I’ve blogged about superfoods before (here, here, and here). The latest issue of New Scientist, has an excellent article on diet, “The Good, the Fad, and the Unhealthy,” with several informative sidebars. This one, on omega-3, is worth pondering:

All kinds of foods, from Brussels sprouts to peanut butter and potatoes, have been touted as brain food. Sadly, these are little more than old wives’ tales, but there is one well-known brain food that has solid evidence on its side: fish.

No nutrient has garnered as much supporting evidence for promoting mental health as long-chain omega-3 fatty acids, in particular eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are plentiful in oily fish such as tuna and salmon. And the potential benefits are huge: globally, more working days are lost to depression than any other illness. In the UK, 1 in 10 people are depressed at any given time, and for 1 in 20 it is a lifelong problem (New Scientist, 24 August 2002, p 34).

Back in 1998, Joseph Hibbeln, a psychiatrist and lipid biochemist at the National Institute on Alcohol Abuse and Alcoholism (NIAAA) in Bethesda, Maryland, noticed that depression was up to 60 times rarer in countries such as Taiwan and Japan, where people eat a lot of oily fish, compared with the US and Germany where they don’t. In particular, he noticed that in countries that consume a lot of fish, rates of bipolar disorder, post-natal depression and seasonal affective disorder were lower.

In the intervening years Hibbeln and others have produced an impressive body of evidence that differences in intake of omega-3s are correlated with the prevalence of depressive disorders and that giving omega-3 supplements to depressed patients reduces their symptoms. “Each of these psychiatric disorders has been shown to be improved by omega-3 in double-blind placebo-controlled trials,” says Hibbeln.

You don’t have to be clinically depressed to benefit either. Healthy people with relatively low levels of omega-3 in their bloodstream are more likely to be mildly depressed, pessimistic and impulsive than those with high levels, according to research presented in March by Sarah Conklin of the University of Pittsburgh School of Medicine at the American Psychosomatic Society meeting in Denver, Colorado. “This is not magic,” says Hibbeln, “just the psychiatric manifestations of omega-3 deficiencies in the brain.”

Here’s how it works. The brain is about 60 per cent fat, much of it making up the membranes that envelop nerve cells. The fats we eat influence their composition, and omega-3s are a particularly important membrane component: studies have shown that they make the membrane more fluid and flexible, rendering the cell more receptive to incoming signals. That’s because when neurotransmitters dock with receptor proteins in the membranes, the receptors change shape, triggering a cascade of chemical reactions. The more fluid the membrane, the faster the signal propagates.

Historically the dietary ratio of omega-3s to another class of fatty acid, omega-6s, was about 1:1. In the past century, however, this ratio has shifted to more like 1:10 or 1:15 in western diets because of the introduction of seed oils such as soybean oil. Scientists believe that when the diet is low in omega-3s the brain compensates by substituting an omega-6, which alters the physical properties of membranes.

This omega-6 contains one fewer double bond in its carbon chain than omega-3s, making it more rigid. When the neuron’s membrane contains too much omega-6 the receptors can’t change their shape as easily, making them less responsive to signals. “It’s like putting a dancer in a matrix of marshmallow fluff versus molasses and asking her to move,” says Burton Litman, a membrane biophysicist at NIAAA.

In a recent experiment, Litman found that rats fed a diet deficient in omega-3s lost 80 per cent of the DHA in the membranes of their retinal cells, replaced with omega-6s. It also impaired their visual signalling (The Journal of Biological Chemistry, vol 279, p 31098). The retina’s light-sensitive rhodopsin molecules, which generate the first signal in the visual cascade when light hits them, were unable to change shape as easily. With fewer rhodopsin molecules responding, the chemical pathway that produces the nerve impulse to the brain was less efficient and the final signal was weaker.

Hibbeln says that this probably happens in other parts of the central nervous system too. “I assume similar situations in other neuronal systems, [substituting other fats] slows down the system. You can’t imagine that it is good,” he says.

Omega-3s also seem to have another benefit, by promoting neuronal growth and so allowing the brain to repair damage. In a paper published in April, neuropsychiatrist Basant Puri at Imperial College London did “before and after” MRI scans on people with schizophrenia and Huntington’s disease who were treated with EPA or a placebo (International Review of Psychiatry, vol 18, p 149). After six months those in the placebo group had clearly lost cerebral tissue, but in patients given the supplements, the amount of grey and white matter increased enormously. “This demonstrates that the brain uses these fatty acids amongst other things to sprout new dendrites and increase its mass,” says Puri. These patients also experienced “amazing” improvements in cognition, he adds, with better short-term memory, arithmetic ability and concentration levels.

Preliminary studies with omega-3 supplements have also shown encouraging results for schizophrenia, borderline personality disorder, dyslexia, autism, attention deficit hyperactivity disorder, obsessive compulsive disorder and dyspraxia, among others. However, cautions psychiatrist Bruce Cohen at Harvard Medical School, there are too few studies in most of these areas to be sure.

Omega-3s may also help the brain because they are anti-inflammatories, says Cohen. Both EPA and DHA are known to dampen inflammation in cardiovascular disease and may also suppress inflammatory processes in the brain, which are a hallmark of major psychotic disorders and dementias.

That too is the hypothesis for the brain-healing properties of brightly-coloured fruits and vegetables. Researchers speculate that this stems in turn from their antioxidant properties. Many mental illnesses are accompanied by oxidative stress to the brain caused by free radicals. This often leads to physical destruction of brain tissue, which is believed to trigger cognitive decline and low-level inflammation.

Preliminary research in rats suggest a diet high in antioxidants can slow down and to some degree reverse age-related behavioural, cognitive and motor decline. For example, a string of studies from Tufts University in the US show that spinach, blueberries and strawberries are particularly effective at slowing cognitive decline.

Evidence has also been building in favour of eating other foods rich in antioxidants, including ginger, green tea, coffee and turmeric.

However, rather than just one superfood emerging, researchers agree it is more likely that a constellation of different foods will be most effective at keeping the brain healthy - once again underscoring the value of a balanced diet.

Don’t forget, though, that the benefit of antioxidants is obtained from eating foods, not supplements. For more information about the benefits and actions of omega-3, download this .PDF file by clicking the link.

UPDATE: I get my wild-salmon oil capsules via the Web, from here (usually) or from here. I take two of the 1000mg capsules with breakfast, two more with dinner.