In this dose-response trial of healthy, older persons, we found that serum folate concentration progressively increased with higher doses of supplemental folic acid. Among all participants and in those with the highest homocysteine concentrations at baseline, there was no dose-response relation between folic acid intake and homocysteine level. When analyses were restricted to persons with the lowest plasma folate concentrations at baseline, there was a borderline-significant trend in homocysteine reduction with increasing doses of folic acid.
Our finding of an increase in serum folate concentration with increasing folic acid intake is consistent with results from observational studies (30
), as well as randomized clinical trials of fortified foods (32
) or supplements (35
). Most relevant to our findings are results from another dose-response study carried out in older adults (26
), where 4 weeks of supplementation with placebo and folic acid at doses of 100 μg/day and 400 μg/day produced increases in serum folate concentrations.
Our results complement those of other studies examining the homocysteine-lowering effect of folic acid. These include 2 meta-analyses (2
) and 5 dose-response trials (23
). Results from the 2 dose-response trials that were conducted in healthy, older adults are most relevant to our trial (23
). Although our results are in contrast to those of these 2 trials, they are consistent with those of the first Homocysteine Lowering Trialists’ Collaboration meta-analysis (2
), which showed no differences in the homocysteine-lowering effect among higher folic acid doses (800, 1,000, or 2,000 μg/day). In this meta-analysis, the investigators concluded that the reduction in homocysteine concentration achieved was comparable (~25%) for folic acid doses of 500–1,000 μg/day, 1,000–3,000 μg/day, and >3,000 μg/day (2
). The second Homocysteine Lowering Trialists’ Collaboration meta-analysis included lower doses of folic acid (<500 μg/day) and found differences in their homocysteine-lowering effects (38
). Differences were seen between 200 μg/day and 400 μg/day and between 400 μg/day and 800 μg/day. These results for lower doses of folic acid are in contrast to our findings. Unlike our study, the meta-analyses included persons from a wide age range (17–92 years) with a median preintervention homocysteine concentration of 10.5 μmol/L.
There are 3 potential reasons for the lack of effect of folic acid on homocysteine levels seen in our study. First, our study population was healthier than expected, with relatively high serum folate concentrations and low homocysteine concentrations at baseline. The degree of homocysteine response to folic acid intervention depends on preintervention concentrations of both folate and homocysteine. There is also a nonlinear relation between folate and mean homocysteine levels and a plateau beyond which homocysteine concentrations remain stable (1
). Our trial baseline homocysteine concentrations (8.3 μmol/L) were lower than those seen in the dose-response study by van Oort et al. (26
) (range of 10.9–12.0 μmol/L by dose group) and in the dose-response study by Rydlewicz et al. (23
) (range of 9.5–10.8 μmol/L by dose group). We conducted subgroup analyses in this study to evaluate the effect of the intervention in persons with the lowest preintervention concentrations of folate. We did not have sufficient statistical power for subgroup analyses; thus, sample sizes for these analyses were small. However, we found a near-significant (P
= 0.06) dose response among persons in the lowest tertile of serum folate concentration at baseline. Furthermore, the greatest homocysteine reductions in this study were seen among persons with the highest baseline homocysteine concentrations who took 100 μg/day (−2.3 μmol/L) or 400 μg/day (−2.8 μmol/L).
The second potential reason for the lack of effect of folic acid in our study is the possibility that the homocysteine-lowering effect of folic acid may have been attenuated by participants’ intake of fortified foods during the fortification transition period. Early fortification of foods may have occurred in the Baltimore area. Although we intended to start this study prior to fortification, industry initiation of fortification was done earlier than the mandate for 1998 and occurred throughout our data collection period. The extent of the effect of fortification on our results is unknown, but it is possible that it influenced our findings. Evidence from clinical trials on cardiovascular disease suggests that fortification has influenced the homocysteine-lowering effect of folic acid (15
). Trials conducted in countries with mandatory fortification show narrower differences in homocysteine outcomes for study groups than trials conducted in countries without fortification (2
The third potential reason for the lack of effect of folic acid in this study is the possibility that supplementation with vitamin B6
, or B2
is also necessary for a reduction in homocysteine levels to occur. There are 2 pathways involved in homocysteine metabolism. One pathway is remethylation, and it requires folic acid and vitamin B12
coenzymes. The other pathway is transsulfuration, which requires a vitamin B6
coenzyme. Riboflavin may also be used in the metabolic cycle. Since folic acid fortification began, there have been scientific reports suggesting that other B vitamins have an important role in lowering homocysteine concentration (35
). More specifically, associations between vitamin intakes and homocysteine levels have shifted from being primarily dependent on folate status to being more dependent on B-vitamin status (35
). Results from these studies suggested that the homocysteine-lowering effect of a regimen including folic acid and B vitamins was highly dependent on the action of vitamin B12
. In this study, we did not monitor intake of vitamin B6
, or B2
In summary, although no dose-response relation was seen between folic acid supplementation and homocysteine concentration, our data indicate that healthy, older adults can improve their folate status through supplementation. The benefits, risks, and appropriate dose of folic acid supplementation continue to be investigated for various health outcomes (40
). These data suggest that a folic acid dose as low as 100 μg/day is effective in raising serum folate concentrations, and increasing folic acid dose leads to greater increases in blood folate concentration, with no evidence of a threshold. In the folic acid fortification era, daily supplementation comparable to usual dietary intake or even high-dose supplements does not lower homocysteine concentrations in healthy, older adults. Older adults with low folate status may benefit from folic acid supplementation.