‘What we stick in our mouths matters to our mental health,’ says a leading light in this new field. So what should we be eating?

Felice Jacka’s work showing that junk food shrinks the brain was motivated by personal experience. Growing up in Melbourne, Australia, Jacka struggled with anxiety and panic disorders; by the time she enrolled at art school, she was accustomed to regular bouts of depression, too, leaving her “devoid of happy feelings and unable to experience pleasure”.

But in her late 20s Jacka managed to recover and stay well by focusing on her diet, exercise and sleep. The effect was so marked that it inspired her to put her life as an artist on hold in order to dedicate herself to studying the effects of diet on mental health.

She is now head of the Food and Mood Centre at Deakin University in Australia, and president of the International Society for Nutritional Psychiatry – a relatively new field of research, applying a rare scientific rigour to the link between diet and mental health. For her PhD study in 2010, Jacka found that women whose diets were higher in vegetables, fruit, fish and wholegrains, with moderate amounts of red meat, were less likely to have depression or anxiety disorders than those who consumed a typical western diet of processed foods, pizza, chips, burgers, white bread and sweet drinks.

Her study made the cover of the American Journal of Psychiatry; shortly afterwards, studies in Spain and the UK identified similar trends. Today Jacka is at the forefront of nutritional psychiatry, studying large samples of populations for indications of the impact of entire diets (not individual ingredients) on mental health. Correlations cannot prove causality outright, but by replicating results repeatedly, risk factors can be identified and studied further. Her new book, Brain Changer, is a straight-talking, evidence-based antidote (complete with recipe ideas for good mental health) to the bloggers and self-styled dietary experts who, she says, have “brought nutrition research into disrepute”.

“When I first started, people were terribly sceptical – they thought it was just rubbish,” says Jacka. “In psychiatry, people are trained to think about particular molecules in the brain that can be targeted by certain drugs and they’ve lost sight of the bigger picture – the body as a whole complex system.”

More than 150 peer-reviewed scientific papers later, Jacka has amassed evidence from all over the world showing that “what we stick in our mouths matters to our mental health”.

For her so-called Smiles trial, published in 2017, Jacka recruited 67 moderate to severely depressed people with unhealthy diets. Half received seven sessions with a clinical dietician while the others received “social support”, involving friendly conversation. After 12 weeks, one-third of those who had received nutritional support were in remission, compared with 8% of those who had had the social support. Scientifically speaking, says Jacka, “it was a pretty big difference”.

She is keen to snuff out fanciful ideas about quick fixes, and food trends dressed up as panaceas – the success resulted not from “clean eating” or coconut oil, but from following standard advice for a healthy, balanced diet. As an added bonus, the participants spent a little less on food than they did on their original diets – and probably ended up with bigger brains, too. In a 2015 study of 250 older Australians, Jacka found that the less healthy their diets, the smaller their left hippocampuses (the brain region linked with emotional regulation and mental health); the finding was more recently replicated in the Netherlands with 4,000 older adults.

Overall, Jacka has found that simply following a healthier diet – without other lifestyle modifications such as exercise, but taking into account things such as education, income, bodyweight and other health behaviours – results in a 30% reduced risk of depression.

That healthier diet may vary from country to country, but research has shown that, regardless of where you live, eating closer to a traditional, pre-industrial diet rich in plant foods, fish, unrefined grains and fermented foods, with less meat and highly palatable processed and snack foods, reduces your risk of depression. It could be the Mediterranean diet or Japanese cuisine rich in fish, seaweed, green tea and tofu, Jacka writes: “There’s not just one healthy way to eat.”

One unexpected finding of her PhD study, for example, was that cutting out red meat led to poorer mental health among the 1,000 participants. “We saw in our data a very clear pattern around too little or too much being problematic,” she says. “A tiny amount – three or four palm-sized servings [65-100g] a week – was associated with about half the probability of having a depressive or anxiety disorder. I suspect there’s probably individual variation in how much people need.” The need is probably greater in young women who are menstruating, adds Jacka (who does not eat red meat “for ethical reasons”).

Grain-fed beef (common in the US) is also less healthy than grass-fed beef, which is higher in the fatty acids that have been linked to improved mental health.

As for fish, eating it about three times a week is a component of many healthy diets – but there are unlikely to be extra benefits from eating more, Jacka writes. Fish-oil supplements can be helpful for some people with severe clinical depression, “but it’s definitely not a panacea for the wider population,” she says. “Have sardines on your toast, or some mackerel, mussels, or – if you can afford it – oysters.”

Jacka is the first to admit that we may never understand how individual ingredients of our diet combine to influence the brain: “The complexity of the human body is mind-boggling.” And because even the most beige western diet comprises countless individual chemicals invisibly interacting with each other, “we can’t even begin to measure all their effects”.

Cottage cheese, for example, is often trotted out as a mood-boosting food because it is rich in tryptophan, which is essential for creating serotonin, the “happy hormone”. If only nutrition were that simple. Scientists have failed to find any evidence that eating foods (or supplements) rich in tryptophan affects mental health, with other amino acids in foods restricting its journey from stomach to brain.

It is the trillions of micro-organisms such as bacteria and yeasts that live in our guts that help convert tryptophan into serotonin, and they can be encouraged by consuming fermented foods such as sauerkraut, kimchi and kefir, and fibre from many different plant sources. But, crucially, it is not enough just to eat well: you need to consume all the nutrients and fibre your body needs – and avoid significant consumption of refined and heavily processed foods. “People are eating all this stuff that’s toxic and detrimental to their brain health,” says Jacka. One reason to avoid high-sugar diets, for example, is that they lead to an increase in the same inflammation markers that are raised in people with depression.

The gut microbiome is also key to regulating inflammation. Recent animal studies have shown that depression can be transmitted through fecal microbial transplants (“It’s poo in a pill, or ‘crapsules’”); Jacka is currently investigating whether good mental health can be transmitted the same way.

However it might be achieved, she is convinced of the need to address what she sees as the current mental health “disaster”. Unlike most risk factors for depression (including genes, poverty, trauma and abuse), diet is something we can modify – yet only about 10% of the population eat an adequately healthy diet, says Jacka. “The fact that we’ve got something under our nose that could potentially address a good proportion of the burden of depression is really important.”

The elephant in the room is the global food industry. “Big Food has completely altered the food environment so that unhealthy foods are the cheapest, most ubiquitous, heavily marketed, [most] difficult to resist and socially acceptable – as a result, the [world’s] health has gone down the toilet.”

The lack of political will to address this reflects the size of the corporations involved. “It’s just so powerful and influential, bigger than the tobacco industry,” she says.

Is Jacka gearing up to be the Erin Brockovich figure who takes them on? “That would be my life’s dream.”

With half of all mental-health disorders setting in by age 14, the importance of diet is especially relevant to young people – but as in the UK and US, Australian teenagers are eating multiple servings of junk foods every day, says Jacka: “This is not an occasional biccie with your tea.”

Messages about obesity and health problems in a distant, abstract future do not seem to be influencing people’s eating behaviours, but they might act on the knowledge that these same foods could be making them unhappy, says Jacka. “It’s much more in your face: ‘It’s going to affect me now.’”

Brain Changer: The Good Mental Health Diet by Professor Felice Jacka is published by Pan Macmillan, Australia