AJCN Tufts Nutrition Symposium, Boston Sept 24-26
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Purchase Article
Right arrow View Shopping Cart
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Related articles in AJCN
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Morris, M. S.
Right arrow Articles by Selhub, J.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Morris, M. S.
Right arrow Articles by Selhub, J.
Agricola
Right arrow Articles by Morris, M. S.
Right arrow Articles by Selhub, J.
American Journal of Clinical Nutrition, Vol. 85, No. 1, 193-200, January 2007
© 2007 American Society for Nutrition

ORIGINAL RESEARCH COMMUNICATION

Folate and vitamin B-12 status in relation to anemia, macrocytosis, and cognitive impairment in older Americans in the age of folic acid fortification 1,2,3,4

Martha Savaria Morris, Paul F Jacques, Irwin H Rosenberg and Jacob Selhub

1 From the Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging, Tufts University, Boston, MA

See corresponding editorial on page 3.

2 Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views or policies of the US Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the US government.

3 Supported by USDA agreement no. 58-1950-9-001 and NIH no. R03 AG021536-01.

4 Address reprint requests to MS Morris, Jean Mayer USDA Human Nutrition Research Center on Aging, Tufts University, 711 Washington Street, Room 901D, Boston, MA 02111. E-mail: martha.morris{at}tufts.edu.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Background:Historic reports on the treatment of pernicious anemia with folic acid suggest that high-level folic acid fortification delays the diagnosis of or exacerbates the effects of vitamin B-12 deficiency, which affects many seniors. This idea is controversial, however, because observational data are few and inconclusive. Furthermore, experimental investigation is unethical.

Objective:We examined the relations between serum folate and vitamin B-12 status relative to anemia, macrocytosis, and cognitive impairment (ie, Digit Symbol-Coding score <34) in senior participants in the 1999–2002 US National Health and Nutrition Examination Survey.

Design:The subjects had normal serum creatinine concentrations and reported no history of stroke, alcoholism, recent anemia therapy, or diseases of the liver, thyroid, or coronary arteries (n = 1459). We defined low vitamin B-12 status as a serum vitamin B-12 concentration <148 pmol/L or a serum methylmalonic acid concentration >210 nmol/L—the maximum of the reference range for serum vitamin B-12–replete participants with normal creatinine.

Results:After control for demographic characteristics, cancer, smoking, alcohol intake, serum ferritin, and serum creatinine, low versus normal vitamin B-12 status was associated with anemia [odds ratio (OR): 2.7; 95% CI: 1.7, 4.2], macrocytosis (OR: 1.8; 95% CI: 1.01, 3.3), and cognitive impairment (OR: 2.5; 95% CI: 1.6, 3.8). In the group with a low vitamin B-12 status, serum folate >59 nmol/L (80th percentile), as opposed to ≤59 nmol/L, was associated with anemia (OR: 3.1; 95% CI: 1.5, 6.6) and cognitive impairment (OR: 2.6; 95% CI: 1.1, 6.1). In the normal vitamin B-12 group, ORs relating high versus normal serum folate to these outcomes were <1.0 (Pinteraction < 0.05), but significantly <1.0 only for cognitive impairment (0.4; 95% CI: 0.2, 0.9).

Conclusion:In seniors with low vitamin B-12 status, high serum folate was associated with anemia and cognitive impairment. When vitamin B-12 status was normal, however, high serum folate was associated with protection against cognitive impairment.

Key Words: Aging • anemia • cognition disorders • folate • fortified food • nutrition surveys • vitamin B-12 deficiency


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Since January of 1998, the US Food and Drug Administration has required the folic acid fortification of all enriched cereal-grain products. The main goal of this program is the prevention of neural tube birth defects (1). Before the program's inception, concern was expressed over the possibility that an increased folic acid intake might delay the diagnosis of vitamin B-12 deficiency or even exacerbate its neurologic and neuropsychiatric effects (2, 3). The elderly are of particular concern because of age-related declines in vitamin absorption and extraction of vitamin B-12 from protein (4-11) and age-related increases in autoimmunity against intrinsic factor or the gastric parietal cells that produce it (11-13).

Fears of harm from fortification to seniors and other Americans with low vitamin B-12 status are based on early case reports of pernicious anemia that detail the alleviation of anemia but the precipitation or exacerbation of neurologic or neuropsychiatric sequelae after folic acid administration—an early form of treatment based on the mistaken idea that a lack of folate was the problem (8, 14). Modern reports on the effects of high folic acid intakes are infrequent because of the rarity of identified cases of vitamin B-12 deficiency affected by oversupplementation or mistreatment with folic acid. However, close study of the original case reports showed no proof that folic acid therapy exacerbated central nervous system (CNS)—related symptoms (15, 16), and modern case reports and studies suggest that folic acid therapy did not cure anemia either (17-20). Such data support the idea that folic acid can be safely added to foods in moderation, but recent editorials have stressed the need for systematic study of the hypothesis that high folate intakes cause harm (21, 22).

Data collected in the most recent National Health and Nutrition Examination Survey (NHANES) afforded us the opportunity to study interrelations between serum folate and vitamin B-12 status in relation to anemia, macrocytosis, and cognitive impairment in the age of folic acid fortification.


