Folate status and its relation with one-carbon metabolites in reproductive aged and lactating women

Abstract

Folate plays an important role in foetal and infant growth and development, contributing to the synthesis of DNA nucleotides and the donation of methyl groups in one-carbon metabolism involved in epigenetic regulation of DNA expression. As a result, the requirements for folate are increased during pregnancy and lactation. Folate interacts with several other nutrients in the one-carbon metabolism pathway, including vitamin B12, choline and betaine, which may further impact folate requirements. To date, several studies have investigated the relationships between these nutrients and folate during pregnancy, however, no published study has investigated these relationships in lactating women. This thesis examines the metabolism of folate in the one-carbon metabolism pathway with a focus on reproductive-age and lactating women in settings of higher and lower folate environments. The overall aim of this thesis was to determine the effect of high or low folate status on the metabolism of folate and other one-carbon metabolites, primarily choline and betaine, and their interrelationships with other one-carbon metabolites, including the folate status marker, homocysteine.

The first study herein aimed to determine the relationship between folate status and individual blood folate forms in lactating women supplemented with high-dose folic acid supplements. Folate forms were measured in 117 healthy Canadian lactating women supplemented with 1000 µg folic acid/d beginning in gestation and continued into lactation. At 8-wks postpartum, median serum and red blood cell folate concentrations concentrations were high at 79.3 and 2793 nmol/L, respectively. Serum folic acid was significantly positively associated with serum total folate when concentrations were ≥70 nmol/L (r=0.892, P<0.001), but not <70 nmol/L (r=0.264, P=0.087). These results suggest that there is a serum folate status above which folic acid is inefficiently converted to 5‑methyltetrahydrofolate (5‑methylTHF), the active folate vitamer. Consideration should be given to the reformulation of pregnancy multivitamins and folic acid supplements toward a lower dose. The next part of the thesis (Chapter 4) involved developing a liquid chromatography-tandem mass spectrometry method to accurately measure choline, betaine, methionine, methionine sulfoxide, homocysteine, and cysteine in low volume plasma samples. In doing so, we were able to establish a procedure to successfully measure all analytes with the exception of dimethylglycine. The procedure for measurement of these analytes is described herein as well as measurement of their concentrations in samples from reproductive aged women, lactating women, and their 8-wk old exclusively breastfed infants. Next, to determine if moderate folic acid supplementation results in changes to choline and betaine status, we performed a secondary analysis of one-carbon metabolism-related nutrients using samples from a randomised placebo controlled trial involving 144 healthy reproductive aged women (Chapter 5). Women were randomly assigned to either placebo, 140 µg folic acid/d, or 400 µg folic acid/d over a 40-wk period. Analysis of folate and one-carbon metabolism-related nutrients indicated baseline associations of plasma folate with plasma betaine (β=0.20, P=0.015); however, there was no evidence to support that folate supplementation at the assigned moderate doses provided altered choline or betaine status. In addition, plasma folate was significantly negatively correlated with plasma total homocysteine at baseline (β=-0.33, P<0.001), however, there was no evidence of a relationship between choline or betaine status with homocysteine concentration. Given the wide variation in intakes of women from countries with and without mandatory folic acid food fortification, these interactions should be assessed across a range of folate intakes – in particular high dose folic acid supplementation typically seen among North American women. Lastly, we determined the interrelationships among folate and other nutrients involved in the one-carbon metabolism pathway in a cross-sectional group of healthy lactating New Zealand women at 8-wks postpartum (Chapter 6). Similar with in reproductive aged women, a positive association between serum total folate and plasma betaine was found (β=0.31, P=0.009); however, in contrast, plasma betaine was significantly negatively associated with plasma homocysteine (β=-0.19, P=0.047). No evidence of an association between choline and homocysteine concentration existed, contrary to findings of other studies. Given the cross-sectional nature of the data, these relationships require further investigation using a randomised placebo controlled trial design. In conclusion, our findings highlight a potential serum folate threshold above which folic acid concentrations are increased in blood and demonstrate a need for further research assessing high dose supplementation in lactating women. At low to moderate doses of folic acid intake, our data do not justify the re-evaluation of recommended dietary intake levels for reproductive aged or lactating women based on interactions occurring in folate metabolism with other one-carbon metabolism related nutrients. Although there was an indication that folic acid concentration may be related with betaine status during lactation, further research is required to confirm whether folate status affects choline and betaine metabolism in lactating women.