Perhaps more easily understood than nutrigenomics, nutrigenetics is the science behind single-gene/ single food compound relationships. One of the best-described examples is the relationship between folate and the gene for MTHFR - 5,10-methylenetetrahydrofolate reductase.

MTHFR has a role in supplying 5-methylenetetrahydrofolate, which is necessary for the re-methylation of homocysteine to form methionine. Methionine is essential to many metabolic pathways include production of neurotransmitters and regulation of gene expression. Folate is essential to the efficient functioning of this MTHFR.

There is a common polymorphism in the gene for MTHFR that leads to two forms of protein: the wild type(C), which functions normally, and the thermal-labile version (T), which has a significantly reduced activity. People with two copies of the wild-type gene (CC) or one copy of each (CT) appear to have normal folate metabolism. Those with two copies of the unstable version (TT) and low folate accumulate homocysteine and have less methionine, which increases their risk of vascular disease and premature cognitive decline.

Supplemented with folic acid (or increased intake of folate from food sources), these individuals quickly metabolise the excess homocysteine restoring their methionine levels to normal.

Currently we are aware of about 20 genes that that have polymorphisms that appear to confer a significant disadvantage, which may be overcome with dietary modification. Businesses like Sciona Inc. (www.sciona.com) and Genelex Inc. (www.genelex.com) in the US base their services on this knowledge and the scientific literature that supports it.

There are, however, a number of wider issues to consider. In the first place, genotypes that confer a substantial survival disadvantage are not usually preserved in a population. Those that are, have often been shown to offer some other benefit. For example, it was widely assumed that both the genes for sickle cell and thalassemias were for the individuals involved tragic and for the population at large disadvantageous. We now know that individuals with a single copy of these genes have inherent protection from malaria, which is endemic in regions where these mutations are most commonly found.

The fact that the most common polymorphism for the MTHFR gene is present in 15-20% of European population must at least raise the question why it and the other genes have persisted so successfully. Secondly, these 15-20 genes represent 0.1% of human genes; Ensembl (www.ensembl.org) estimates there are 22218 protein coding genes in the human genome. Currently, we neither know how or which of these genes interact with one another nor the consequences of modifying the response of a few on the majority and the effects of that on our immediate or long term health. Thus, while increasing your intake of folate may be beneficial in the long term, it may be shown at some point in the future that increased intake has unforeseen risks for some individuals or sub-populations.