   SUBJECTS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Study population
NHANES monitors the nation's health and nutritional status. The survey is currently implemented as a continuous annual survey and uses a complex multistage probability design to select a representative sample of the noninstitutionalized US civilian population. To increase the precision of estimates derived from the survey, adolescents, the elderly, Mexican Americans, and blacks are oversampled. The protocols for conducting the NHANES were approved by the institutional review board of the National Center for Health Statistics, Centers for Disease Control and Prevention, and informed consent was obtained from all participants (23). Consistent with NHANES analytic guidelines, we combined data from the 2 most recent surveys into a single data set (1999–2002) (24).

Trained interviewers used a computer-assisted personal interview system to interview participants in their homes. The participants were also asked to report to a mobile examination center (MEC) to provide further interview data and undergo a physical examination that included phlebotomy. A detailed description of blood collection and processing can be found in the NHANES Phlebotomy Manual (25). Although the survey included people of all ages, we focused our attention on seniors (ie, those aged ≥60 y)—the only group whose cognitive function was assessed.

We excluded seniors with serum creatinine concentrations (based on the Jaffe reaction) indicative of renal dysfunction (ie, men, >131 µmol/L; women, >115 µmol/L) and who reported recent anemia therapy or a history of stroke, heavy alcohol use, or diseases of the liver, kidney, or coronary arteries. Of 3706 senior survey participants, 1684 were eligible, 1078 were ineligible, and eligibility status could not be determined for 944. Complete information for analyses pertaining to anemia was available for 1458 seniors, and complete information for analyses pertaining to cognitive function was available for 1302 seniors.

Assessment of anemia and macrocytosis
Anemia and macrocytosis were assessed on the basis of hemoglobin concentrations and mean cell volumes, which were measured at the mobile examination center laboratory with a MAXM hematology flow cytometer (Beckman Coulter Inc, Fullerton, CA). Anemia was defined according to World Health Organization criteria (ie, hemoglobin <12 g/dL for women and <13 g/dL for men) (26). We defined macrocytosis as a mean cell volume ≥99 fL.

Assessment of cognitive function
The cognitive function of seniors was assessed by using a version of the Digit Symbol-Coding subtest of the Wechsler Adult Intelligence Scale III—a screening test designed to detect cognitive impairment in adults and children (27). In the test, participants copy symbols that are paired with numbers. Using the key provided at the top of the exercise form, the participant draws the symbol under the corresponding number. The score, which declines with age (28), is the number of correct symbols drawn within 120 s. One point is given for each correctly drawn symbol completed within the time limit for a maximum score of 133. Use of the test in the most recent NHANES was based on its reputation as a more sensitive measure of dementia than the Mini-Mental State Examination (29). According to NHANES documentation, aptitudes needed for a high score are response speed, sustained attention, visual spatial skills, associative learning, and memory (29). However, research suggests that speed is the prime determinant of performance on the test (28). We defined cognitive impairment as having attained a test score <34—the 20th percentile of the distribution.

Biochemical measurements
Blood samples were analyzed at the Inorganic Toxicology and Nutrition Branch of the Division of Laboratory Sciences, National Center for Environmental Health. Serum concentrations of folate and vitamin B-12 were measured by using the Quantaphase II Radioassay Kit (Bio-Rad Laboratories, Anaheim, CA). Serum methylmalonic acid (MMA) was measured by gas chromatography–mass spectrometry with cyclohexanol derivatization (30). Serum homocysteine was analyzed by using a commercially available fluorescence polarization immunoassay kit (Abbott Laboratories, Abbott Park, IL) on the Abbott IMx analyzer (31). Serum ferritin was measured by using the QuantaImmune Ferritin IRMA Kit (Bio-Rad Laboratories). Serum creatinine concentration was based on the Jaffe reaction, and serum glucose was determined by using a hexokinase enzymatic method.

Classification of subjects according to vitamin B-12 and folate status
We defined low vitamin B-12 status as a low serum vitamin B-12 or elevated serum MMA concentration. We defined low serum B12 as a value below the conventionally applied cutoff for deficiency of 148 pmol/L. We defined elevated MMA as a serum MMA concentration above the recently published reference range (ie, 60–210 nmol/L) for serum vitamin B-12–replete survey participants with normal serum creatinine concentrations (32). It should be noted that a similar strategy applied to data from the previous NHANES led to a cutoff point of 370 nmol/L (33). Pfeiffer et al (32) attributed the different MMA values obtained in the 2 surveys to differences in the laboratories conducting the analyses and the matrices and methods used. For powerful tests of interactions between vitamin B-12 status and serum folate in relation to anemia, macrocytosis, and cognitive impairment, we used serum folate as a continuous variable. In other analyses we defined high folate status as a serum folate concentration >59 nmol/L—the 80th percentile of the distribution of the seniors.

Statistical analyses
Data analyses were performed by using SUDAAN release 9.0 (Research Triangle Institute, Research Triangle Park, NC) with appropriate 4-y sampling weights to account for the survey's complex sampling design (24). P < 0.05 was considered statistically significant for all tests. Except for the percentages displayed in Table 1Go, and where otherwise indicated, the results were obtained after multivariate adjustment for age, sex, race-ethnicity (as determined by combining responses to questions on race and Hispanic origin;23), education (<high school diploma, high school diploma, >high school diploma), current cigarette smoking status, alcohol intake, self-reported history of cancer, and serum concentrations of creatinine and ferritin.


