MENTAL HEALTH

Mental disorders, particular anxiety and depression, are serious problems for individuals, families and societies. Having had my share of mental health problems, I sympathise with anyone who has been in the same boat. Unfortunately, research suggests they are increasing, particularly in young people (1). It is important to know that depression isn’t just a brain disorder, it’s a whole-of-body disorder with a dysfunctional immune system (officially called chronic, low grade systemic inflammation). This arises when a predisposition acts in concert with many common stresses such as excessive working hours, lack of exercise, poor sleep and, of course, a poor diet. Life events like birth, moving house and divorce are common precipitators.

I think we are all familiar with the link between food and mood. My colleague Dr Philippa Wall drew my attention to the link between carbohydrates and mood in the 1990s (2). The idea was that carbohydrates made us feel calm and happy because they facilitated more serotonin crossing the blood-brain barrier. Serotonin of course, is that feel good neurotransmitter or chemical messenger, that plays a crucial role in regulating not just mood, but sleep and appetite. We know that imbalances in serotonin levels in the brain lead to serious states of anxiety, insomnia and eventually depression.

Drugs that are used in the treatment of depression often target pathways in the brain that increase serotonin levels in the brain (3). But wouldn’t it make sense if there were components in foods that did the same thing? We know that serotonin is synthesised from the amino acid tryptophan which is found in large amounts in milk (4). But crossing the blood-brain barrier is no easy feat, so adding additional carbohydrates to milk, such as honey, is thought to facilitate the process. Because serotonin is also converted to melatonin, the hormone that regulates the circadian rhythm, products like malted milk powder and Horlicks™ are added to milk for a night cap (see the May 2025 edition of GI News for more on this topic).

The more important question is whether our everyday diet can really affect our mental health? According to Professor Felice Jacka, a Distinguished Professor of Nutritional Psychiatry, and founder of the Food and Mood Centre at Deakin University, the answer is a resounding yes (5). I know that the very idea of diet affecting mental health was met with deep scepticism by psychiatrists and psychologists at the time. But she stood her ground, undertaking careful observational and randomised controlled trials that proved that higher quality diets could reduce the rate of depression.

Professor Jacka led an important study of more than 20,000 mothers and their children that showed that the children of mothers who ate a not-so-healthy diet during pregnancy had higher levels of behaviours that are linked to mental disorders (6). This finding has since been replicated in other large cohort studies of maternal diet during pregnancy, finding that unhealthy diets were related to emotional/behavioural and cognitive outcomes. Her team also found that the children’s diets during the first years of life were associated with behavioural markers of mental health, suggesting that maternal diet during pregnancy and early life diet are both important in modulating the risk for mental health problems in children as they grow. This is consistent with what we see in animal experiments where unhealthy diets fed to pregnant animals result in many changes to the brain and behaviour in offspring. It’s critical that we think about preventing mental disorders in the first place.

However, in observational studies, like these, it is difficult to disentangle cause-and-effect. It’s possible that mental ill health makes us choose an unhealthy diet. Professor Jacka therefore undertook a randomised controlled trial (called SMILES) and was the first to show that a good diet could successfully treat clinical depression, even severe cases (7). Since then, a meta-analysis based on 16 randomised controlled trials with more than 45,000 participants confirmed that a good diet can improve symptoms of depression (8).

What about the glycemic index of carbohydrate foods? The brain’s essential source of energy is glucose, and it would make sense for blood glucose spikes and troughs to play a role in brain health. In 2016, a team from Iran, Australia and Canada studied over 3,000 Iranian adults who completed a food questionnaire and a validated anxiety, depression and health questionnaire (9). The researchers found greater odds of depression (44% more cases) in those with who had a higher dietary GI (top third of all participants) vs those with lower GI (bottom third). Similarly, the odds of having anxiety were 52% greater. In both cases, women were more affected than men. But interestingly, higher carbohydrate diets were associated with lower odds for all mental disorders. This type of study, a cross-sectional study, is not ideal for proving that the GI is related to mental health. For that we need to await more randomised controlled trials.

In the meantime, we can rest assured that a diet rich in fruit, vegetables, wholegrain foods (half of them low GI), legumes, nuts and oily fish will help give us the best chance of a long, happy and healthy life. Use good oils like olive and Canola. Consume red meat in moderation. You shouldn’t deny yourself your favourite foods, whatever they are. For me, I make weekends the time to enjoy butter on my sourdough bread with eggs and bacon. I add mushrooms, tomato and spinach if I can. And death-by-chocolate is my dessert-of-choice!

And talking about celebrations and favourite foods…please toast us in any way you wish because July 2025 marks the 20th Anniversary of GI News. GIN and tonic anyone

Read more:

1. Australian Institute of Health and Welfare. Australia’s youth: Mental illness. Australian Government, 2021.
2. Wurtman. Carbohydrate craving. Relationship between carbohydrate intake and disorders of mood. Drugs. 1990.
3. Cleveland Clinic. Antidepressants. 2023.
4. Robinson and colleagues. The effects of melatonin, serotonin, tryptophan and NAS on the biophysical properties of DPPC monolayers. Biochimica et Biophysica Acta (BBA) – Biomembranes, 2020.
5. Felice Jacka, OAM.
6. Jacka and colleagues. Maternal and early postnatal nutrition and mental health of offspring by age 5 years: a prospective cohort study. J Am Acad Child Adolesc Psychiatry, 2013.
7. O’Neil and colleagues. A randomised, controlled trial of a dietary intervention for adults with major depression (the “SMILES” trial): study protocol. BMC Psychiatry, 2013 SMILES.
8. Firth and colleagues. The Effects of Dietary Improvement on Symptoms of Depression and Anxiety: A Meta-Analysis of Randomized Controlled Trials. Psychosom Med, 2019.
9. Fahimeh and colleagues. Glycemic index, glycemic load, and common psychological disorders. The American Journal of Clinical Nutrition, 2016.

Emeritus professor Jennie Brand-Miller held a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney until she retired in December 2022. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.