B vitamins, homocysteine, and neurocognitive function in the elderly

¹ From the Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, and the University of California at Davis.

² The contents of this publication do not necessarily reflect the views or policies of the US Department of Agriculture, nor does the mention of trade names, commercial products, or organizations imply endorsement by the US Government.

³ Supported in part by the US Department of Agriculture, Agricultural Research Service under contract no. 53-3K06-01.

⁴ Address reprint requests to J Selhub, Jean Mayer US Department of Agriculture Human Nutrition Research Center on Aging, Tufts University, 711 Washington Street, Boston, MA 02111. E-mail: jselhub{at}hnrc.tufts.edu

ABSTRACT  
Evidence of the importance of the B vitamins folic acid, vitamin B-12, and vitamin B-6 for the well-being and normal function of the brain derives from data showing neurologic and psychologic dysfunction in vitamin deficiency states and in cases of congenital defects of one-carbon metabolism. The status of these vitamins is frequently inadequate in the elderly and recent studies have shown associations between loss of cognitive function or Alzheimer disease and inadequate B vitamin status. The question that arises is whether these B vitamin inadequacies contribute to such brain malfunctions or result from aging and disease. From a theoretical standpoint, these inadequacies could give rise to impairment of methylation reactions that are crucial to the health of brain tissue. In addition or perhaps instead, these inadequacies could result in hyperhomocysteinemia, a recently identified risk factor for occlusive vascular disease, stroke, and thrombosis, any of which may result in brain ischemia. Advances in the understanding of this putative relation between inadequate vitamin status and loss of cognitive function in the elderly are likely to be slow and may depend on the outcomes of both prospective studies and longitudinal studies in which nutritional intervention is provided before cognitive decline occurs.

Key Words: Alzheimer disease • cognitive function • cognitive decline • dementia • folate • folic acid • vitamin B-12 • vitamin B-6 • homocysteine • hyperhomocysteinemia • brain • B vitamins • elderly • aging

INTRODUCTION  
No other organ system in the body has a greater minute-to-minute dependence on its nutrient supply than the central nervous system. In turn, that system has a profound effect on dietary intake. Current theories describe functions of brain receptors for cholecystokinins, opioid-like endorphins (1), and serotonin that appear to influence eating behavior and satiety. In animal studies, the number and function of such receptors have been found to decline with age. However, the importance of these observations with respect to declining appetite in the elderly is uncertain. In addition, there are well-documented declines in olfactory functions that may influence eating behavior and the taste threshold of the elderly (2).

The central nervous system requires a constant supply of glucose, and adequate brain function and maintenance depend on almost all the essential nutrients. For those B vitamins that participate in one-carbon metabolism (ie, folate, vitamin B-12, and vitamin B-6), deficiency of or congenital defects in the enzymes involved in these pathways is associated with severe impairment of brain function (Table 1).

View this table:
TABLE 1.. Neurologic and behavioral dysfunctions associated with defects in one-carbon metabolism¹  
Although severe vitamin deficiencies and congenital defects are rare, milder subclinical vitamin deficiencies are not uncommon in the elderly. Interest is increasing in learning the extent to which these mild, reversible deficiencies contribute to some decline in cognitive function in the later years of life. This article reviews currently available data that relate to aging, B vitamin status, and cognitive decline. Other reviews of these topics were published elsewhere (3–5). A review by Nourhashemi et al in this supplement addresses more directly the relation between B vitamin status and Alzheimer disease (6).

AGING AND DECLINE IN B VITAMIN STATUS  
One of the most striking age-related changes in gastric histology and function is the increasing prevalence with aging of atrophic gastritis with hypochlorhydria or achlorhydria. Based on various studies of elderly people, the prevalence of atrophic gastritis ranges from 20% to 50% depending on how the diagnosis is made and which definitions are used. In the Framingham Heart Study, the prevalences of atrophic gastritis among 60–69-y-olds and those >80 y were found to be 24% and 37%, respectively; the criteria used were serum pepsinogen I and II concentrations as measured by radioimmunoassay (7).

The physiologic consequences of atrophic gastritis include changes in gastric emptying and decreased secretion of intrinsic factor. However, the stomach appears to have a large reserve capacity for intrinsic factor secretion. Only in the most severe cases of gastric atrophy does intrinsic factor secretion become a rate-limiting factor for vitamin B-12 absorption. Nevertheless, atrophic gastritis has been reported to limit the bioavailability of vitamin B-12, although not because of impaired intrinsic factor secretion. Rather, the cause may be impaired release of vitamin B-12 from food proteins and peptides due to impaired acid secretion and reduced digestion by pepsin. Another potential effect of atrophic gastritis is bacterial overgrowth in the stomach and proximal small bowel, which in turn can reduce vitamin B-12 bioavailability because some types of bacteria take up vitamin B-12 for their own use.