View this table:
[in this window]
[in a new window]

  		TABLE 1 Characteristics of eligible senior participants in the National Health and Nutrition Examination Survey (1999–2002) by vitamin B-12 status 1

 
We first used SUDAAN PROC REGRESS, SUDAAN PROC CROSSTAB, and SUDAAN PROC RLOGIST to describe the study subjects via least-squares means and proportions and to perform comparisons between those with low and normal vitamin B-12 status.

Our primary data analyses addressed the hypothesis that the effects of vitamin B-12 and folate on study outcomes modified each other. We tested this hypothesis by evaluating statistical interactions between vitamin B-12 status and folate status in relation to macrocytosis, anemia, and cognitive impairment using multivariate models for the 3 outcome variables that included terms for vitamin B-12 category, serum folate (continuous), and their interaction along with the potentially confounding factors identified above (full model). For subjects in the 2 vitamin B-12–status categories, we also used SUDAAN PROC RLOGIST to estimate the odds ratios (ORs) and associated 95% CIs relating high versus normal serum folate to anemia and cognitive impairment. We generated these estimates from both the full multivariate model and a model controlled only for age, sex, and race-ethnicity (basic model). To summarize the interactions we found, we created a single variable with 4 levels (ie, abnormal for serum folate alone, abnormal for vitamin B-12 status alone, abnormal for both vitamins, and normal for both vitamins). We then used multivariate logistic regression models to estimate ORs (95% CI) for anemia and cognitive impairment that compared each abnormal group with the group that was normal for both vitamins.

To shed light on homocysteine's influence on our findings, we used SUDAAN PROC REGRESS and the full model plus terms for diabetes and serum glucose (factors inversely related to homocysteine and cognition) to estimate the multivariate-adjusted prevalence of hyperhomocysteinemia (>13 µmol/L) in each of the 4 B vitamin status categories. We then estimated the ORs (95% CI) relating B vitamin status to anemia and cognitive impairment after controlling for hyperhomocysteinemia.

Finally, we used SUDAAN ROC REGRESS and the full multivariate model to examine the association between serum vitamin B-12 and log-transformed serum folate as well as potential effect modification by both supplement use and low-versus-normal vitamin B-12 status as previously defined. For this purpose we divided subjects according to self-reported supplement use (yes or no) and determined quartiles of serum vitamin B-12 separately for each group. For trend tests and tests of interaction, we modeled serum vitamin B-12 as a continuous variable created by assigning each subject the median of his or her quartile category. We also examined the interaction between supplement use and low-versus-normal vitamin B-12 status as previously defined. To graphically display the association between serum vitamin B-12 and serum folate, we plotted least-square geometric mean serum folate for the serum vitamin B-12 quartile categories.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Subject description: characteristics and study outcomes
The mean age of the participants was 70 ± 0.32 y, and the mean (±SEM) Digit Symbol-Coding score was 49 ± 0.7. Sixty-seven percent reported using dietary supplements, 14% were smokers, and 20.7% had a serum folate concentration >59 nmol/L. Anemia affected 4.5% of the seniors, and 4% were macrocytic. Just under 3% had serum vitamin B-12 concentrations <148 pmol/L, but 25% met our definition of low vitamin B-12 status (ie, serum vitamin B-12 <148 pmol/L or serum MMA >210 nmol/L, the upper limit of the reference range for serum vitamin B-12–replete participants with normal serum creatinine). Seniors with a combination of low vitamin B-12 status and high serum folate concentrations accounted for {approx}4% of the participants.

Subject characteristics in relation to anemia, macrocytosis, and cognitive impairment
After multivariate adjustment, anemia was marginally significantly related to cancer (P = 0.07) and significantly related to age, serum creatinine, non-Hispanic black race-ethnicity, and nonsmoking status. (The masking of anemia by cigarette smoking is consistent with previously reported findings from NHANES II; 34.) Macrocytosis was marginally significantly related to alcohol intake (P = 0.06) and significantly related to age, cigarette smoking, and serum creatinine. Macrocytosis was inversely related to serum ferritin. Cognitive impairment was significantly associated with age, nonwhite race-ethnicity, and not having attained a high school diploma.

Vitamin B-12 status in relation to subject characteristics and study outcomes
Non-Hispanic black subjects were significantly less likely than were non-Hispanic white subjects to have low vitamin B-12 status. Subject characteristics and health problems directly associated with low vitamin B-12 status after multivariate adjustment were age, female sex, not having attained a high school diploma, cigarette smoking, nonuse of dietary supplements, serum creatinine, anemia, macrocytosis, and cognitive impairment (Table 1Go).

Interaction between vitamin B-12 status and serum folate in relation to anemia and cognition
Vitamin B-12 status interacted significantly with serum folate concentration in relation to anemia (P = 0.03) and cognitive impairment (P < 0.001), but not with macrocytosis (P = 0.14). In all subjects combined, high folate status was not significantly associated with macrocytosis (OR: 1.7; 95% CI: 0.7, 4.0).

