What we eat can have a material impact on how we feel and perform. Many of you, I’m sure, have experienced this. The lethargy and negativity after eating fast food or cake. The energy and positivity after a meal of fresh, nutritious produce. Food affects mood. We know that. We’ve felt it. But does diet contribute to serious mood disorders, such as depression?

The evidence certainly suggests that it does. Eating a lot of processed food (as we do in the UK) increases the risk of depression. Whereas eating a diet of fresh, whole foods can prevent, as well as treat, depression, by providing sufficient nutrients, improving gut health, and keeping inflammation and oxidative stress at bay.

In a study of middle-aged adults working in the UK civil service, a diet rich in processed foods (including sweetened desserts, chocolates, fried food, processed meat, pies, refined grains, high-fat dairy products and condiments) was associated with an increased risk of developing depression 5 years later. In a similar study in Spain, higher consumption of fast food (including hamburgers, sausages, and pizza) and higher consumption of commercial baked goods (including muffins, doughnuts, and croissants) were both independently associated with an increased risk of becoming depressed 6 years later.

Fortunately, a healthy diet appears to have the opposite effect. The UK study also found that a diet rich in whole foods (including fish, vegetables, and fruit) was associated with a reduced risk of developing depression 5 years later. And a similar association was found in a US population adhering to a Mediterranean-style diet.

These findings are interesting, because they suggest that our dietary choices now may influence our future risk of depression. But they don’t tell us much about causation. For that we need randomised controlled trials. And it just so happens there are some. Quite a few actually. And they show that eating more fresh, nutrient-dense foods, and less processed junk food, can reduce depressive symptoms in people with and without clinical depression. In other words, a healthy diet can both prevent and treat depression.

Let’s dig into some of the more recent trials a bit further.

The SMILES trial from 2017 (acronym-derived names seem to be de rigueur for clinical trials these days!) showed that personalised dietary advice – based on a Mediterranean-style diet – for 3 months improved depression in people with major depressive disorder. It even led to remission in about one third of people (compared to only 8% in the control group).

Two years later, the HELFIMED trial (there it is again!) showed that a Mediterranean-style diet for 3 months, plus fish oil supplementation for 6 months, reduced depression and improved quality of life in people with self-reported depression. In this one, they also provided fortnightly food hampers and cooking workshops.

In the same year as HELFIMED, another trial showed that depression symptoms in people without clinical depression can be reduced by adopting a healthy, Mediterranean-style diet for just 3 weeks. They checked in on the participants 3 months later, and the benefits had been maintained.

How can diet have such a profound effect on mental health? There are probably several mechanisms at work here. Firstly, whole foods – such as meat, fish, and vegetables – have far higher concentrations of essential nutrients than do processed foods. Essential nutrients, including vitamins and minerals, are crucial to all aspects of health, including mood. Secondly, it’s well established that whole food diets improve gut health, and moderate inflammation and oxidative stress, all of which appear to contribute to better mental health.

Depression is a complex condition with many contributing causal factors, of which diet is only one. But it’s one that we can take immediate control of if we want to.