Although reports of claims that a particular gene has been found for a particular disease are often inaccurate, genetic studies can be used to identify reliable ways of improving health through simple interventions, such as moderate dietary change.

For example, studying the associations between genes carried by mothers and the occurrence of spina bifida (a neural tube defect) in their children could have shown that folate pills taken before the women get pregnant would have prevented the disease.

The report's authors, Professors George Davey Smith and Shah Ebrahim, call this method 'Mendelian randomisation', after Gregor Mendel, the monk who discovered the laws of genetic inheritance in the 1860s.

Genes that produce well-defined effects in the body can be used to uncover the effects of exposures that are difficult to study because they are closely linked with other variables. For example, simply comparing folate intake in mothers with and without children suffering from a neural tube defect is complicated because folate intake is lower among smokers, poorer people, people who drink heavily or have other dietary deficiencies. These links make it difficult to identify low folate as the important factor in causing neural tube defects - such diseases could be caused by one of these other factors.

A gene (known as MTHFR) exists in some people in a form that makes them resistant to dietary folate intake. Women who carry this gene are no more likely to be smokers, heavy drinkers or poor than are woman without this gene. However, these women do have a higher risk of having a baby with a neural tube defect. This provides strong evidence that by taking folate supplements, mothers would reduce the risk of neural tube defects in their children.

Professor Ebrahim said: "The report shows the important role genetic research could play in informing public health policies. Investment in the human genome project may contribute to understanding and preventing the adverse effects on human health of modifiable exposures such as diet, alcohol consumption and smoking - and not just explain our individual susceptibility to disease."

"Mendelian randomization": can genetic epidemiology contribute to understanding environmental determinants of disease? Int J Epidemiol 2003 vol 32 no 1 pp 1-22

The International Journal of Epidemiology is a key journal in the field of epidemiology and public health, published six times per year by Oxford University Press. It is edited at the Department of Social Medicine, University of Bristol, which is a leading centre for epidemiology, health services research and public health in the UK and was one of only three to be awarded the top 5* grade in the 2001 Research Assessment Exercise.