Among subjects classified as having normal vitamin B-12 status, high serum folate compared with normal serum folate was associated with protection from cognitive impairment. Specifically, the multivariate-adjusted OR (95% CI) relating serum folate >59 nmol/L versus a lower value to cognitive impairment was 0.4 (0.2, 0.9). The OR (95% CI) relating high versus normal serum folate to anemia was 0.6 (0.1, 2.4). Among subjects classified as having low vitamin B-12 status, high serum folate compared with normal serum folate was directly related to both anemia (OR: 3.1; 95% CI: 1.5, 6.6) and cognitive impairment (OR: 2.6; 95% CI: 1.1, 6.1). These estimates were very similar to those generated from the basic model. With the basic model, ORs (95% CI) relating high serum folate to anemia and cognitive impairment were 0.5 (0.2, 1.6) and 0.4 (0.2, 0.7) for the group with normal vitamin B-12 status and 3.0 (1.4, 6.2) and 2.6 (1.1, 6.1) for the group with low vitamin B-12 status.

Data displayed in Table 2Go summarize the interaction between serum folate and vitamin B-12 status in relation to anemia and cognitive impairment and address the potential role of homocysteine in the associations. The tabulated data show that, compared with having normal status for both vitamins, having high serum folate status alone was associated with a reduced prevalence of anemia and a significantly reduced prevalence of cognitive impairment. Furthermore, having low vitamin B-12 status, regardless of serum folate, was associated with a significantly increased prevalence of both anemia and cognitive impairment. The worst combination was low vitamin B-12 status and high serum folate. Specifically, anemia and cognitive impairment were observed {approx}5 times as often in the group with that combination as they were in the group with normal vitamin B-12 status and normal serum folate.


View this table:
[in this window]
[in a new window]

  		TABLE 2 Interaction between vitamin B-12 status and serum folate in relation to anemia and cognitive impairment in eligible senior participants in the National Health and Nutrition Examination Survey (1999–2002) 1

 
After multivariate adjustment, hyperhomocysteinemia was associated with an increased prevalence of both cognitive impairment (OR: 1.9; 95% CI: 1.1, 3.4) and anemia (OR: 2.0; 95% CI: 1.01, 3.8). When hyperhomocysteinemia was controlled for, associations between low vitamin B-12 status alone and study outcomes were slightly reduced in magnitude and marginally statistically significant. However, the ORs relating the combination of low vitamin B-12 status and high serum folate to anemia and cognitive impairment remained large and statistically significant (Table 2Go). Furthermore, although hyperhomocysteinemia was common in the group that was abnormal for both vitamins, the prevalence of hyperhomocysteinemia in that group was significantly lower than that in the group with low vitamin B-12 status but normal serum folate (P < 0.001).

Association between serum vitamin B-12 and serum folate
We observed a direct association (Ptrend < 0.001) between serum vitamin B-12 and serum folate (Figure 1Go) regardless of supplement use (Pinteraction = 0.751) or low-versus-normal vitamin B-12 status as previously defined (Pinteraction = 0.502). Geometric mean serum folate was 34.1 nmol/L (95% CI: 31.6, 36.9) for subjects classified as having a low vitamin B-12 status based on both serum vitamin B-12 and serum MMA. It was 39.2 nmol/L (95% CI: 37.7, 40.8) for subjects with normal vitamin B-12 status, and this difference (P = 0.003) was not significantly affected by supplement use (Pinteraction = 0.228).


Figure 1
View larger version (9K):
[in this window]
[in a new window]

  		FIGURE 1. Association between serum vitamin B-12 and serum folate in senior participants in the National Health and Nutrition Examination Survey (1999–2002) who had no evidence of renal dysfunction or history of stroke, alcoholism, recent anemia therapy, or diseases of the liver, thyroid, or coronary arteries (n = 1457). Points represent least-squares geometric means adjusted for age, sex, race-ethnicity, education, cancer, smoking, alcohol intake, and serum concentrations of ferritin and creatinine for supplement users and nonusers combined. Error bars represent 95% CIs. Quartile categories 1, 2, 3, and 4 are <271, 271–366, 367–484, and >484 pmol/L for supplement users and <208, 208–281, 282–350, and >350 pmol/L for nonusers, respectively. The direct association between serum vitamin B-12 and serum folate (Ptrend < 0.001) did not vary with supplement use (Pinteraction < 0.751).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
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, 3). 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, 16, 19, 35, 36), 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, 40), 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, 16). 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, 43, 44). Despite a marked increase in the folate status of Americans as a result of fortification (45, 46), 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 (47-50) or hyperhomocysteinemia (48, 49, 51-54) 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, 49, 57, 58) 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, 63). 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, 65), 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 (66-70).

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 (72-76) that is particularly helpful in identifying so-called preclinical or subtle cases (77-79). Although the cognitive function test administered in the NHANES is not specific for the cognitive impairment that results from vitamin B-12 deficiency (73, 80-82), 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.


