alongside their treatments, so it’s definitely not an either/or situation.

Antidepressants can be incredibly useful for some. Psychotherapy can be very useful for others. But good nutrition underpins all of it. Nutrition is the petrol for our bodies and brains. Nothing will work properly without good-quality petrol.

Do you find gluten to be an issue for mental health?

It’s an issue of processed foods, and gluten is added to a lot of processed foods. There is little evidence that suggests gluten itself is a problem, but a very small study by colleagues of mine (who are experts in this field) suggested that gluten can promote depressive symptoms for a small number of people who have non-celiac gluten sensitivity. You really can’t hang much on a single study, though, particularly when it’s small.

Research evidence suggests the possibility that some people who perceive themselves to be sensitive to gluten are actually sensitive to components of grains called FODMAPS. (FODMAPS is an acronym for types of short-chain carbohydrates found in certain foods, including grains like wheat.)

Responding to FODMAPS indicates an unhealthy gut microbiome, because FODMAPS are fermentable foods that your bacteria like. The FODMAP diet is designed to cut out FODMAP-containing foods in the short-term; it’s like an exclusion diet, and then you gradually reintroduce foods to see what is triggering symptoms. There’s a lot of research being done on what we can do better to help people reintroduce foods good for their gut microbiota rather than completely avoid them.

Consuming probiotics or fermented foods when reintroducing these foods can help. It’s not about avoiding them long-term. When people exclude gluten from their diet, they often exclude a lot of sources of fiber, and therefore the health of their gut goes down the toilet, so to speak.

Is there any research on how nutraceuticals and probiotics can help with mental health and depression?

There’s been quite a bit of research done, but most of it has not shown compelling evidence, with the exception of fish oil for clinical depression. Nutraceuticals and supplements will never replace a healthy diet, particularly when you understand the role of the gut and the importance of diet to the gut.

There is evidence around very high-dose vitamin and mineral supplements for ADHD, and there are two studies that looked at probiotics. They’re both very recent. In one study of 380 women, those who received probiotics during pregnancy had lower rates of postnatal depression than those who received a placebo.

Another very important study that was just published showed that people with bipolar disorder who consume probiotics maintained longer times in which they stayed well and didn’t go back to the hospital with manic episodes, compared with those who received the placebo.

We’re also about to start a trial looking at fecal microbial transplants for depression. We’re taking poop from a healthy person with no metabolic disease or psychiatric disease, and we’ll be screening them for all sorts of different bugs. We turn it into a pill and we give it to people with depression. Because we know from animal research that if you take poop from a person with depression and give it to a mouse or a rat, it will induce depressive-type behaviors.

That means poop from a depressed person can make an animal depressed, so we think taking healthy stool and giving it to people with depression may be helpful.

What are some tips for improving mental health through nutrition?

The more diverse the plant foods in your diet, the more diverse your gut microbiota will be. That means you’ll have a healthier, stronger, and more robust gut. Also, the types of fats you eat are important. Fish, olive oil, and the fats that you get from nuts and avocados—they’re the ones to prioritize.

If you are having gut symptoms because you’re consuming high-fiber foods when you haven’t had them before, just introduce them slowly, but try to consume probiotics and fermented foods alongside them. It will give you the bacteria and all the things that your gut needs to adapt.

Your gut microbiota is your primary source of adaptation to your environment. It’s also your detoxification engine. New research is suggesting that our gut microbiota deals with things like mercury and persistent organic pollutants in the environment, so the healthier your gut is, the more you’ll be able to deal with all of the environmental toxins that you’re exposed to in daily life.

Really, feeding your gut is the most important thing you can do. If you look after your gut and make sure that the health of your gut is optimized, then that will flow into health benefits for every aspect of your physical, mental, and brain health.

Professor Felice Jacka is the director of Deakin University’s Food & Mood Centre and the founder and president of the International Society for Nutritional Psychiatry Research. Professor Jacka has pioneered a program of research that examines how individuals’ diets and other lifestyle behaviors interact with the risk for mental health problems. This research is being carried out with the ultimate goal of developing new evidence-based prevention and treatment strategies for mental disorders. She has published extensively in journals in the mental health field including the American Journal of Psychiatry, World Psychiatry, BMC Medicine, Schizophrenia Bulletin, and Lancet Psychiatry.

This article is for informational purposes only, even if and to the extent that it features the advice of physicians and medical practitioners. This article is not, nor is it intended to be, a substitute for professional medical advice, diagnosis, or treatment and should never be relied upon for specific medical advice. The views expressed in this article are the views of the expert and do not necessarily represent the views of goop.