B-vitamins, genotype and disease causality

Despite a great deal of research effort there is still considerable uncertainty surrounding the importance of the B-vitamins in health and disease. This continuing uncertainty is partly a result of the difficulty of measuring intake, confounding in observational studies and the very large numbers required to evaluate primary prevention in randomised controlled trials. Consequently, genetic data are increasingly being used to infer nutritional effects on health and even in the formulation of nutrition policy using the approach of ‘mendelian randomisation’. Genetic information has already contributed greatly to the understanding of B-vitamin metabolism and the heterogeneity of responses to intake. It has the potential to provide further nutritional insights and to assist in the elucidation of causal mechanisms, but it is important that genetic data is not viewed as an alternative to nutritional information, both are necessary when addressing nutritional problems. Similarly, the interpretation of nutrient and biomarker status in some experimental designs may require knowledge of genotype. Formal tests of gene–gene and gene–nutrient interaction are of limited value in nutritional studies and the formulation of policy. Graphical representation of diet–genotype–health data greatly assists in the elucidation of the nature of genetic effects, their interaction with nutrition and the implications for nutrition policy.