Australasian Science Magazine

Recent studies have associated a better diet quality with reduced depression.

By Lesley MacDonald-Wicks

There is substantial interest in the role of nutrition in preventing and treating depression.

One of the more alarming health statistics in Australia is the rising rate of depression. In 2007, 45% of Australians experienced one of the common mental disorders (CMD), including depression or anxiety. The most alarming aspect of CMDs is an early onset, often in adolescence or early adulthood.

Just as alarming is the rise in overweight and obesity in Australia – currently 60% of the population – and the rapid trajectory of weight gain in young women aged18–35 years. While there appears to be a similar age of onset for both conditions, you have to ask whether there are any other commonalities.

Can Omega-3 Alleviate Depression?

There is increasing interest in the role of nutrition in depression. Studies have looked at the role of individual nutrients in the prevention or treatment of depressive illness. Zinc, magnesium and some of the B group vitamins, in particular folate, have potentially biologically plausible roles in depression, with many studies showing associations between these nutrients and depression. One of the most extensively studied nutrients in relation to CMDs is the omega-3 fats.

Usually we divide omega-3 fats into two categories: alpha- linolenic acid (ALA), which is the essential and shorter chain version, and the long chain (LC) omega-3 fats eicosapentanoic acid and docosahexanoic acid. ALA is a type of fat found in some nuts and seeds, such as walnuts and flaxseed, and in canola-based margarine, while the LC omega-3 fats are primarily found in seafood (e.g. deep-sea fish). There is a lot of discussion about LC omega-3 fats and health, but most commonly this is in relation to heart disease and not usually in relation to depressive illnesses.

Yet there is substantial supporting evidence that LC omega-3 fats could be used as one part of preventing or treating depressive illness. It is biologically plausible that LC omega-3 fats have a role in depression, with research suggesting many possible mechanisms, including the amount of stored LC omega-3 in the body and consumed from foods affecting mental health status. Observational and cross-sectional studies have shown an association with LC omega-3 status and depressive symptoms. Randomised controlled trials also show beneficial effects of LC omega-3 fats on symptoms of depression.

However, many studies have shown no association, so the science is not conclusive. A very recent systematic review comparing all the evidence on omega-3 fats in depressive illness showed that while the evidence is mounting, the size of any effect is small and only seen in populations with severe depression. However, there is no evidence of harm. Currently, there is little evidence supporting a role in the prevention of depression, and no conclusive statement can be made at this time for using LC omega-3 fats in the treatment of depression. But watch this space.

The relationship between diet and depression is likely to be broader than one nutrient. Research at the University of Newcastle indicates a role for other types of fat, such as ALA in reducing the risk of depressive illness, and omega-6 and omega-9 (olive oil) in reduced risk of anxiety disorders.

Diet Quality

CMDs are complex disorders, and it is perhaps naive to think that one nutrient could be the answer to treating them. Recent work from Deakin University in Australia is focusing on overall diet quality and the role of diet as a whole with depression.

Recent studies in relation to depressive illness indicate that a higher diet quality score is associated with reduced depression. Diet quality is a way of measuring the quality of the diet, depending on how often core food groups such as fruit, vegetables, low fat dairy products, lean meat, legumes and whole grains are eaten. The higher the diet quality score, the closer the eating pattern resembles national dietary guidelines of suggested ways to eat in a healthier manner.

Similarly, some researchers have calculated an unhealthy diet quality score, measuring the amount of unhealthy high fat and high sugar foods eaten, and found an association between unhealthy diet scores and increased risk of depressive illness and symptoms of depression. This seems to be true in both adult and adolescent groups. Given adolescence is the time of life that depressive symptoms commence, it is important to relate lifestyle and dietary patterns to the emergence of the condition.

Dietary Patterns

Dietary patterns refer to overall habitual dietary intake. One of the most common dietary patterns involves what is known as a “prudent” diet (a high intake of fruit and vegetables, whole grains, lean meat, and low fat dairy products) compared with a “western” diet (a high intake of processed foods, fried food and take away, high sugar foods and processed grains). The prudent diet is linked to a reduced risk of chronic disease such as heart disease, Type 2 diabetes and some cancers compared with the western diet. Recently it was also linked to a reduced risk of depressive illness and possibly reduced risk of anxiety, although evidence for anxiety disorders was lower.

These studies need to be looked at with caution. There are many differences in the way studies describe the prudent diet and the western diet, and these cloud comparisons of the studies and make it difficult to make recommendations about eating a specific type of dietary pattern and reducing the risk of depressive illness.

One common dietary pattern is the Mediterranean diet, which is characterised by the use of olive oil (a mono-unsaturated fat), fish (omega-3 fats), whole grains (B group vitamins), vegetables, fruit and legumes. A recent paper described the Mediterranean dietary pattern as having a protective relationship with depression. This association needs to be tested in a randomised controlled trial before we can make definitive statements about this dietary pattern and the prevention or treatment of depression.

The Role of Exercise

Physical activity is widely reported to have beneficial effects on mental health status, and has been associated with benefits in the prevention and treatment of CMDs. A recent systematic review of studies in adolescents in the school setting indicated that there is a small but significant overall benefit to physical activity in relation to depression, with little evidence of any adverse events.

Can Diet Treat or Prevent Depression?

There is some favourable research evidence for using diet in the treatment of depression, with caution recommended in interpreting the evidence so far. It may be that there are particular nutrients that help to protect us from depression, However, it is more likely that your entire dietary pattern and lifestyle will turn out to be more important.

Some of the best evidence supports a dietary pattern with a generous amount and variety of vegetables and fruit, and only limited amounts of energy-dense, nutrient-poor “junk food” and sugar-sweetened soft drinks, as the best way to obtain any dietary benefits when it comes to preventing or treating depressive illness.

The prudent diet’s focus on good quality fat, along with regular physical activity, seems to be the sensible approach. This lifestyle pattern may protect you from depressive illness and other chronic conditions like obesity, heart disease,Type 2 diabetes and some cancers at the same time.

Dr Lesley MacDonald-Wicks is a Senior Lecturer in Nutrition and Dietetics at the University of Newcastle, and has a research interest in omega-3 fats.