Feeling depressed? Food could be the cause
Modern diets may be one cause of mental disorders and depression, as well as neurodegenerative disease. Here’s how you can lessen your risk.
Evidence is now incontrovertible that a diet high in ultra-processed food is devastating for human metabolic health. But the effect of the inflammation and insulin resistance in humans that highly processed foods tend to trigger across years of high consumption is not isolated to the body’s digestive systems.
It’s well established in scientific literature that there is a link between diet quality, brain health and susceptibility to suffer mental illness and neurodegenerative disease. So why have we been so slow in adopting what seems to be an inexpensive and highly accessible intervention for a widely suffered condition such as depression when numerous studies have shown reversal of even significant depression through dietary changes and even better success when combined with exercise?
Why are we not telling people as they age that reducing inflammation in the brain through diet could have a large impact on susceptibility to dementia?
It no doubt boils down partly to a lack of awareness across the community of the emerging field known as nutritional psychiatry, which explores linkages between overall diet quality and common disorders such as depression and anxiety. And part of the blame for the lack of communication on the link between diet and brain health to consumers lies at the feet of general practitioners.
GPs at the heart of the issue
Most Australians see a GP at least once a year. Many of these patients are metabolically unhealthy with markers of inflammation growing by the year if they only knew it.
There are a growing number of GPs who are interested in lifestyle medicine and increasingly are practising it. But far too many GPs fail to read scientific literature even when a willing and highly educated patient presents it to them, and GPs carry a large part of the blame for doling out antidepressant medication at what is a highly concerning rate.
GPs will argue that they spend enormous amounts of time on mental health matters with patients, and of course they do, but discussions of the links between diet, inflammation and brain health, and diet prescriptions accordingly, are rare even though the literature is clear that such changes can reverse depression in some cases.
Lifestyle interventions for mental illness are not a novel idea. The Royal Australian and New Zealand College of Psychiatry clinical guidelines for the treatment of mood disorders, for the first time anywhere, now positions “foundational” medicine – involving a healthy diet, movement and sleep patterns – centrally.
And this year the World Psychiatric Association and the American Psychiatric Association chose to focus on lifestyle psychiatry as their key themes.
The reason is clear: although mental illness accounts for the second highest burden of disease in many countries including Australia, pharmacotherapy is ineffective for many and can come with enormous harm, while exercise and other lifestyle therapies have high efficacy and little downside if done safely. Drug therapies, including antidepressants and antipsychotics, and psychotherapy and cognitive behavioural therapy have been said to avert less than half the burden of disease.
Now there is consistent observational and interventional data to suggest diet may be a modifiable risk factor for mental illness.
There is increasing understanding among scientists and researchers as to the neurobiological mechanisms that are likely to be modulated by diet.
These mechanisms include inflammation (in substantial part triggered by the consumption of highly processed food, refined carbohydrates and unhealthy fats), oxidative stress (or the release of harmful “free radical” molecules that cause inflammation), the gut microbiome, epigenetic modifications and neuroplasticity.
“What we do know from a very, very large body of evidence around the world is that people who have higher diet quality, higher in whole foods and lower in ultra-processed and unhealthful foods, have about a 30 to 35 per cent reduction in risk for having depression,” Deakin University nutritional psychiatry distinguished professor Felice Jacka says. “And we see this prospectively; it doesn’t seem to be explained by reverse causality such as people eating a particular way because of their mental illness.”
Jacka, who is the founder and director of Deakin’s Food & Mood Centre, says: “We have published full clinical trials showing that if you take people with depression, even moderate to severe clinical depression, and you help support them to improve their diet, that will have a very profound benefit to their depressive symptoms.”
While studies decades ago centred on single-factor dietary effects such as whether fish oil supplements were associated with a reduction in symptoms of depression, to understand the links between nutrition and brain health researchers during the past 15 years or so have looked at the whole diet and its interaction with numerous underlying mechanisms in which the body responds to certain foods.
As described in a 2020 paper in Nature Molecular Biology, “the mechanisms of action associating diet with health outcomes are complex, multifaceted, interacting and not restricted to any one biological pathway”.
Epigenetics, the health of the gut microbiota, and the profound effect of obesity if it exists can all come into play. But primarily it is the role of inflammation and oxidative stress inside the brain that has attracted the most scrutiny in nutritional psychiatry.
A subfield known as metabolic psychiatry, aimed at reducing these toxic processes in the brain through a ketogenic diet, also has sprung up more recently.
Why do some foods cause inflammation in our bodies?
Of greatest concern in Western countries where the food industry is powerful and mass produces industrial foods that are of low nutrition, high sugar, low fibre and high in refined carbohydrates is that the consumption of these ultra-processed foods are horrendous not only for people’s metabolic systems but also brain health.
Consumption of too many of these types of foods leads to persistently high blood glucose and insulin levels in the body that eventually results in insulin resistance. Insulin resistance can affect not only the pancreas but also the brain, resulting in a brain flooded with glucose but with not enough insulin, so it struggles to generate enough power but is left coping with an excess of glucose that can’t be converted into energy.
This high blood glucose floods the brain with inflammation and oxidative stress, damaging its mitochondria (the cells responsible for turning glucose into energy), as American psychiatrist and author Georgia Ede says.
