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Original Contribution |

Dietary Folate and Vitamin B12 Intake and Cognitive Decline Among Community-Dwelling Older Persons FREE

Martha Clare Morris, ScD; Denis A. Evans, MD; Julia L. Bienias, ScD; Christine C. Tangney, PhD; Liesi E. Hebert, ScD; Paul A. Scherr, PhD, ScD; Julie A. Schneider, MD
[+] Author Affiliations

Author Affiliations: Rush Institute for Healthy Aging (Drs Morris, Evans, Bienias, and Hebert), Departments of Internal Medicine (Drs Morris, Evans, Bienias, and Hebert), Preventive Medicine (Dr Morris), and Clinical Nutrition (Dr Tangney), and Rush Alzheimer’s Disease Center (Dr Schneider), Rush University Medical Center, Chicago, Ill; and Division of Adult and Community Health, Centers for Disease Control and Prevention, Atlanta, Ga (Dr Scherr).


Arch Neurol. 2005;62(4):641-645. doi:10.1001/archneur.62.4.641.
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Published online

Background  Deficiencies in folate and vitamin B12 have been associated with neurodegenerative disease.

Objective  To examine the association between rates of age-related cognitive change and dietary intakes of folate and vitamin B12.

Design  Prospective study performed from 1993 to 2002.

Setting  Geographically defined biracial community in Chicago, Ill.

Participants  A total of 3718 residents, 65 years and older, who completed 2 to 3 cognitive assessments and a food frequency questionnaire.

Main Outcome Measure  Change in cognitive function measured at baseline and 3-year and 6-year follow-ups, using the average z score of 4 tests: the East Boston Tests of immediate and delayed recall, the Mini-Mental State Examination, and the Symbol Digit Modalities Test.

Results  High folate intake was associated with a faster rate of cognitive decline in mixed models adjusted for multiple risk factors. The rate of cognitive decline among persons in the top fifth of total folate intake (median, 742 μg/d) was more than twice that of those in the lowest fifth of intake (median, 186 μg/d), a statistically significant difference of 0.02 standardized unit per year (P = .002). A faster rate of cognitive decline was also associated with high folate intake from food (P for trend = .04) and with folate vitamin supplementation of more than 400 μg/d compared with nonusers (β = −.03, P<.001). High total B12 intake was associated with slower cognitive decline only among the oldest participants.

Conclusions  High intake of folate may be associated with cognitive decline in older persons. These unexpected findings call for further study of the cognitive implications of high levels of dietary folate in older populations.

Vitamin B12 and folate have long been studied in relation to neurologic function. In addition to the well-established importance of vitamin B12 deficiency to neurologic disease, a number of cross-sectional studies observed lower cognition among persons with insufficient serum or plasma levels of vitamin B1213 and folate.1,4 Furthermore, inadequate levels of either vitamin can lead to elevated plasma homocysteine levels,58 which may be independently associated with Alzheimer disease,9,10 thus raising interest in these vitamins as a means of prevention. As of January 1998, the US Food and Drug Administration required the fortification of grain products with folic acid11 to prevent neural tube defects. The long-term effects of the program on other health outcomes or in special populations are not yet known. We examined the associations of dietary folate and vitamin B12 with 6-year cognitive change in a 1993 to 2002 study in Chicago, Ill.

SUBJECTS

Subjects were participants of the Chicago Health and Aging Project (CHAP), an ongoing study of residents 65 years and older from a geographically defined biracial population.12 A census identified 7813 age-eligible residents of whom 6158 participated in home interviews that included 4 cognitive tests. Of 4930 surviving participants, 4390 (89%) completed the food frequency questionnaire (FFQ) and at least 1 of the 3- and 6-year follow-up interviews. We eliminated 217 persons because of potentially invalid FFQ data and 460 persons whose FFQ was completed more than 2.5 years after baseline, leaving 3718 persons for analysis of cognitive change with a median follow-up of 5.5 years. Three cognitive assessments were obtained for 1946 persons with a median follow-up of 6.3 years. Dietary assessments occurred a median of 1.2 years after baseline, most (86%) before the January 1998 folate fortification. Ninety-one percent had at least 1 follow-up cognitive assessment after this date. The institutional review board of Rush University Medical Center, Chicago, approved the study, and all participants gave written informed consent.

