In this study of older Americans in the age of folic acid fortification, we found direct associations between high serum folate and both anemia and cognitive impairment in subjects with low vitamin B-12 status. Among subjects with normal vitamin B-12 status, on the other hand, high serum folate was associated with protection from cognitive impairment.
Our findings were somewhat consistent with predictions of harm to vitamin B-12–deficient seniors from the US government’s folic acid fortification program (2
). However, the positive association we found between high folate status and anemia among older Americans with low vitamin B-12 status was unexpected.
Two related ideas have been expressed in the literature about impaired CNS function in the elderly from folic acid fortification. Both scenarios follow from hypothesized effects of unmetabolized folic acid in the circulation. Normally, folate circulates in the body as 5-methyltetrahydrofolate (5-MTHF) (11
). Folic acid, the form of folate in supplements and in fortified foods, can be converted to 5-MTHF as it passes through the intestinal mucosa (11
). However, capacity for this conversion is limited, such that repeated high dosing with folic acid can result in the appearance of unmetabolized folic acid in the bloodstream (35
). Circulating unmetabolized folic acid has been suggested to either merely delay diagnosis by curing vitamin B-12 deficiency anemia (6
), a key clinical sign (the so-called “masking” effect), or cause rapid deterioration of CNS function (8
). The simultaneous curing of anemia and exacerbation of CNS effects is potentially explicable by a hypothesized stimulatory effect of unmetabolized folic acid on DNA synthesis. According to the methyl-folate trap hypothesis (14
), the hematologic and neuropsychiatric consequences of vitamin B-12 deficiency result from the loss of vitamin B-12’s function as a cofactor for the enzyme methionine synthase—the catalyst for the remethylation of homocysteine to methionine. The result is not only a lack of S
-adenosyl methionine (SAM), the CNS methyl donor (37
), but also the failure of methyl folate to be converted to tetrahydrofolate (THF), and consequently, a deficiency of folate precursors needed for DNA synthesis and red blood cell maturation (38
). According to theory, this deficiency can be overcome by the direct conversion of folic acid to THF (11
), but the cost is further depletion of SAM because of the demand for methionine imposed by protein synthesis (14
Advocates of the addition to food of more folic acid than the US government currently requires continue to question the importance of masking to the consequences of vitamin B-12 deficiency (39
), particularly in light of increasing knowledge of the clinical heterogeneity of vitamin B-12 deficiency and the predominance of atypical presentations in the elderly (41
). More recently, the phenomenon of masking, itself, has been questioned—one case report shows that macrocytosis persisted and anemia worsened after folic acid treatment of sickle cell disease (18
) and another study reported no effect of fortification on the proportion of vitamin B-12–deficient veterans with anemia (17
The idea that high folic acid intake exacerbates neurologic and neuropsychiatric effects of vitamin B-12 deficiency is also controversial. Dickinson failed to find evidence of this phenomenon after closely scrutinizing the historical pernicious anemia case reports (15
). However, on the basis of not only case reports, but also on the basis of animal data and the known metabolic interaction between folate and vitamin B-12, the Food and Nutrition Board of the Institute of Medicine called the evidence “suggestive” (42
Our findings are most consistent with Reynolds’s (8
) conclusion from both original data and case reviews that folic acid precipitates both hematologic and neuropsychiatric manifestations of vitamin B-12 deficiency. Thus, our results are at odds with the idea that folic acid stimulates cell division at the expense of homocysteine remethylation, particularly in light of the lower prevalence of hyperhomocysteinemia in subjects with a low vitamin B-12 status and high serum folate compared with those with a low vitamin B-12 status and normal serum folate.
Our findings also support the often-expressed idea that many seniors would actually benefit from more folate (16
). Despite a marked increase in the folate status of Americans as a result of fortification (45
), we found a strong inverse relation between high folate status and cognitive impairment among vitamin B-12–replete subjects. Perhaps because of the diverse cognitive function tests used and the different domains assessed, previous studies of relations between folate status (47
) or hyperhomocysteinemia (48
) and cognition have yielded mixed results. In one recently published study, the subjects aged >65 y whose total folate intake at baseline exceeded 400 μg/d had a more rapid cognitive decline over 6 y of follow-up than did the participants with intakes <201 μg/d (55
). However, other prospective studies linked low folate intake (56
) or low circulating folate concentrations (48
) with an elevated risk of cognitive decline.
If folate causally affects cognitive function, its benefits, like those of vitamin B-12, may relate to its role in homocysteine remethylation. Remethylation leads to SAM production. Furthermore, homocysteine or its metabolites may damage neurons or cause vascular disease (59
)—a leading cause of dementia (60
). In subjects with a normal vitamin B-12 status, high serum folate was associated with protection from hyperhomocysteinemia, but this did not entirely explain the inverse relation between high serum folate and cognitive impairment.
Given our study’s cross-sectional design, a direct association between folate status and cognition could, theoretically, reflect the adverse effects of cognitive impairment on diet. However, such reverse causation could not explain the link between high folate status and poor cognition among subjects with low vitamin B-12 status. Although the methyl-folate trap hypothesis predicts normal or high serum folate in vitamin B-12 deficiency (61
), this phenomenon probably does not explain our results either. The higher serum folate supposedly results from a failure of polyglutamation, a modification of THF that facilitates intracellular folate retention (62
). Indeed, folate was lost from tissues in rats made vitamin B-12 deficient by nitrous oxide (64
). However, in our study, as in previous human investigations (19
), serum concentrations of folate and vitamin B-12 were directly, not inversely, related, regardless of vitamin B-12 status and dietary supplement use.
In addition to its large size and general population base, the strengths of our study included the availability of data on MMA and our control of key confounders. Although some data were self-reported, such that residual confounding might remain, we tried to exclude low intelligence (as indicated by educational level), stroke, and coronary artery disease as causes of poor cognition; iron deficiency and cancer as causes of anemia; and renal impairment as a cause of high MMA (66
The lack of a gold standard indicator of low vitamin B-12 status presents a challenge to all investigators of this nutritional problem (71
). It is currently accepted that clinically significant vitamin B-12 deficiency can occur in the elderly at serum vitamin B-12 concentrations >148 pmol/L (41
), and the serum MMA concentration is considered to be a sensitive and specific diagnostic tool (72
) that is particularly helpful in identifying so-called preclinical or subtle cases (77
). Although the cognitive function test administered in the NHANES is not specific for the cognitive impairment that results from vitamin B-12 deficiency (73
), the strong association we found between low scores and low vitamin B-12 status attests to its ability to capture cognitive impairment due to this cause. On the other hand, the availability of this single marker prevented us from evaluating associations between folate status and other neurologic and neuropsychiatric effects. We also cannot say definitively that the associations we found were due to unmetabolized folic acid, because only serum total folate was measured.
In conclusion, we undertook this investigation to shed light on long-held but evolving ideas about the effects of folic acid fortification on the elderly. We found a higher prevalence of both anemia and cognitive impairment in association with high serum folate in older Americans with a low vitamin B-12 status. We encourage further study of these relations and their underlying mechanisms and hope our findings both inform the continuing debate about folic acid fortification and influence efforts to detect and treat low vitamin B-12 status in seniors.