“The glucose will stick to important components of cells inside the brain – proteins and lipids and nucleic acids like DNA,” Ede says.
“This stickiness will cause these molecules to become crippled and dysfunctional. There will be certain advanced glycation end products generated. Advanced glycation end products are largely responsible for premature ageing of tissues throughout the body, including the skin and the brain and so forth. So too much sugar will lead to a lot of problems with cellular function within the brain. The brain mounts an immune response to clear away these sticky clusters. And that’s where you get the inflammation and oxidative stress. The brain will deliberately release inflammatory cytokines and oxygen free radicals as the first response to other first responders in the immune response to sound the alarm – we’ve got a problem here, we need to clear away these problematic clusters.”
The effect is also replicated in the gut and thus amplified in the brain. Highly refined foods with sugars provide easy nutrition for bacteria in our gut, leading to unchecked bacterial overgrowth. This triggers the release of inflammatory chemicals that are absorbed into our bloodstream. When we consume harmful fats, especially trans fats, the slow breakdown of these substances also releases chemicals that promote inflammation. Refining grains – that is, removing the fibre from whole grains – also promotes bacterial overgrowth.
“So we have this chronic inflammatory and excessive oxidative stress situation inside the brain,” says Harvard-trained Ede, author of the book Change Your Diet, Change Your Brain.
“If it persists or occurs too often it can lead to a whole cascade of very damaging effects inside the brain including neurotransmitter imbalances, but especially something called glutamate excitotoxicity, where a level of a particular neurotransmitter in the brain called glutamate can rise to a level 100 times its baseline.
“We see high glutamate levels in certain psychiatric conditions, the best example being bipolar disorder. When people have high glutamate levels, the brain is on high alertness. It’s really in overdrive, it’s a sort of hyper-excitable state, where the brain is too active and too reactive. So you can see lots of anxiety, insomnia and irritability.
“And you see this in epilepsy as well; there are a lot of similarities. So the brain is simply overstimulated by this being in this emergency state of mind too often.”
As well as affecting the functioning of key neurotransmitters, a pro-inflammatory diet that triggers oxidative stress in the brain affects functioning of nerve pathways. Inflammation can trigger the hypothalamus to instruct the adrenal glands to release the stress hormone cortisol. Having high levels of stress hormones affects our mental state.
Other deep areas of our brain including the hippocampus constantly develop new nerve pathways or alter existing ones, a process known as neuroplasticity. Chemical messengers (brain-derived neurotrophic factor) control these deep brain responses. There is good evidence that a healthy diet is responsible for normal nerve growth in our brains and an unhealthy pro-inflammatory diet reduces the amount of BDNF in our nervous system. This derails the healthy function of our brain in response to new experiences.
The nutrient tryptophan also is involved in nerve function and is absorbed from the colon. There is good evidence that absorption from the foods we eat – chicken and milk, for example – is affected by bacterial growth in the gut. When we have abnormal gut function from an unhealthy diet it affects tryptophan absorption into our bodies and this has a negative effect on nerve functions.
All of these changes in the body in response to what an individual eats cause epigenetic changes that overlay an individual’s DNA and change the body’s protein function. There is strong evidence that diet early in life, including a mother’s diet while a child is in utero, affects the way our DNA functions. Poor diet and lack of exercise – sources of inflammation and oxidative stress – can change a child’s DNA for life and promote poorer brain and nerve function, and mental health issues such as depression across the lifespan.
All of the processes described above are also risk factors for the development of neurodegenerative diseases including Alzheimer’s and Parkinson’s. The rise of a highly processed industrial diet and the brain inflammation it triggers may be a factor supercharging the prevalence of these conditions alongside the ageing population.
‘The elephant in the room’
While further study into the effects of diet on the brain and application of the principles of nutritional psychiatry may be welcome for patients, Jacka points out that it will not deal with what she calls “the elephant in the room”.
“The elephant in the room is our industrialised food system that costs the globe upwards of $20 trillion a year in its negative effects on human health,” Jacka says.
“And it also costs us trillions in loss of biodiversity and costs to the environment. That loss of biodiversity in the environment, in the soil, in our food, directly relates to the loss of biodiversity in humans inside our microbiomes.
“And we’re seeing this massive increase in disorders that are related to the loss of microbiome health – things such as food allergies, multiple sclerosis, rheumatoid arthritis, type one diabetes, cancers in young people and, of course, things like ADHD, autism spectrum disorder and auto-immune diseases.
“As people who are working in this field, we strongly suspect the rise of these conditions relates to the impact of industrialised food systems on the microbiome, through encodings in the food chain, through loss of microbes in the soil and in our food, loss of nutrients, loss of phytochemicals.
“The fact that we have no decent policies to improve our industrial food environment because the power of lobbyists is so huge is disgraceful. You’re talking about the biggest industries in the world.
“In America 70 per cent of food in the food chain is ultra-processed. The power of those lobbyists to influence or stop any form of food policy reform is immense.
“And if you think about the cost to human health, to productivity, to wellbeing, for the environment, in Australia, consider if there is one single thing in place to prevent that or to improve that. It’s sort of such a contrast with our very tough stance on other areas, like tobacco control.
“It just doesn’t make sense.”