   ACKNOWLEDGMENTS
 
JS was responsible for the study concept. MSM and JS were responsible for the research design. MSM designed and carried out the data analyses and drafted the manuscript. MSM, PFJ, IHR, and JS critically revised the article for important intellectual content. None of the authors had a conflict of interest.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 SUBJECTS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. US Food and Drug Administration, Health and Human Services. Food standards: amendment of standards of identity for enriched grain products to require addition of folic acid (final rule–21 CFR Part 101). Fed Reg 1996;61:8781–97.
  2. Rush D. Periconceptional folate and neural tube defect. Am J Clin Nutr 1994;59(suppl):511S–5S; discussion 515S–6S.[Medline]
  3. Herbert V, Bigaouette J. Call for endorsement of a petition to the Food and Drug Administration to always add vitamin B-12 to any folate fortification or supplement. Am J Clin Nutr 1997;65:572–3.[Free Full Text]
  4. Herbert V. Vitamin B12. In: Ziegler EE, Filer LJ, eds. Present knowledge in nutrition. 7th ed. Washington, DC: International Life Sciences Institute Press, 1996:191–205.
  5. Rothenberg SP. Increasing the dietary intake of folate: pros and cons. Semin Hematol 1999;36:65–74.[Medline]
  6. Koehler KM, Pareo-Tubbeh SL, Romero LJ, Baumgartner RN, Garry PJ. Folate nutrition and older adults: challenges and opportunities. J Am Diet Assoc 1997;97:167–73.[Medline]
  7. Reynolds EH, Bottiglieri T, Laundy M, et al. Subacute combined degeneration with high serum vitamin B12 level and abnormal vitamin B12 binding protein. New cause of an old syndrome. Arch Neurol 1993;50:739–42.
  8. Reynolds EH. Benefits and risks of folic acid to the nervous system. J Neurol Neurosurg Psychiatry 2002;72:567–71.[Abstract/Free Full Text]
  9. Selhub J, Bagley LC, Miller J, Rosenberg IH. B vitamins, homocysteine, and neurocognitive function in the elderly. Am J Clin Nutr 2000;71(suppl):614S–20S.
  10. Saltzman JR, Russell RM. The aging gut. Nutritional issues. Gastroenterol Clin North Am 1998;27:309–24.
  11. Scott JM. Folate and vitamin B12. Proc Nutr Soc 1999;58:441–8.[Medline]
  12. Carmel R. Cobalamin, the stomach, and aging. Am J Clin Nutr 1997;66:750–9.[Abstract/Free Full Text]
  13. FAO/WHO. Human vitamin and mineral requirements. In: Human vitamin and mineral requirements. Bangkok, Thailand: FAO/WHO, 2002.
  14. Scott JM, Weir DG. The methyl folate trap. A physiological response in man to prevent methyl group deficiency in kwashiorkor (methionine deficiency) and an explanation for folic-acid induced exacerbation of subacute combined degeneration in pernicious anaemia. Lancet 1981;2:337–40.
  15. Dickinson CJ. No reliable evidence that folate is harmful in B-12 deficiency. Br Med J 1995;311:949.[Medline]
  16. Dickinson CJ. Does folic acid harm people with vitamin B12 deficiency? QJM 1995;88:357–64.[Abstract/Free Full Text]
  17. Mills JL, Von Kohorn I, Conley MR, et al. Low vitamin B-12 concentrations in patients without anemia: the effect of folic acid fortification of grain. Am J Clin Nutr 2003;77:1474–7.[Abstract/Free Full Text]
  18. Dhar M, Bellevue R, Carmel R. Pernicious anemia with neuropsychiatric dysfunction in a patient with sickle cell anemia treated with folate supplementation. N Engl J Med 2003;348:2204–7.[Free Full Text]
  19. Metz J, McNeil AR, Levin M. The relationship between serum cobalamin concentration and mean red cell volume at varying concentrations of serum folate. Clin Lab Haematol 2004;26:323–5.[Medline]
  20. Liu S, West R, Randell E, et al. A comprehensive evaluation of food fortification with folic acid for the primary prevention of neural tube defects. BMC Pregnancy Childbirth 2004;4:20.[Medline]
  21. Shane B. Folate fortification: enough already? Am J Clin Nutr 2003;77:8–9.[Free Full Text]
  22. Rosenberg IH. Science-based micronutrient fortification: which nutrients, how much, and how to know? Am J Clin Nutr 2005;82:279–80.[Free Full Text]
  23. National Center for Health Statistics, National Health and Nutrition Examination Survey. Version current 1 September 2005. Internet: http://www.cdc.gov/nchs/data/nhanes/gendoc.pdf (accessed 1 September 2005).
  24. National Center for Health Statistics, National Health and Nutrition Examination Survey. Version current 31 December 2005. Internet: http://www.cdc.gov/nchs/data/nhanes/nhanes_03_04/nhanes_analytic_guidelines_dec_2005.pdf (accessed 31 December 2005).
  25. National Center for Health Statistics, National Health and Nutrition Examination Survey. Version current 1 September 2005. Internet: http://www.cdc.gov/nchs/data/nhanes/frequency/phdoc.pdf (accessed 1 September 2005).
  26. WHO/UNICEF/UNU, eds. Iron deficiency anaemia assessment, prevention, and control: a guide for programme managers. Geneva, Switzerland: World Health Organization, 2001.
  27. Wechsler D. Wechsler Adult Intelligence Scale—III. San Antonio, TX: The Psychological Corporation, 1997.