DIETARY ASSESSMENTS

Diet was assessed using a modified Harvard FFQ13 that inquired about usual intake during the past year of 139 food items and vitamin supplements. Post-1997 estimates of folate intake reflect the folate fortification. Nutrient composition for each food was multiplied by frequency of intake and summed over all food items to estimate daily nutrient intake. All nutrients were energy adjusted using the regression residual method.14 In a validation study of 232 randomly selected CHAP participants using repeated 24-hour dietary recalls as a comparison, correlations were 0.70 and 0.50 for total folate reproducibility and validity, respectively, and 0.50 and 0.38 for total vitamin B12.15

COGNITIVE ASSESSMENT

The 4 cognitive tests included the East Boston Tests of immediate and delayed recall (0 to 12 ideas recalled),16 the Mini-Mental State Examination (0 to 30 correct items),17 and the Symbol Digit Modalities Test18 of perceptual speed and attention (0 to 96 correct items). We computed z scores for the 4 tests and averaged the scores for a global measure of cognitive function.

OTHER COVARIATES

Nondietary variables were collected at participants’ baseline interview and included the following: age (in years), sex, race (black or white), education (in years), cigarette smoking (ever or never), smoking pack-years, number of depressive symptoms,19 heart disease (self-reported history of myocardial infarction, digitalis use, or angina pectoris20), hypertension (self-reported history or measured blood pressure ≥160 mm Hg systolic or ≥95 mm Hg diastolic or use of antihypertensive medications), stroke history (self-report), and diabetes mellitus (self-report or antidiabetic medication use). Information on medications was based on interviewer inspection.

STATISTICAL ANALYSIS

We used mixed-effects21 models in SAS statistical software22 to estimate effects of vitamin B12 and folate on the annual rate of change in cognitive score. The model explicitly accounts for individual differences in initial level of cognition and its correlation with rate of change. Energy-adjusted folate and vitamin B12 intakes were modeled in quintiles. Other energy-adjusted nutrients were modeled as continuous log-transformed variables. Model coefficients (β) represent the difference in slopes (rates of cognitive change per year) for an upper quintile of intake compared with the referent lowest quintile. Effect modification was examined by including terms in the model for all 2-way and 3-way interaction terms among the covariate, intake of folate or vitamin B12, and time.

Multivitamins were consumed by 33% of the analyzed cohort of 3718 persons. Folic acid (folate) and cobalamin (vitamin B12) supplementation were mostly through multivitamins (99% and 97%, respectively), with use of an individual folic acid vitamin indicated in 10 persons and cobalamin in 42 persons.

Persons with high intake of total folate (from food and supplements) tended to have a more favorable risk profile for cognitive change than persons with low intake, with more years of education, higher baseline cognitive scores, and greater consumption of vitamin E and vitamin C (Table 1>). They were also less likely to have a history of heart disease or hypertension. The mean cognitive score at baseline for the cohort was 0.18 (range, −3.5 to 1.6), with an overall decline in score of 0.04 standardized unit per year.

Table Graphic Jump LocationTable 1. Baseline Characteristics of 3718 Participants of the Chicago Health and Aging Project, 1993-2002*

Persons with higher intake of total folate had a marginally significantly faster rate of cognitive decline in the age-adjusted model (Table 2>). With further adjustment for demographic factors, education, and intakes of vitamin E in food and total vitamin C, the rate differences for persons in the fourth and fifth quintiles compared with the first quintile were increased and statistically significant (Table 2>). The estimates changed little with additional adjustment for cardiovascular factors, including history of stroke, heart disease, hypertension, diabetes, smoking, alcohol use, and depressive symptoms (β = −.01, P = .09 for the fourth quintile; and β = −.02, P = .005 for the fifth quintile). The estimates also did not change when we added control for total intakes of vitamin B6, thiamin, and riboflavin in separate multiple-adjusted models.

Table Graphic Jump LocationTable 2. Difference in the Estimated Rate of Change in Cognitive Score During 6 Years by Quintile of Folate Intake Among 3718 Participants of the Chicago Health and Aging Project, 1993-2002

Food intake of folate was not associated with cognitive change in the age-adjusted model, but with multiple adjustment, higher intake was associated with faster cognitive decline (Table 2>). The rate difference for persons in the fifth quintile (median intake, 382 μg/d) compared with the first quintile (median intake, 175 μg/d) was statistically significant (Table 2>). When we further adjusted for the cardiovascular factors, the difference in rates for persons in the fifth quintile compared with the first quintile was reduced to −0.01 (P = .07).

