5 Epidemiology and trace elements

Basically, epidemiology is the making of measurements of known reproducibility, in a bias-free manner, on representative samples of subjects drawn from defined communities. Epidemiology has become a relatively precise science and its value in medicine is widely appreciated. So too are its limitations: the difficulties in achieving a high response rate, in identifying and controlling confounding factors in the examination of an association, and the ultimate difficulties in distinguishing causation from association. While the value of community-based studies seems to be recognized by those interested in man and his environment, the need for the strict application of epidemiological procedures, and the limitations imposed on conclusions drawn from studies in which these procedures have been compromised, does not seem to be adequately understood.

There are certain known links between trace elements in the environment and disease: for example the level of iodine in soil and water and the prevalence of goitre; the level of fluoride in water and the prevalence of dental caries. The investigation of other possible associations is difficult for a number of reasons, including interrelationships between trace elements, confounding of trace element levels (and disease) with social and dietary factors, and the probability that relationships are generally weak.

Two conditions in which associations are likely are cardiovascular disease and cancer. Despite research along a number of lines, the relevance of trace elements to cardiovascular disease is not clear, and certainly the apparent association with hardness of domestic water supply seems unlikely to be causal. The same general conclusion seems reasonable for cancer, and although there are a very few well established associations which are likely to be causal, such as exposure to arsenic and skin cancer, the role of trace elements is obscure, and likely to be very small.