Commentary

A risk factor may show disease causation; if it suggests a plausible intervention, then it is all the more interesting. It is even better if that intervention is preventive. Robust risk factors for Alzheimer’s disease are now well established: increased age, Down’s syndrome, poor education, and apolipoprotein E status. These risks are not easily altered. Biological risk factors that can be altered are especially attractive. Some, such as oestrogen use and middle-life vascular risk factor control, have accumulating evidence and suggest obvious, if awkward, interventions. The homocysteine story includes a plausible mechanism for causing Alzheimer’s disease, and the suggested intervention (dietary folate supplementation) is cheap, easy, and available to all. Folate prevents neural tube defects, a finding that has already led to dietary supplementation with folate in the US.

The study by Seshadri et al is hypothesis generating and prospectively studies 1092 participants from the Framingham Study, of whom 111 subsequently developed dementia. The wide confidence intervals of the RRs reported in the study suggest the need for caution in interpreting these results. Can they be explained because of inadequate adjustment for possible confounding factors? In addition, the findings of Seshadri et al are contrary to those from the other large prospective study of homocysteine and cognition.¹ Some of these uncertainties can be resolved by doing larger studies that provide estimates of the association between homocysteine and dementia in a range of different circumstances (such as at different levels of other risk factors and at different times in the evolution of risk). Analogy with plasma risk factors in cardiovascular disease is instructive. Initial studies overestimated the role of homocysteine, and a recent synthesis of the evidence suggests that its role remains uncertain,² even though > 10 times the amount of information is available for cardiovascular disease than for dementia. Understanding the exact nature of the relation between homocysteine and dementia is important because the implications of this link are far reaching.