  28. Joy S, Kaplan E, Fein D. Speed and memory in the WAIS-III Digit Symbol–coding subtest across the adult lifespan. Arch Clin Neuropsychol 2004;19:759–67.[Medline]
  29. National Center for Health Statistics, National Health and Nutrition Examination Survey. Household Interview Questionnaire. Version current 1 September 2005. Internet: http://www.cdc.gov/nchs/data/nhanes/nhanes_01_02/cfq_b_doc.pdf (accessed 1 September 2005).
  30. Pfeiffer CM, Gunter EW. Automated assay for methylmalonic acid (MMA) in plasma. Clin Chem 1999;45:A166 (abstr 593).
  31. Shipchandler MT, Moore EG. Rapid, fully automated measurement of plasma homocyst(e)ine with the Abbott IMx analyzer. Clin Chem 1995;41:991–4.[Abstract/Free Full Text]
  32. Pfeiffer CM, Caudill SP, Gunter EW, Osterloh J, Sampson EJ. Biochemical indicators of B vitamin status in the US population after folic acid fortification: results from the National Health and Nutrition Examination Survey 1999–2000. Am J Clin Nutr 2005;82:442–50.[Abstract/Free Full Text]
  33. Morris MS, Jacques PF, Rosenberg IH, Selhub J. Elevated serum methylmalonic acid concentrations are common among elderly Americans. J Nutr 2002;132:2799–803.[Abstract/Free Full Text]
  34. Nordenberg D, Yip R, Binkin NJ. The effect of cigarette smoking on hemoglobin levels and anemia screening. JAMA 1990;264:1556–9.[Abstract]
  35. Kelly P, McPartlin J, Goggins M, Weir DG, Scott JM. Unmetabolized folic acid in serum: acute studies in subjects consuming fortified food and supplements. Am J Clin Nutr 1997;65:1790–5.[Abstract/Free Full Text]
  36. Mills JL. Fortification of foods with folic acid–how much is enough? N Engl J Med 2000;342:1442–5.[Free Full Text]
  37. Weir DG, Scott JM. Brain function in the elderly: role of vitamin B12 and folate. Br Med Bull 1999;55:669–82.[Abstract/Free Full Text]
  38. Koury MJ, Ponka P. New insights into erythropoiesis: the roles of folate, vitamin B12, and iron. Annu Rev Nutr 2004;24:105–31.[Medline]
  39. Wald NJ, Law MR, Morris JK, Wald DS. Quantifying the effect of folic acid. Lancet 2001;358:2069–73.[Medline]
  40. Oakley GP, Jr. Let's increase folic acid fortification and include vitamin B-12. Am J Clin Nutr 1997;65:1889–90.[Free Full Text]
  41. Carmel R. Current concepts in cobalamin deficiency. Annu Rev Med 2000;51:357–75.[Medline]
  42. Food and Nutrition Board, Institute of Medicine. Dietary reference intakes for thiamin, riboflavin, niacin, vitamin B6, folate, vitamin B12, pantothenic acid, biotin and choline. Washington, DC: National Academy Press, 1998.
  43. Oakley GP Jr. Vitamin B-12 and folic acid supplementation (reply to V Herbert). Am J Clin Nutr 1997;66:1479–80.[Medline]
  44. Tucker KL, Mahnken B, Wilson PW, Jacques P, Selhub J. Folic acid fortification of the food supply. Potential benefits and risks for the elderly population. JAMA 1996;276:1879–85.
  45. Choumenkovitch SF, Jacques PF, Nadeau MR, Wilson PW, Rosenberg IH, Selhub J. Folic acid fortification increases red blood cell folate concentrations in the Framingham study. J Nutr 2001;131:3277–80.[Abstract/Free Full Text]
  46. Choumenkovitch SF, Selhub J, Wilson PW, Rader JI, Rosenberg IH, Jacques PF. Folic acid intake from fortification in United States exceeds predictions. J Nutr 2002;132:2792–8.[Abstract/Free Full Text]
  47. D'Anci KE, Rosenberg IH. Folate and brain function in the elderly. Curr Opin Clin Nutr Metab Care 2004;7:659–64.[Medline]
  48. Ramos MI, Allen LH, Mungas DM, et al. Low folate status is associated with impaired cognitive function and dementia in the Sacramento Area Latino Study on Aging. Am J Clin Nutr 2005;82:1346–52.[Abstract/Free Full Text]
  49. Ravaglia G, Forti P, Maioli F, et al. Homocysteine and folate as risk factors for dementia and Alzheimer disease. Am J Clin Nutr 2005;82:636–43.[Abstract/Free Full Text]
  50. Bryan J, Calvaresi E, Hughes D. Short-term folate, vitamin B-12 or vitamin B-6 supplementation slightly affects memory performance but not mood in women of various ages. J Nutr 2002;132:1345–56.[Abstract/Free Full Text]
  51. Garcia A, Zanibbi K. Homocysteine and cognitive function in elderly people. CMAJ 2004;171:897–904.[Abstract/Free Full Text]
  52. Ellinson M, Thomas J, Patterson A. A critical evaluation of the relationship between serum vitamin B, folate and total homocysteine with cognitive impairment in the elderly. J Hum Nutr Diet 2004;17:371–83 (quiz 385–7).[Medline]
  53. Schafer JH, Glass TA, Bolla KI, Mintz M, Jedlicka AE, Schwartz BS. Homocysteine and cognitive function in a population-based study of older adults. J Am Geriatr Soc 2005;53:381–8.[Medline]
  54. Garcia A, Haron Y, Pulman K, Hua L, Freedman M. Increases in homocysteine are related to worsening of stroop scores in healthy elderly persons: a prospective follow-up study. J Gerontol A Biol Sci Med Sci 2004;59:1323–7.[Abstract/Free Full Text]
  55. Morris MC, Evans DA, Bienias JL, et al. Dietary folate and vitamin B12 intake and cognitive decline among community-dwelling older persons. Arch Neurol 2005;62:641–5.[Abstract/Free Full Text]
