Evaluating the research
In reality, this would be impractical and prohibitively expensive.
Nutrition researchers rely instead on biomarkers to indicate the likelihood of developing disease. For example, heterocyclic amines are often used in colon cancer studies. These proteins significantly increase the risk of developing the disease. Changes in biomarkers allow researchers to suggest whether a supplement modifies risk. Very few researchers would state that supplement X will prevent cancer or heart disease; instead they talk about risk.
Even well-designed studies are difficult to interpret, as people have different lifestyles, genetic make-up and family history. We know how much of a nutrient is needed to prevent deficiency, but we do not yet know what levels would achieve optimum health. The only convincing evidence is that a healthy diet with plenty of fruit and vegetables can reduce the chances of dying from heart disease or cancer.
Dr Toni Steer is a nutritionist at the Medical Research Council's Human Nutrition Research unit, Cambridge