Only recently have scientists begun to explore the relationship between nutrition and mental health. While the science is relatively new and much of it limited to observational studies that do not prove cause and effect, so far the findings are consistent and compelling: What you eat – and don’t eat – can have a powerful impact on mental health.

According to the Canadian Mental Health Association, one in five Canadians will experience a mental-health condition such as depression, anxiety and bipolar disorder in his or her lifetime. Among other factors that contribute to mental illness, our changing diet is thought to play a role. In a 2014 paper published in the Journal of Physiological Anthropology, Australian scientists said the transition away from the whole-foods diet our grandparents ate – one based on nutrient-rich vegetables, fruits and whole grains – to a steady fare of nutrient-poor, high-calorie and highly processed foods has been associated with increases in depression and other mental disorders.

The changing nutrient content of even our healthy foods may also play a role. A 2009 analysis conducted by the Biochemical Institute at the University of Texas at Austin highlighted evidence that during the past 50 to 70 years the mineral content of fruits and vegetables – many of which are thought to play a role in brain health – has been declining, possibly because of modern agricultural practices.

Recent studies have connected a “healthy” dietary pattern to a lower risk of depression, anxiety, bipolar disorder and suicide in adults. For example, a 2014 review of 21 studies from the University of Newcastle in Australia concluded that a high intake of fruits, vegetables, fish and whole grains protected against depression. A 2013 study published in the British Journal of Psychiatry found that, among nearly 90,000 Japanese men and women, a diet characterized by more vegetables, fruit, potatoes, soy, seaweed and fish correlated with a lower risk of suicide.

Researchers have turned up similar findings in children and teenagers. Last year, a review of seven studies by researchers at the Innovation in Mental and Physical Health and Clinical Treatment Strategic Research Centre at the School of Medicine at Deakin University in Australia revealed a consistent trend between a higher intake of nutrient-dense foods, including vegetables, salads, fruits and fish, and lower rates of depression, low mood, emotional problems and anxiety. The analysis also found a consistent and positive relationship between an “unhealthy” dietary pattern – a higher intake of saturated fat, sugar, refined starches and processed foods – and poorer mental health in youth.

The link between nutrition and mental health may even start in the womb. Research suggests that an unhealthy diet during pregnancy and early childhood, characterized by processed foods, refined cereals, sugary drinks, high-calorie snacks and desserts, increases the risk of attention problems, aggressive behaviour and anxious and depressive symptoms in children.

The growing body of evidence connecting better diet quality with better mental health is exciting and promising. However, decisive randomized controlled trials – the gold standard of scientific evidence – are needed to determine if improving your diet will improve your mental health. One such trial is currently under way in Australia to determine if a Mediterranean-like diet can ease depression.

Even so, the current evidence linking nutrition to mental health is so convincing that, in a paper published in The Lancet this year, a panel of international experts suggested that diet is “as important to psychiatry as it is to cardiology, endocrinology and gastroenterology.”

It’s time to get back to basics and eat like our grandparents did.

How diet may affect the brain

Diet is thought to have a direct impact on many biological pathways that underpin depression and other mental-health disorders.

The anti-inflammatory properties of nutrients in fruits, vegetables, whole grains, nuts and oily fish may influence concentrations of brain chemicals that regulate emotions and cognition.

In a study published this year in JAMA Psychiatry, researchers from the Centre for Addiction and Mental Health in Toronto found that a measure of brain inflammation in people experiencing clinical depression was increased by 30 per cent.

Omega-3 fatty acids found in fish oil are essential for the integrity of brain cell membranes. An imbalance of these fats may alter how brain cells communicate with one another.

Antioxidants, including vitamins C and E, beta-carotene and many natural plant compounds, are thought to reduce free radical damage, or oxidative stress, to brain cells that may influence mental health. A 2004 study from Japan correlated higher levels of free radical damage with depressive symptoms in women. And in 2009, researchers from the University of British Columbia found higher levels of oxidative stress in patients with bipolar disorder and schizophrenia.

The B vitamin folate is needed for the production of serotonin, a brain chemical (neurotransmitter) responsible for maintaining mood balance. Folate, along with vitamins B12 and B6, is also thought to protect brain function by reducing levels of an amino acid called homocysteine. (A high homocysteine level is believed to cause a deficiency of certain brain chemicals, which may contribute to depression.)

A diet based on nutrient-packed whole foods also increases the level of brain-derived neurotrophic factor (BDNF), a protein that improves mood, attention and learning, promotes brain cell growth and lowers brain inflammation. In contrast, a diet high in animal (saturated) fat has been shown to lower BDNF.

Certain foods that feed our “good” gut bacteria, known as prebiotics, may also be linked to better mental health since gut microbes synthesize most of the body’s serotonin. Prebiotic foods include whole grains, artichokes, asparagus, bananas, garlic, onions, chicory root and yogurt.

This article first appeared in The Globe and Mail, 27 May 2015.