Other consequences of atrophic gastritis include increased pH in the stomach and proximal small intestine. For example, mean (± SEM) pH measured at the ligament of Trietz was 7.1 ± 0.1 in subjects with atrophic gastritis compared with 6.6 ± 0.1 in a group of healthy elderly subjects (8). This increase in pH seems small but has been shown to significantly limit folic acid absorption; the optimum pH for active folate uptake is 6.3 (9).

In our studies of the original Framingham Heart Study cohort (subjects aged 67–93 y), we found a high prevalence of inadequate B vitamin status. The percentages of subjects with inadequate B vitamin status were 30% for folate, 20–25% for vitamin B-12, and 20% for vitamin B-6 (10). Homocysteine metabolism is closely associated with metabolism of folate, vitamin B-12, and vitamin B-6 (Figure 1); high plasma concentrations of this amino acid indicate disruption of its metabolism (10).

View larger version (12K):
FIGURE 1. . One-carbon metabolism in brain tissue. PLP, pyridoxal-5'-phosphate; FAD, flavin adenine dinucleotide; THF, tetrahydrofolate.

 

RELATION OF B VITAMINS TO COGNITIVE FUNCTIONS  
As discussed above, the evidence supporting neurologic effects of folate, vitamin B-12, and vitamin B-6 is derived from studies of clinical vitamin deficiencies in both humans and laboratory animals, as well as the effects of homozygous mutations of genes that encode the enzymes of folate metabolism (Table 1). Epidemiologic evidence linking low vitamin status or intake with decline in neurocognitive function in the elderly was first described by Goodwin et al (11). These authors showed that healthy elderly subjects who had low blood concentrations or intakes of folate, vitamin B-12, vitamin C, and riboflavin scored poorly on tests of memory and nonverbal abstract thinking. Other studies (see Table 2) have, for the most part, reiterated these epidemiologic associations between vitamins and neuropsychologic functions, although the methods of assessment were different and correlations were not always statistically significant. The extent to which these and other manifestations of brain function impairment can be ascribed to diminished vitamin status in the elderly is unclear. Goodwin et al (11) pointed out that their study subjects ìwere not mentally impaired and none of them was diagnosed as having dementia at the previous three-year complete medical evaluation which included mental status testing.î Neurocognitive impairment in this study was defined on the basis of comparisons between normal and abnormal scores within the same population. It is also noteworthy that some studies have reported improvement in cognitive performance after supplementation with these vitamins (13, 28–31). Lindenbaum et al (13) found significant improvement in neuropsychiatric functions among cobalamin-deficient patients after vitamin B-12 supplementation and Martin et al (31) reported cognitive recovery after vitamin B-12 supplementation in patients with cobalamin deficiency states of short duration (< 1 y). These results support the possibility that poor vitamin status is partially responsible for the cognitive decline seen in some elderly persons.

View this table:
TABLE 2.. Cognitive dysfunction as related to low vitamin status and high homocysteine concentrations¹  

ONE-CARBON METABOLISM AND BRAIN FUNCTION  
Possible biochemical interpretations of the putative effects of low vitamin status on cognitive decline can be made on the basis of the pathway shown in Figure 1. The pathway of one-carbon metabolism is characterized by the generation of one-carbon units, normally from serine, made active through association with tetrahydrofolate. The resulting 5,10-methylenetetrahydrofolate is subsequently used for the synthesis of thymidylate and purines (used for nucleic acid synthesis) and of methionine, which is used for protein synthesis and biological methylations. It is believed that methionine synthesis is the most crucial part of the pathway for the health of brain tissue. This synthesis is preceded by the irreversible reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate in a reaction that is catalyzed by the flavin-containing methylenetetrahydrofolate reductase. Subsequently, 5-methyltetrahydrofolate serves a substrate to methylate homocysteine in a reaction that is catalyzed by a vitamin B-12-containing methyltransferase. Homocysteine is also methylated by betaine in a reaction not involving vitamin B-12; however, this reaction is confined mostly to the liver.

A considerable proportion of methionine is activated by ATP to form S-adenosylmethionine (SAM), which serves primarily as a universal methyl donor in a variety of reactions. In the brain, SAM-dependent methylations are extensive and the products of these reactions include neurotransmitters (catecholamines and indoleamines), phospholipids, and myelin (32–35). One hypothesis proposes that the loss of neurocognitive function in the elderly is due in part to impaired methylation reactions in brain tissue. Because a considerable amount of SAM derives from methionine formed through the methylation reaction involving folate, vitamin B-12, and homocysteine, the hypothesis states that the observed association between loss of cognitive function and inadequate vitamin status is due to lower production of SAM (3, 36–40). Studies that have shown the efficacy of SAM as an antidepressant have provided some support for this hypothesis (41–46).

