UNDER THE MICROSCOPE:We are told to maintain a healthy diet but how can we choose, with an overabundance of contradictory advice, asks William Reville
HEALTHY EATING is a perennially popular media topic. We are bombarded with detailed advice on what to and what not to eat. On the whole, this advice seems to have little positive effect as evidenced by the widespread problem of obesity. Most advice concerns eating to promote physical health. We hear less about the effect of diet on behaviour and intelligence and it is this topic that I will address today. I draw heavily on a review paper published by David Benton in The Psychologist (October 2008).
I am not fond of fish, particularly oily fish, which "repeats" on me for hours afterwards. My wife exhorts me to eat more fish because it is a good source of the omega unsaturated fatty acids. We are unable to synthesise these "essential" fatty acids in the body and they must be supplied in the diet. They play a vital role in the membranes in our cells, making them more "fluid".
Membrane fluidity is particularly important in facilitating communication between nerve cells. Our brains are packed with nerve cells, and communication between them underpins brain function. The brain is a very prominent organ that develops rapidly in the last third of pregnancy and in the first two years after birth. At birth the brain constitutes 10 per cent of body weight, but in the adult it is only 2 per cent. There is therefore reason to fear that an inadequate supply of fatty acids during early brain development could have an adverse effect.
Research does not provide a clear answer to this latter question. One study cited by Benton correlated higher intake of seafood during pregnancy with improved pro-social behaviour, fine motor control and higher verbal intelligence at eight years of age. However, a more rigorous blind-trial comparing infants fed with cows' milk formula that either was or was not supplemented with fatty acids found little evidence that fatty acids supplementation enhanced cognitive function. Nor is there substantial evidence that fatty acid supplementation benefits older children diagnosed with attention deficit hyperactivity disorder.
The notion that fish is a good food for the brain is widely believed. In 2006, Durham County Council gave fatty acid supplements to 2,000 children hoping that GCSE examination results would improve. They did not.
There is a widespread feeling that our diet contains inadequate amounts of vitamins and minerals, largely attributed to modern agricultural methods and refined foods. Many people take vitamin and mineral supplements. Benton cites evidence that vitamin and mineral supplementation improves the intelligence of children. The improvement is found with non-verbal measures of intelligence, but never with verbal. Non-verbal intelligence, called fluid intelligence, is thought to reflect biological potential. This could be affected by nutrients. Verbal intelligence is crystallised intelligence consisting of specific information and vocabulary.
However, Benton warns that there are no easy conclusions despite these promising results. We still don't know exactly which nutrients are important and it appears that only a minority of children respond. It may be that only the poorly nourished respond positively and that children on adequate diets may not need these supplements.
Many people cannot tolerate certain foods. The immune system is involved in about 20 per cent of negative reactions to foods. In these cases the body treats some foods as foreign proteins and generates antibodies against them. Nuts, shellfish and strawberries are common foods that generate allergic responses in some people.
In one study of children whose parents believed that they responded negatively to diet, it was found that cows' milk caused an adverse reaction in 64 per cent of these children, chocolate in 59 per cent, grapes in 49 per cent, wheat in 49 per cent, oranges in 45 per cent, cows' cheese in 40 per cent, and eggs in 39 per cent. These findings cannot be generalised to all children.
Table sugar - sucrose - gets a very bad press. It is felt that since sucrose is a source of energy its consumption can cause hyperactivity. However, research shows no evidence that sucrose has an adverse effect on children's behaviour.
Benton concludes that "human behavioural problems as a reflection of diet may offer false hope to both child and parent as, although there is good evidence that diet can influence behaviour, we lack the information to give precise advice. There is a risk that taking an exclusively dietary approach will delay more effective treatment. Self diagnosis is to be discouraged and professional advice should be obtained before concluding that there is a dietary problem."
The welter of dietary information and advice in the popular media is totally confusing. In my opinion, there is no need for the average person, who is not suffering from a medical problem, to worry at all about this advice. The golden rule on diet and healthy living is very simple - eat a wide variety of foods in moderation, and take plenty of aerobic exercise.
• William Reville is associate professor of biochemistry and public awareness of science officer at UCC - understandingscience.ucc.ie