In an analysis restricted to 2546 persons who did not take a multivitamin, the multiple-adjusted effect estimate for the fifth quintile for food intake of folate was β = −.02 (P = .06). In further analyses of the folic acid dose from vitamin supplements, persons who consumed in excess of 400 μg/d had a significantly faster rate of cognitive decline than supplement nonusers (Table 3>).

Table Graphic Jump LocationTable 3. Differences in the Rate of Cognitive Change (β) During 6 Years by Dose Level of Folate Vitamin Supplementation, Chicago Health and Aging Project, 1993-2002

Intake of vitamin B12, with or without vitamin supplementation, was not significantly associated with cognitive change in the age- or multiple-adjusted models or with adjustment for folate intake. However, when we examined the possibility of interactive effects, we observed a small, statistically significant interaction between total intake of vitamin B12 and older age (P for interaction = .009). In the multiple-adjusted model that included folate intake, the rate of decline for an average 80-year-old who consumed a supplemental dose of 20 μg/d of vitamin B12 was 25% slower than the rate of a similar person who consumed the recommended dietary allowance of 2.4 μg/d. By contrast, the estimated rates of decline for an average 70-year-old did not differ by vitamin B12 intake.

To investigate the possibility that high folate intake might be a marker of poor health, we first restricted the analyses to 2706 persons who reported good or excellent health at baseline, but there were no appreciable differences in the estimated effects (multiple-adjusted β = −.02, P = .007 for fifth quintile of folate intake). Next, we reanalyzed the data after excluding persons from the entire cohort whose baseline cognitive scores were in the bottom 15% of the distribution, and again, there were no material changes in the effect estimates (fifth quintile β = −.02, P = .005).

In this large population-based study of older persons, we observed slower decline in cognitive function among persons with higher intake of vitamin B12, but only among the oldest participants. Unexpectedly, persons with high intake of folate had faster rate of decline.

Vitamin B12 plays an important role in the maintenance of neuronal cell function, and deficiency in this nutrient can result in neurologic damage characterized in part by memory loss and confusion.23 Vitamin B12 deficiency is particularly a problem in older populations because of decreased dietary absorption.24 Except for some malabsorption syndromes, serum vitamin B12 is positively associated with intake, particularly when the intake is from multivitamins or fortified food.25 These well-established relations are supportive of our finding of slower cognitive decline with high intake of vitamin B12 among the oldest participants.

The observed association between high folate intake and cognitive change was opposite to our hypothesized direction. We expected a slower rate of decline because low folate status is a cause of elevated homocysteine levels,57 which some9,10 but not all studies26 have found to be associated with increased risk of Alzheimer disease, the primary reason for cognitive decline. Consistent with the folate-homocysteine relationship, deficient folate intake levels would be the relevant range for cognitive decline, but this is likely a rare occurrence in the United States since the folic acid fortification program.

Previous studies on this topic are not comparable to the present study in 2 important ways. First, most studies measured cognition only at one point and cannot distinguish between lifetime differences in cognitive ability and decline related to aging. Second, none of these studies measured the association with cognition after the folic acid fortification program, whereas 91% of the analyzed CHAP participants had follow-up cognitive assessments during this period.

The CHAP study has a number of strengths that lend confidence to the validity of the observed associations. The large number of participants and the longitudinal analytic design allowed for the detection of small within-person cognitive change and the ability to separate baseline cognitive level from age-related decline. The observed associations did not appear to be due to differences in health or poor recall of diet. Notably, the known confounders were each associated with higher folate intake and slower cognitive decline; thus, the potential for residual confounding is in the opposite direction of the folate association. The validity of the findings is further supported by the specificity of the associations with vitamin B12 and folate and not with other B vitamins, even though they commonly occur together in food and supplements.

A limitation of the study is the reliance on the FFQ as the sole measure of intake, with no corresponding measure of tissue concentrations of these micronutrients, which prevents examination of whether vitamin deficiencies can account for the results. In addition, due to the potential for spurious associations in an observational study design, we must caution against a causal interpretation of the findings.

