CHILDREN have a lot to contend with these days, not least a tendency for their pushy parents to force-feed them omega-3 oils at every opportunity. These are supposed to make children brainier, so they are being added to everything from bread, milk and pasta to baby formula and vitamin tablets. But omega-3 is just the tip of the nutritional iceberg; many nutrients have proven cognitive effects, and do so throughout a person's life, not merely when he is a child.

Fernando Gómez-Pinilla, a fish-loving professor of neurosurgery and physiological science at the University of California, Los Angeles, believes that appropriate changes to a person's diet can enhance his cognitive abilities, protect his brain from damage and counteract the effects of ageing. Dr Gómez-Pinilla has been studying the effects of food on the brain for years, and has now completed a review, just published in Nature Reviews Neuroscience, that has analysed more than 160 studies of food's effect on the brain. Some foods, he concludes, are like pharmaceutical compounds; their effects are so profound that the mental health of entire countries may be linked to them.

Last year, for example, the Lancet published research showing that folic-acid supplements—sometimes taken by pregnant women—can help those between 50 and 70 years old ward off the cognitive decline that accompanies ageing. In a study lasting three years, Jane Durga, of Wageningen University in the Netherlands, and her colleagues found that people taking such supplements did better on measures of memory, information-processing speed and verbal fluency. That, plus evidence that folate deficiency is associated with clinical depression, suggests eating spinach, orange juice and Marmite, which are all rich in folic acid.

Another suggestion from Dr Gómez-Pinilla's review is that people should eat more antioxidants. That idea is not new. Antioxidants are reckoned by many to protect against the general effects of ageing. Vitamin E, for example, which is found in vegetable oils, nuts and green leafy vegetables, has been linked (in mice) with the retention of memory into old age, and also with longer life.

Dr Gómez-Pinilla, however, gives the antioxidant story a particular twist. The brain, he observes, is peculiarly susceptible to oxidative damage. It consumes a lot of energy, and the reactions that release this energy also generate oxidising chemicals. Moreover, brain tissue contains a great deal of oxidisable material, particularly in the fatty membranes surrounding nerve cells.

That suggests, among other things, the value of a diet rich in berries. These have been shown to have strong antioxidant effects, though only a small number of their constituents have been evaluated in detail. One group that has been evaluated, the polyphenols, has been shown in rodents to reduce oxidative damage and to boost the ability to learn and retain memories. In particular, these chemicals affect changes in response to different types of stimulation in the hippocampus (a part of the brain that is crucial to the formation of long-term memories, and which is the region most affected by Alzheimer's disease). Another polyphenol, curcumin, has also been shown to have protective effects. It reduces memory deficits in animals with brain damage. It may be no coincidence that in India, where a lot of curcumin is consumed (it is the substance that makes turmeric yellow), Alzheimer's disease is rarer than elsewhere.

Peas of mind

Though the way antioxidants work in the brain is not well known, Dr Gómez-Pinilla says it is likely they protect the synaptic membranes. Synapses are the junctions between nerve cells, and their action is central to learning and memory. But they are also, he says, the most fragile parts of the brain. And many of the nutrients associated with brain function are known to affect transmission at the synapses.

An omega-3 fatty acid called docosahexaenoic acid (DHA), for example, provides membranes at synaptic regions with “fluidity”—the capacity to transport signals. It also provides “plasticity”—a synapse's capacity to change. Such changes are the basis of memory. Since 30% of the fatty constituents of nerve-cell membranes are DHA molecules, keeping your DHA levels topped up is part of having a healthy brain. Indeed, according to the studies reviewed by Dr Gómez-Pinilla, the benefits of omega-3s include improved learning and memory, and resistance to depression and bipolar disorder, schizophrenia, dementia, attention-deficit disorder and dyslexia.

Omega-3s are found in oily fish such as salmon, as well as in walnuts and kiwi fruit, and there is a strong negative correlation between the extent to which a country consumes fish and its levels of clinical depression. On the Japanese island of Okinawa, for example, people have a strikingly low rate of mental disorder—and Okinawans are notable fish eaters, even by the standards of a piscivorous country like Japan. In contrast, many studies suggest that diets which are rich in trans- and saturated fatty acids, such as those containing a lot of deep-fried foods and butter, have bad effects on cognition. Rodents put on such diets show declines in cognitive performance within weeks.

In the past few years, several studies have looked at the effect of adding omega-3s to people's diets—particularly those of children. One such, carried out in the British city of Durham, was controversial in that it was funded by a maker of children's omega-3 supplements and did not include a control group being given a placebo. Despite the publicity this study has received, Ben Goldacre, author of a book called “Bad Science” that includes an investigation of it, says the results will not be released.

Work by other researchers, however, has suggested such supplements do improve the performance and behaviour of school-age children with specific diagnoses such as dyslexia, attention-deficit disorder and developmental co-ordination disorder. Moreover, although more work is needed to elucidate the effects of omega-3s on healthy school-age children, Dr Gómez-Pinilla says that younger children whose mothers took fish-oil supplements (which contain omega-3s) when they were pregnant and while they were breast-feeding do show better cognitive performance than their unsupplemented contemporaries.

Eating well, then, is one key to a healthy brain. But a word of warning—do not overeat. This puts oxidative stress on the brain and risks undoing all the good work those antioxidants have been up to. For those who would like a little practical guidance, The Economist has some suggestions for dinner (see menu). So why not put the Nintendo brain trainer away tonight, and eat your way to intelligence instead?