Another warning about supplements comes from a finding that older adults taking more than twice the recommended daily allowance (RDA) of folate were more likely to have peripheral neuropathy if they had a common genetic variant linked to reduced cellular vitamin B12 availability.
B12 deficiency is a common problem for older adults, and those with this gene variant (estimated to occur in one in six Americans) are vulnerable to conditions related to vitamin B12 deficiency even if they consume normal amounts of B12.
The study, involving 171 older adults (60+), found that those with the GG variant of the TCN2 gene were three times more likely to develop peripheral neuropathy. But among those who consumed more than twice the RDA of 800 micrograms of folate, individuals with the gene variant had seven times the risk of peripheral neuropathy compared to those without. There was no significant difference for those consuming less than 800 micrograms of folate a day.
Note that the average daily folate intake for U.S. adults over 50 is in fact more than twice the Recommended Daily Allowance. Moreover, it's estimated that some 35% of Americans take folate supplements.
Hathairat Sawaengsri, Peter R Bergethon, Wei Qiao Qiu, Tammy M Scott, Paul F Jacques, Jacob Selhub, and Ligi Paul Transcobalamin 776C→G polymorphism is associated with peripheral neuropathy in elderly individuals with high folate intake Am J Clin Nutr 2016 104: 6 1665-1670; First published online October 12, 2016. doi:10.3945/ajcn.116.139030