Topics that need consideration with regard to setting nutrient requirements and to considering susceptibility to higher levels of intake of a nutrient include genotyping, epigenetics, and imprinting—including the assessment of effects of single nucleotide polymorphisms (SNPs) on variability in requirements and/or ULs. Earlier in this workshop, Drs. Steven Zeisel and Patrick Stover provided examples, such as the finding that methylation of deoxyribonucleic acid (DNA) at the cytosine position can have a substantial influence on gene expression and that dietary methyl donors can influence the level of methylation to some extent. This is a knowledge area that is important to understand, particularly when it is possible that one can influence not only gene expression but even the total silencing of a gene, resulting in genetic imprinting and some very important phenotypic outcomes. Some interesting modeling challenges can be anticipated.

With regard to the methylene tetrahydrofolate reductase (MTHFR) polymorphisms, the pathway is highly complex; flavin-adenine dinucleotide (FAD) may be able to partially stabilize the heat lability of the variant. Looking at this additional way in which riboflavin interacts with this pathway may serve as another example of how this type of information will enter some of the decision making related to DRIs.

A specific example involves the C677T mutation in the MTHFR gene. Homozygosity for this mutation results in a less active and a more heat-sensitive enzyme protein (Kang et al., 1988). Heat sensitivity results in dissociation of FAD, but the dissociation is prevented by folate substrates (Guenther et al., 1999). The prevalence of the homozygous variant differs among subgroups: the variant exists in about 15 percent of the Caucasian population, is less prevalent in African Americans, and is more prevalent in Hispanics.

Data from Dr. Paul Jacques of Tufts University shows an elevation of homocysteine in the homozygous variant only when folate intake is below the median. This finding raised questions about whether different subgroups would respond differently to intake. The increase in plasma folate by genotype (data provided by Jacques, see Figure 12-1), suggests that the abnormality probably is not a large determinant of the requirement. One possible model of the MTHFR variant related to the folate