We all know by now that the quality of our diet has a huge effect on our physical health. But Australia's obesity epidemic and physical health decline isn't the only thing we need to be worried about.
We also need to consider our mental health.
In Australia, 45 percent of people will experience a mental health condition in their lifetime. Anxiety is the most common mental health condition which one in four Australians will experience. For depression, on average, one in six people will experience depression at some stage of their lives.
But is there a link? With many Australians' diet being so poor, does this impact our mental health, as well as our physical health?
To find out, The Huffington Post Australia spoke to two health experts: Felice Jacka, associate professor, psychiatric epidemiologist and director of the Food and Mood Centre at Deakin University, the first of its kind in the world. And Chloe McLeod, an accredited practising dietitian and sports dietitian.
"There's a very strong link between quality of diet and the risk of common mental disorders like depression and anxiety," Jacka told HuffPost Australia.
"Depression is very common and extremely disabling. The way diets across the globe have changed over the last 20 or 30 years is profound and very much for the worse. That has had a major impact on the health of the world.
"This evidence base is relatively new. We've conducted a huge amount of research and established a link between diet quality and depression and anxiety in children, adolescents, young adults, adults and the elderly."
There's such potential for a completely new way of preventing and treating mental illness.
The reason why diet has an effect on mental health is due to the way certain dietary patterns influence parts of the brain.
"Diet has a potent impact on brain plasticity," Jacka explained. "There's an area in your brain called the hippocampus and that's central to learning and memory. It's also very important for mental health. In animal studies, we see that manipulating diet has an impact on that part of the brain.
"But we published the first research last year showing this seems to be true in humans, as well.
"We looked at older adults in the ACT and we looked at the size of their hippocampus at two time points and then we looked at the quality of their diets. We took into account a whole range of things that could impact the hippocampus, such as depression and socioeconomic status."
The research found very clear links between the quality of people's diet and the size of their hippocampus.
"So, people's whose diet quality were better had bigger hippocampi, and those whose diets were worse had smaller hippocampi," Jacka said.
It's also our gut microbiota (which used to be called gut flora) which plays a large role in the link between diet and mental health.
"But now we're very much focused on the gut microbiota because all the emerging research in medical science now tells us that the gut microbiota is central to physical health, but we also think to brain health and behaviour," Jacka said.
"And of course, diet is the leading factor that influences the gut microbiota, particularly fibre. So we're running a number of studies looking at the gut microbiome, diet and mental health, and other aspects of physical health. We're also running an intervention trying to target that.
"There's such potential for a completely new way of preventing and treating mental illness."
Dietitian McLeod also believes the nutrients we get -- or don't get -- from food is also a key factor in the link between diet and mental health.
"If you look at other systems in our body, there's certain nutrients that we need for the different functions in our bodies to work effectively," McLeod told HuffPost Australia.
One function many people are familiar with is the way we consume carbohydrates -- it gets digested, absorbed, stored in our muscles and then when we're exercising, it gets used up.
"There's all sorts of other biochemical pathways that are going on at the same time, as well, so if we're not making healthy food choices and eating enough of these other nutrients that our bodies need to function properly, it makes logical sense that if those things aren't present, then it's going to make it harder for us to be mentally healthy as well as physically healthy," McLeod said.
So, what kinds of food are associated with poorer mental health?
"We think that there's not a huge amount to be gained by looking at individual foods because you never eat individual foods in isolation. It's a bit of a red herring," Jacka said.
Many aspects of diets are important, and it all comes down to that very simple adage: Eat food. Not too much. Mostly plants.
"We look at dietary patterns. You see that people tend to have certain patterns and habits when they eat. By looking at those dietary patterns we can see links between mental health."
There are two main things Jacka and her team see very clearly.
"They're quite independent of one another, but of course they are related. One is that if people have higher intakes of junk and processed foods (the foods we know are higher in fat, sugar, carbohydrates, emulsifiers, artificial flavours, etc.) seem to be more likely to have depression, in particular," Jacka said.
"But people with low levels of healthful foods (vegetables, fruits, legumes, fish, lean red meat -- the foods we know sustain health) are also at an increased risk.
"These are independent in that, even if you're eating good food, if you're also eating a lot of junk food, it can be a problem. The opposite is also true: if you don't eat a lot of junk food, but at the same time you're also not eating enough fruits and vegetables, that seems to be a problem."
Jacka also highlights the link between diet during pregnancy and the effect this has on the child's future mental health.
"The fact that unhealthy diet could increase the risk of mental disorders -- and this right across the age range -- pushed us to lead the first study in pregnant women to show what mothers eat during pregnancy is related to their children's mental health," Jacka said.
"We did that in 23,000 Norwegian mothers and their children, and that's now been supported by other studies which have shown the same thing."
A healthy diet, however, doesn't have to be complicated. In fact, we need to keep it simple.
"Many aspects of diets are important, and it all comes down to that very simple adage: 'Eat food. Not too much. Mostly plants'. It doesn't have to be complicated, it's really that simple," Jacka said.
The Mediterranean diet is beneficial in promoting mental health, however it's not the only diet that helps.
"We've published a paper last year and we looked at all the literature and research that's been done now, and what we see is that traditional dietary patterns -- whether they be traditional Mediterranean diet, traditional Norwegian, traditional Japanese and so on -- tend to be associated with better mental health," Jacka said.
"The Mediterranean diet is great because it's very high in plant foods and fibre, legumes, nuts, healthful fats such as olive oil and fish, and that does appear to be associated with many health benefits, including mental health.
"But that's not to say that other forms of healthy, traditional diets are not just as healthful and beneficial."
It's because of this emerging evidence of the strong link between diet and mental health that Jacka urges everyone -- particularly governing bodies -- to acknowledge this and make changes accordingly.
We need very powerful action at the government level to change the food environment to make healthy food the easy option, the most socially acceptable option, and the cheapest option.
"We now know that unhealthy or poor diet is the leading cause of early death across the globe," Jacka said.
"The understanding that this is also a problem for mental health, and not just physical health, has huge implications for public health and the health of the next generation, because mental disorders are extremely disabling, incredibly expensive and we haven't taken that into account when we're looking at the costs associated with unhealthy diets."
Direct action should be starting now. Not in the next decade. Now.
"Direct action should really be starting now. You can see that the costs associated with mental disorders in Australia, as well as the cost of the physical problems that are a direct result of unhealthy diet, we should be taking really urgent action," Jacka said.
"We need very powerful action at the government level to change the food environment to make healthy food the easy option, the most socially acceptable option, and the cheapest option."
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