The mechanism by which high folate intake may increase cognitive decline is not clear. One possibility is that high intake may be masking unrecognized vitamin B12 deficiency. This complication was previously suggested as a potential pitfall of folic acid supplementation in food27 and multivitamins.28 There was also the unresolved question of whether folic acid may exacerbate the neurologic syndrome associated with vitamin B12 deficiency.29 Such concerns were instrumental in the recommended upper limit for folate intake of 1000 μg/d.30 With widespread multivitamin use and folic acid fortification, it is likely that a significant percentage of the population is consuming more than the upper limit and well above the dietary reference intake of 400 μg/d.30 The possibility that high intake of folic acid through multivitamins and fortified food may have deleterious effects on the health of the older population warrants further study.

Correspondence: Martha Clare Morris, ScD, Rush Institute for Healthy Aging, 1645 W Jackson, Suite 675, Chicago, IL 60612 (Martha_C_Morris@rush.edu).

Accepted for Publication: November 29, 2004.

Author Contributions:Study concept and design: Morris, Evans, and Tangney. Acquisition of data: Morris, Evans, Tangney, and Hebert. Analysis and interpretation of data: Morris, Bienias, Tangney, Hebert, Scherr, and Schneider. Drafting of the manuscript: Morris and Bienias. Critical revision of the manuscript for important intellectual content: Morris, Evans, Tangney, Hebert, Scherr, and Schneider. Statistical analysis: Morris, Bienias, Tangney, Hebert, and Scherr. Obtained funding: Morris and Evans. Administrative, technical, and material support: Morris, Evans, and Schneider. Study supervision: Morris.

Funding/Support: This study was supported by grants AG11101 and AG13170 from the National Institute on Aging, Bethesda, Md.

Acknowledgments: We gratefully acknowledge the work of study coordinators Cheryl Bibbs, BA, Michelle Bos, BA, Jennifer Tarpey, BS, and Flavio Lamorticella, BA, theirstaffs, and the analytic programmer Woojeong Bang, MS. We also thank Walter Willett, MD, DrPh, of Harvard University, Cambridge, Mass, and Katherine Tucker, PhD, of Tufts University, Boston, Mass, for invaluable advice on the analytic investigation of these complex nutrient associations.