  56. Corrada MM, Kawas CH, Hallfrisch J, Muller D, Brookmeyer R. Reduced risk of Alzheimer's disease with high folate intake: the Baltimore Longitudinal Study of Aging. Alz Dem J Alz Assoc 2005;1:11–18.
  57. Kado DM, Karlamangla AS, Huang MH, et al. Homocysteine versus the vitamins folate, B6, and B12 as predictors of cognitive function and decline in older high-functioning adults: MacArthur studies of successful aging. Am J Med 2005;118:161–7.[Medline]
  58. Tucker KL, Qiao N, Scott T, Rosenberg I, Spiro A III. High homocysteine and low B vitamins predict cognitive decline in aging men: the Veterans Affairs Normative Aging Study. Am J Clin Nutr 2005;82:627–35.[Abstract/Free Full Text]
  59. Weir DG, Molloy AM. Microvascular disease and dementia in the elderly: are they related to hyperhomocysteinemia? Am J Clin Nutr 2000;71:859–60.[Free Full Text]
  60. Roman GC. Vascular dementia may be the most common form of dementia in the elderly. J Neurol Sci 2002;203–204:7–10.
  61. Lokk J. News and views on folate and elderly persons. J Gerontol A Biol Sci Med Sci 2003;58:354–61.[Medline]
  62. Scott JM, Weir DG, Molloy A, McPartlin J, Daly L, Kirke P. Folic acid metabolism and mechanisms of neural tube defects. CIBA Found Symp 1994;181:180–7; discussion 187–91.[Medline]
  63. Shane B, Stokstad EL. Vitamin B12-folate interrelationships. Annu Rev Nutr 1985;5:115–41.[Medline]
  64. Lumb M, Perry J, Deacon R, Chanarin I. Changes in tissue folates accompanying nitrous oxide-induced inactivation of vitamin B12 in the rat. Am J Clin Nutr 1981;34:2412–7.[Abstract/Free Full Text]
  65. Duthie SJ, Whalley LJ, Collins AR, Leaper S, Berger K, Deary IJ. Homocysteine, B vitamin status, and cognitive function in the elderly. Am J Clin Nutr 2002;75:908–13.[Abstract/Free Full Text]
  66. Lindenbaum J, Rosenberg IH, Wilson PW, Stabler SP, Allen RH. Prevalence of cobalamin deficiency in the Framingham elderly population. Am J Clin Nutr 1994;60:2–11.[Abstract/Free Full Text]
  67. Carmel R, Green R, Jacobsen DW, Rasmussen K, Florea M, Azen C. Serum cobalamin, homocysteine, and methylmalonic acid concentrations in a multiethnic elderly population: ethnic and sex differences in cobalamin and metabolite abnormalities. Am J Clin Nutr 1999;70:904–10.[Abstract/Free Full Text]
  68. Bjorkegren K, Svardsudd K. Serum cobalamin, folate, methylmalonic acid and total homocysteine as vitamin B12 and folate tissue deficiency markers amongst elderly Swedes—a population-based study. J Intern Med 2001;249:423–32.[Medline]
  69. Herrmann W, Schorr H, Bodis M, et al. Role of homocysteine, cystathionine and methylmalonic acid measurement for diagnosis of vitamin deficiency in high-aged subjects. Eur J Clin Invest 2000;30:1083–9.[Medline]
  70. Stabler SP, Allen RH, Fried LP, et al. Racial differences in prevalence of cobalamin and folate deficiencies in disabled elderly women. Am J Clin Nutr 1999;70:911–9.[Abstract/Free Full Text]
  71. Andres E, Loukili NH, Noel E, et al. Vitamin B12 (cobalamin) deficiency in elderly patients. CMAJ 2004;171:251–9.[Abstract/Free Full Text]
  72. Herrmann W, Obeid R, Schorr H, Geisel J. Functional vitamin B12 deficiency and determination of holotranscobalamin in populations at risk. Clin Chem Lab Med 2003;41:1478–88.[Medline]
  73. Clarke R, Refsum H, Birks J, et al. Screening for vitamin B-12 and folate deficiency in older persons. Am J Clin Nutr 2003;77:1241–7.[Abstract/Free Full Text]
  74. Mason JB. Biomarkers of nutrient exposure and status in one-carbon (methyl) metabolism. J Nutr 2003;133(suppl):941S–7S.[Abstract/Free Full Text]
  75. Carmel R, Green R, Rosenblatt DS, Watkins D. Update on cobalamin, folate, and homocysteine. Hematology Am Soc Hematol Educ Program 2003:62–81.
  76. Elin RJ, Winter WE. Methylmalonic acid: a test whose time has come? Arch Pathol Lab Med 2001;125:824–7.[Medline]
  77. Carmel R. Subtle cobalamin deficiency. Ann Intern Med 1996;124:338–40.[Free Full Text]
  78. Carmel R. Subtle and atypical cobalamin deficiency states. Am J Hematol 1990;34:108–14.[Medline]
  79. Carmel R, Sinow RM, Karnaze DS. Atypical cobalamin deficiency. Subtle biochemical evidence of deficiency is commonly demonstrable in patients without megaloblastic anemia and is often associated with protein-bound cobalamin malabsorption. J Lab Clin Med 1987;109:454–63.
  80. Allen RH, Stabler SP, Lindenbaum J. Relevance of vitamins, homocysteine and other metabolites in neuropsychiatric disorders. Eur J Pediatr 1998;157(suppl):S122–6.[Medline]
  81. Stabler SP. Vitamins, homocysteine, and cognition. Am J Clin Nutr 2003;78:359–60.[Free Full Text]
  82. Eastley R, Wilcock GK, Bucks RS. Vitamin B12 deficiency in dementia and cognitive impairment: the effects of treatment on neuropsychological function. Int J Geriatr Psychiatry 2000;15:226–33.[Medline]
Received for publication January 12, 2006. Accepted for publication August 3, 2006.


