It’s often said ‘you are what you eat’.

The links between our diet and our physical health are well established. But what role does your diet play when it comes to your mental health?

You know what it’s like when you treat yourself with something sugary or fatty to pep yourself up when you’re feeling low. Your mood might improve and you might feel a bit energised for a short time, but these feelings pass quickly and you often end up feeling guilty.

But does this mean a poor diet will affect your mental health?

Associate Professor Felice Jacka, principal research fellow at Deakin University, says many studies worldwide have shown that there’s a link between diet and mental health. But this research is still in early stages, so we don’t know unequivocally if unhealthy foods actually cause illnesses such as depression.

What’s not clear is whether your diet affects your mental health or if it’s your mental health that affects your food choices. It’s also possible there’s something else that is associated with eating a healthy diet – such as regular exercise and other lifestyle choices – that is having an impact on people’s mental health, or a combination of these. Or it could be that each of these contributes.

“These are observational studies and all they can tell us is that there’s an association – they can’t prove that it’s a causal association,” explains Jacka.

“To do that you need to do experiments on humans where you manipulate their diets and show that there’s a direct impact on mental health, which is what we’re currently in the process of doing at Deakin [University].”

The science so far

Jacka says the available evidence shows people who eat an overall healthy diet – that is one including lots of fresh, unprocessed and nutrient-dense foods – tend to have better mental health. In particular, research suggests depression and dementia are affected by the quality of our diets across the life span.

She points to research from Japan that found a healthy diet rich in vegetables, fruits, potatoes, soy products, mushrooms, seaweed and fish was associated with a decreased risk of suicide, while a study on men found a high-fat, junk-food diet can within a week impact spatial memory, which is associated with dementia.

“The studies have shown across time that diet is related to the risk for these disorders and it’s certainly associated to prevalence of them,” she says.

Jacka’s own research has found links between diet quality and mental health in adolescents, and in middle-aged women.

Most recently she looked at pregnant women and found those who ate processed foods during pregnancy had children who were more likely to be angry, throw tantrums and have other behavioural problems, which she says are markers of mental health issues.

“We also showed that kids who had higher intakes of healthy, nutrient-dense foods had lower levels of these behaviours,” she explains.

“We’ve seen this in adults, older adults and right across countries.”

Food on the brain

What’s less clear is whether specific nutrients may play a role in your mental health; although there’s evidence to show the following may play a role.

Omega-3 fatty acids: Your brain and central nervous system are made up of about 60 per cent fat, in particular the omega-3s, which are found in oily fish such as salmon, mackerel, herring and sardines. These play a crucial role in healthy brain function and research has shown people with depression have lower levels of some omega-3s in certain parts of their brain.

Omega-3s have been found to help reduce inflammation in the body and brain, which is caused by a poor diet, stress, smoking, lack of vitamin D, obesity and physical inactivity. In some people, inflammation can be a contributing factor to an increased risk of mental illness, explains Professor Michael Berk, Alfred Deakin Chair of Psychiatry at Deakin University.

“Inflammation seems to be a very important pathway, whereby signalling in the brain and regulation of mood is mediated,” he explains.

“We know that a high-fat and high-sugar diet will increase systemic inflammation throughout our body and brain. That’s associated with changes in the brain, such as reductions in the chemicals that make brains cells grow and thrive.”

Carbohydrates: Dr Alan Barclay, dietitian and chief scientific officer at the Glycemic Index Foundation, says the brain needs to be fuelled by high-quality carbohydrates – fruits, vegetables and wholegrains – which are broken down into glucose.

“Our brains depend on glucose and there’s good reason to believe having a steady flow of it to the brain is important for optimal mental functioning,” he explains.

“If you spike your blood glucose, as highly refined carbs do, your body has to overproduce insulin and you’ll come crashing down.”

Nothing to single out

But Berk stresses we shouldn’t single out specific nutrients and other components of food as cure-alls for illness, because there are many others we need just as much in order to be healthy.

“Vast amounts of one are never going to compensate for major deficiencies or gross excess in others,” he warns.

“The most important way to get everything you need is to eat a healthy diet with enough of the good stuff and minimal amounts of the bad.”

He also points out there are many risk factors that cause mental illness, including many we have no control over, such as genetics, childhood trauma and socio-economic standing. However, you can control what you choose to eat and for some people this can make a difference.

Four food rules

Your best bet if you want to maintain your blood-sugar levels, and have the best shot at feeling good mentally and physically, is to eat a good balance of healthy grains, fats, fruit and vegies every day.

“I have four rules for food that I tell my patients,” Berk says.

1. “If it comes from a farm it’s probably good for you; if it comes from a factory it probably isn’t.
2. Does it rot? If it doesn’t, it’s unlikely to be good for you.
3. Does it have a brand? If so, it’s also unlikely to be good for you.
4. Would your great-grandmother have recognised it as food?”

Professor Felice Jacka is from Deakin University’s School of Medicine; Professor Michael Berk is the Alfred Deakin Chair of Psychiatry at Deakin University; Dr Alan Barclay is chief scientific officer at the Glycemic Index Foundation and a spokesperson for the Dietitians Association of Australia. They spoke to Cassie White.

This article first appeared on ‘ABC’ on 2 October 2014.