Goodwin  JSGoodwin  JMGarry  PJ Association between nutritional status and cognitive functioning in a healthy elderly population. JAMA 1983;2492917- 2921
PubMed
Bernard  MANakonezny  PAKashner  TM The effect of vitamin B12 deficiency on older veterans and its relationship to health. J Am Geriatr Soc 1998;461199- 1206
PubMed
Louwman  MWvan Dusseldorp  Mvan de Vijver  FJ  et al.  Signs of impaired cognitive function in adolescents with marginal cobalamin status. Am J Clin Nutr 2000;72762- 769
PubMed
Lindeman  RDRomero  LJKoehler  KM  et al.  Serum vitamin B12, C and folate concentrations in the New Mexico elder health survey: correlations with cognitive and affective functions. J Am Coll Nutr 2000;1968- 76
PubMed
Carmel  RGreen  RJacobsen  DWRasmussen  KFlorea  MAzen  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;70904- 910
PubMed
Stabler  SPAllen  RHFried  LP  et al.  Racial differences in prevalence of cobalamin and folate deficiencies in disabled elderly women. Am J Clin Nutr 1999;70911- 919
PubMed
Riggs  KMSpiro  ATucker  KLRush  D Relations of vitamin B-12, vitamin B-6, folate, and homocysteine to cognitive performance in the Normative Aging Study. Am J Clin Nutr 1996;63306- 314
PubMed
Duthie  SJWhalley  LJCollins  ARLeaper  SBerger  KDeary  IJ Homocysteine, B vitamin status, and cognitive function in the elderly. Am J Clin Nutr 2002;75908- 913
PubMed
Seshadri  SBeiser  ASelhub  J  et al.  Plasma homocysteine as a risk factor dementia and Alzheimer's disease. N Engl J Med 2002;346476- 483
PubMed
Clarke  RSmith  ADJobst  KARefsum  HSutton  LUeland  PM Folate, vitamin B12, and serum total homocysteine levels in confirmed Alzheimer disease. Arch Neurol 1998;551449- 1455
PubMed
Quinlivan  EPGregory  JF  III Effect of food fortification on folic acid intakes in the United States. Am J Clin Nutr 2003;77221- 226
PubMed
Evans  DABennett  DAWilson  RS  et al.  Incidence of Alzheimer's disease in a biracial urban community: relation to apolipoprotein E allele status. Arch Neurol 2003;60185- 189
PubMed
Morris  MCColditz  GAEvans  DA Response to a mail nutritional survey in an older bi-racial community population. Ann Epidemiol 1998;8342- 346
PubMed
Willett  WStampfer  MJ Total energy intake: implications for epidemiologic analysis. Am J Epidemiol 1986;12417- 27
PubMed
Morris  MCTangney  CCBienias  JLEvans  DAWilson  RS Validity and reproducibility of a food frequency questionnaire by cognition in an older biracial sample. Am J Epidemiol 2003;1581213- 1217
PubMed
Scherr  PAAlbert  MSFunkenstein  HH  et al.  Correlates of cognitive function in an elderly community population. Am J Epidemiol 1988;1281084- 1101
PubMed
Folstein  MFFolstein  SEMcHugh  PR “Mini-Mental State”: a practical method for grading the cognitive state of patients for the clinician. J Psychiatr Res 1975;12189- 198
PubMed
Smith  A Symbol Digit Modalities Test Manual—Revised.  Los Angeles, Calif: Western Psychological; 1984
Kohout  FJBerkman  LFEvans  DACornoni-Huntley  J Two shorter forms of the CES-D depression symptoms index. J Aging Health 1993;5179- 193
PubMed
Rose  GABlackburn  HGillum  RFPrineas  RJ Cardiovascular Survey Methods.  Geneva, Switzerland: World Health Organization; 1982:162-165
Laird  NWare  J Random-effects models for longitudinal data. Biometrics 1982;38963- 974
PubMed
SAS Institute Inc SAS/STAT User's Guide Version 8.  Cary, NC: SAS Institute Inc; 1997
Kinsella  LJRiley  DE Nutritional deficiencies and syndromes associated with alcoholism.  In: Goetz  CG, Pappert  EJ, eds. Textbook of Clinical Neurology. Philadelphia, Pa: WB Saunders Co; 1999:803-806
Carmel  R Cobalamin, the stomach, and aging. Am J Clin Nutr 1997;66750- 759
PubMed
Tucker  KLRich  SRosenberg  IH  et al.  Plasma vitamin B-12 concentrations relate to intake source in the Framingham Offspring Study. Am J Clin Nutr 2000;71514- 522
PubMed
Crystal  HAOrtof  EFrishman  WHGrudber  AHershman  DAronson  M Serum vitamin B12 levels and incidence of dementia in a healthy elderly population: a report from the Bronx Longitudinal Aging Study. J Am Geriatr Soc 1994;42933- 936
PubMed
Savage  DGLindenbaum  J Neurological complications of acquired cobalamin deficiency: clinical aspects. Baillieres Clin Haematol 1995;8657- 678
PubMed
Crosby  WH The danger of folic acid in multivitamin preparations. Mil Med 1960;125233- 235
PubMed
Reynolds  EH Benefits and risks of folic acid to the nervous system. J Neurol Neurosurg Psychiatry 2002;72567- 571
PubMed
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; 1998:196-305

Figures

Tables

Table Graphic Jump LocationTable 1. Baseline Characteristics of 3718 Participants of the Chicago Health and Aging Project, 1993-2002*
Table Graphic Jump LocationTable 2. Difference in the Estimated Rate of Change in Cognitive Score During 6 Years by Quintile of Folate Intake Among 3718 Participants of the Chicago Health and Aging Project, 1993-2002
Table Graphic Jump LocationTable 3. Differences in the Rate of Cognitive Change (β) During 6 Years by Dose Level of Folate Vitamin Supplementation, Chicago Health and Aging Project, 1993-2002