Related articles in AJCN:

Folic acid fortification: the good, the bad, and the puzzle of vitamin B-12
A David Smith
AJCN 2007 85: 3-5. [Full Text]  



This article has been cited by other articles:


Home page
 J. Nutr.Home page
A. D. Dangour, E. Breeze, R. Clarke, P. S. Shetty, R. Uauy, and A. E. Fletcher
Plasma Homocysteine, but Not Folate or Vitamin B-12, Predicts Mortality in Older People in the United Kingdom
J. Nutr., June 1, 2008; 138(6): 1121 - 1128.
[Abstract] [Full Text] [PDF]

Home page
 Clin. Chem.Home page
P. M. Ueland and S. Hustad
Homocysteine and Folate Status in an Era of Folic Acid Fortification: Balancing Benefits, Risks, and B-vitamins
Clin. Chem., May 1, 2008; 54(5): 779 - 781.
[Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
A D. Smith, Y.-I. Kim, and H. Refsum
Is folic acid good for everyone?
Am. J. Clinical Nutrition, March 1, 2008; 87(3): 517 - 533.
[Abstract] [Full Text] [PDF]

Home page
 J. Nutr.Home page
A. H. Lichtenstein, H. Rasmussen, W. W. Yu, S. R. Epstein, and R. M. Russell
Modified MyPyramid for Older Adults
J. Nutr., January 1, 2008; 138(1): 5 - 11.
[Abstract] [Full Text] [PDF]

Home page
 Proc. Natl. Acad. Sci. USAHome page
J. Selhub, M. S. Morris, and P. F. Jacques
In vitamin B12 deficiency, higher serum folate is associated with increased total homocysteine and methylmalonic acid concentrations
PNAS, December 11, 2007; 104(50): 19995 - 20000.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
R. Clarke, J. Birks, E. Nexo, P. M Ueland, J. Schneede, J. Scott, A. Molloy, and J. G. Evans
Low vitamin B-12 status and risk of cognitive decline in older adults
Am. J. Clinical Nutrition, November 1, 2007; 86(5): 1384 - 1391.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
L. Hoey, H. McNulty, N. Askin, A. Dunne, M. Ward, K. Pentieva, J. Strain, A. M Molloy, C. A Flynn, and J. M Scott
Effect of a voluntary food fortification policy on folate, related B vitamin status, and homocysteine in healthy adults
Am. J. Clinical Nutrition, November 1, 2007; 86(5): 1405 - 1413.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
I. Brouwer and P. Verhoef
Folic acid fortification: is masking of vitamin B-12 deficiency what we should really worry about?
Am. J. Clinical Nutrition, October 1, 2007; 86(4): 897 - 898.
[Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
L. B Bailey
The rise and fall of blood folate in the United States emphasizes the need to identify all sources of folic acid
Am. J. Clinical Nutrition, September 1, 2007; 86(3): 528 - 530.
[Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
C. M Pfeiffer, C. L Johnson, R. B Jain, E. A Yetley, M. F. Picciano, J. I Rader, K. D Fisher, J. Mulinare, and J. D Osterloh
Trends in blood folate and vitamin B-12 concentrations in the United States, 1988 2004
Am. J. Clinical Nutrition, September 1, 2007; 86(3): 718 - 727.
[Abstract] [Full Text] [PDF]

Home page
 BMJHome page
E. Reynolds
Clarify the neurological risks
BMJ, July 28, 2007; 335(7612): 171 - 171.
[Full Text] [PDF]

Home page
 NEJMHome page
P. De Wals, F. Tairou, M. I. Van Allen, S.-H. Uh, R. B. Lowry, B. Sibbald, J. A. Evans, M. C. Van den Hof, P. Zimmer, M. Crowley, et al.
Reduction in Neural-Tube Defects after Folic Acid Fortification in Canada
N. Engl. J. Med., July 12, 2007; 357(2): 135 - 142.
[Abstract] [Full Text] [PDF]

Home page
 Proc. Natl. Acad. Sci. USAHome page
W. He, H. Wang, L. C. Hartmann, J.-X. Cheng, and P. S. Low
In vivo quantitation of rare circulating tumor cells by multiphoton intravital flow cytometry
PNAS, July 10, 2007; 104(28): 11760 - 11765.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
R. J Berry, H. K Carter, and Q. Yang
Cognitive impairment in older Americans in the age of folic acid fortification
Am. J. Clinical Nutrition, July 1, 2007; 86(1): 265 - 267.
[Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
M. S. Morris, P. F Jacques, I. H Rosenberg, and J. Selhub
Reply to RJ Berry et al
Am. J. Clinical Nutrition, July 1, 2007; 86(1): 267 - 268.
[Full Text] [PDF]

Home page
 Am. J. Clin. Nutr.Home page
A D. Smith
Reply to RJ Berry et al
Am. J. Clinical Nutrition, July 1, 2007; 86(1