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The comment and scientific evidence put forward by Caroline Relton and colleagues in 'Folate status and genetic selection' provides yet further evidence that mandatory population measures to increase folic acid could select the MTHFR 677T allele and create a 'folic acid habit'. An increased prevalence of the 677TT genotype selected for by elevated periconceptional folic acid levels means an increased risk for vascular disease and certain cancers in later life unless folate supplementation is maintained as a life-long measure. I would agree that over a number of generations, the 677TT genotype will probably increase in populations where pre-pregnancy levels of folate are high. Indeed, the selection of mutations that influence human fitness can happen quite rapidly: Haemochromatosis is a genetic disease that shows a progressive iron overload and is associated with a C282T mutation in the HLA-H gene. This gene has reached a high frequency in a very short time (it arose around 60 generations ago). This positive selection might well have improved the reproductive fitness of C282T heterozygote carriers by reducing premature delivery, and therefore offspring with low birth weight, which can be a feature of iron deficiency during pregnancy1. The deleterious effects of this disease only appear in one's 50s and 60s, and therefore do not affect the selective advantage of this mutation, which obviously only occurs during the reproductive phase of the human lifecycle. In terms of human evolutionary pressures, there are many examples of gene polymorphisms that influence how nutrients are metabolized and that are likely to have shaped our species. Why should folate be any different? It is after all one of the most important of all the vitamins, given its crucial role in the elaboration and function of DNA2.