References

Goodwin  JSGoodwin  JMGarry  PJ Association between nutritional status and cognitive functioning in a healthy elderly population. JAMA 1983;2492917- 2921
PubMed
Bernard  MANakonezny  PAKashner  TM The effect of vitamin B12 deficiency on older veterans and its relationship to health. J Am Geriatr Soc 1998;461199- 1206
PubMed
Louwman  MWvan Dusseldorp  Mvan de Vijver  FJ  et al.  Signs of impaired cognitive function in adolescents with marginal cobalamin status. Am J Clin Nutr 2000;72762- 769
PubMed
Lindeman  RDRomero  LJKoehler  KM  et al.  Serum vitamin B12, C and folate concentrations in the New Mexico elder health survey: correlations with cognitive and affective functions. J Am Coll Nutr 2000;1968- 76
PubMed
Carmel  RGreen  RJacobsen  DWRasmussen  KFlorea  MAzen  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;70904- 910
PubMed
Stabler  SPAllen  RHFried  LP  et al.  Racial differences in prevalence of cobalamin and folate deficiencies in disabled elderly women. Am J Clin Nutr 1999;70911- 919
PubMed
Riggs  KMSpiro  ATucker  KLRush  D Relations of vitamin B-12, vitamin B-6, folate, and homocysteine to cognitive performance in the Normative Aging Study. Am J Clin Nutr 1996;63306- 314
PubMed
Duthie  SJWhalley  LJCollins  ARLeaper  SBerger  KDeary  IJ Homocysteine, B vitamin status, and cognitive function in the elderly. Am J Clin Nutr 2002;75908- 913
PubMed
Seshadri  SBeiser  ASelhub  J  et al.  Plasma homocysteine as a risk factor dementia and Alzheimer's disease. N Engl J Med 2002;346476- 483
PubMed
Clarke  RSmith  ADJobst  KARefsum  HSutton  LUeland  PM Folate, vitamin B12, and serum total homocysteine levels in confirmed Alzheimer disease. Arch Neurol 1998;551449- 1455
PubMed
Quinlivan  EPGregory  JF  III Effect of food fortification on folic acid intakes in the United States. Am J Clin Nutr 2003;77221- 226
PubMed
Evans  DABennett  DAWilson  RS  et al.  Incidence of Alzheimer's disease in a biracial urban community: relation to apolipoprotein E allele status. Arch Neurol 2003;60185- 189
PubMed
Morris  MCColditz  GAEvans  DA Response to a mail nutritional survey in an older bi-racial community population. Ann Epidemiol 1998;8342- 346
PubMed
Willett  WStampfer  MJ Total energy intake: implications for epidemiologic analysis. Am J Epidemiol 1986;12417- 27
PubMed
Morris  MCTangney  CCBienias  JLEvans  DAWilson  RS Validity and reproducibility of a food frequency questionnaire by cognition in an older biracial sample. Am J Epidemiol 2003;1581213- 1217
PubMed
Scherr  PAAlbert  MSFunkenstein  HH  et al.  Correlates of cognitive function in an elderly community population. Am J Epidemiol 1988;1281084- 1101
PubMed
Folstein  MFFolstein  SEMcHugh  PR “Mini-Mental State”: a practical method for grading the cognitive state of patients for the clinician. J Psychiatr Res 1975;12189- 198
PubMed
Smith  A Symbol Digit Modalities Test Manual—Revised.  Los Angeles, Calif: Western Psychological; 1984
Kohout  FJBerkman  LFEvans  DACornoni-Huntley  J Two shorter forms of the CES-D depression symptoms index. J Aging Health 1993;5179- 193
PubMed
Rose  GABlackburn  HGillum  RFPrineas  RJ Cardiovascular Survey Methods.  Geneva, Switzerland: World Health Organization; 1982:162-165
Laird  NWare  J Random-effects models for longitudinal data. Biometrics 1982;38963- 974
PubMed
SAS Institute Inc SAS/STAT User's Guide Version 8.  Cary, NC: SAS Institute Inc; 1997
Kinsella  LJRiley  DE Nutritional deficiencies and syndromes associated with alcoholism.  In: Goetz  CG, Pappert  EJ, eds. Textbook of Clinical Neurology. Philadelphia, Pa: WB Saunders Co; 1999:803-806
Carmel  R Cobalamin, the stomach, and aging. Am J Clin Nutr 1997;66750- 759
PubMed
Tucker  KLRich  SRosenberg  IH  et al.  Plasma vitamin B-12 concentrations relate to intake source in the Framingham Offspring Study. Am J Clin Nutr 2000;71514- 522
PubMed
Crystal  HAOrtof  EFrishman  WHGrudber  AHershman  DAronson  M Serum vitamin B12 levels and incidence of dementia in a healthy elderly population: a report from the Bronx Longitudinal Aging Study. J Am Geriatr Soc 1994;42933- 936
PubMed
Savage  DGLindenbaum  J Neurological complications of acquired cobalamin deficiency: clinical aspects. Baillieres Clin Haematol 1995;8657- 678
PubMed
Crosby  WH The danger of folic acid in multivitamin preparations. Mil Med 1960;125233- 235
PubMed
Reynolds  EH Benefits and risks of folic acid to the nervous system. J Neurol Neurosurg Psychiatry 2002;72567- 571
PubMed
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; 1998:196-305

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