Upon transfer of its methyl group, SAM is converted to S-adenosylhomocysteine (SAH), which is subsequently hydrolyzed to homocysteine and adenosine. This hydrolysis is a reversible reaction that favors SAH synthesis. Thus, in the state of folate or vitamin B-12 deficiency, inability to methylate homocysteine leads to SAH accumulation. SAH is a potent inhibitor of the various SAM-dependent methylations. Hence, the impaired methylations resulting from lower rates of SAM synthesis are augmented by the intracellular accumulation of SAH.

HOMOCYSTEINE AND NEUROCOGNITIVE DYSFUNCTION  
Plasma homocysteine may be considered a functional indicator of B vitamin status, including that of folate and vitamin B-12 and, to a lesser extent, vitamin B-6. High plasma homocysteine concentrations can be largely attributed to inadequate status of these vitamins (47). Data from several laboratories indicate that plasma homocysteine increases with age independent of vitamin status, and that hyperhomocysteinemia is highly prevalent in the elderly (10).

Interest in the relation between neurocognitive dysfunction and plasma homocysteine concentrations arose from the growing epidemiologic evidence suggesting that mild elevations of this amino acid in the plasma are associated with increased risk of occlusive vascular disease, stroke, and thrombosis (48). The theory that elevated plasma homocysteine concentrations are related to cognitive dysfunction arose from results of several studies that showed associations between cognitive dysfunction and hyperhomocysteinemia (17, 48, 49). Riggs et al (24) investigated the relations between plasma concentrations of folate, vitamin B-12, vitamin B-6, and homocysteine and scores on a battery of cognitive tests in 70 men, aged 54–81 y, participating in the Normative Aging Study. Lower folate and vitamin B-12 concentrations were associated with poorer spatial copying skills (P = 0.003 and 0.04, respectively). In addition, plasma homocysteine concentration, which is inversely correlated with plasma folate and vitamin B-12 concentrations, was a stronger positive predictor (P = 0.0009) of spatial copying performance than either folate or vitamin B-12 concentrations.

A study by Bell et al (17) showed that elderly patients with depression who had lower cognitive screening test scores had significantly higher homocysteine concentrations than did either younger depression patients or elderly depression patients with normal cognitive screening test scores. Another study, by Joosten et al (26), showed that patients with Alzheimer disease had higher total plasma homocysteine concentrations than did age-matched healthy controls. However, these authors found no difference in total plasma homocysteine concentrations between the patients with Alzheimer disease and age-matched hospitalized patients. The significance of this observation is unclear. A case-control study of 164 patients with a clinical diagnosis of dementia of Alzheimer type, including 76 patients with histologically confirmed Alzheimer disease, showed that homocysteine concentrations were higher and serum folate and vitamin B-12 concentrations were lower in these patients than in matched control subjects (n = 108; 27).

CONCLUSIONS  
Evidence of the importance of folate, vitamin B-12, and vitamin B-6 in neurocognitive and other neurologic functions derives from reported cases of severe vitamin deficiencies, particularly pernicious anemia, and homozygous defects in genes that encode for enzymes of one-carbon metabolism. The neurologic dysfunctions seen in these cases allow for biochemical interpretations of the roles of vitamins in neurophysiology. The extent to which these interpretations are applicable to the observed epidemiologic relations between inadequate vitamin status (or inadequate vitamin intake) and neuropsychologic dysfunctions remains unclear. Advances in the understanding of this complex area are likely to be slow and may depend on the outcomes of both prospective studies and early nutritional interventions provided before signs of neurocognitive decline occur.

REFERENCES  

1. Morley JE, Levine AS, Yim GK, Lowy MT. Opioid modulation of appetite. Neurosci Biobehav Rev 1983;7:281–305.
2. Schiffman SS, Hornack K, Reilly D. Increased taste thresholds of amino acids with age. Am J Clin Nutr 1979;32:1622–7.
3. Parnetti L, Bottiglieri T, Lowenthal D. Role of homocysteine in age-related vascular and non-vascular diseases. Aging (Milano) 1997; 9:241–57.
4. Bottiglieri T. Folate, vitamin B12, and neuropsychiatric disorders. Nutr Rev 1996;54:382–90.
5. Bottiglieri T, Crellin RF, Reynolds EH. Folates and neuropschiatry. In: Bailey L, ed. Folate in health and disease. New York: Marcel Dekker, 1994:435–62.
6. Nourhashémi F, Gillette-Guyonnet S, Andrieu S, et al. Alzheimer disease: protective factors. Am J Clin Nutr 2000;71(suppl):643S–9S.
7. Krasinski SD, Russell RM, Samloff IM, et al. Fundic atrophic gastritis in an elderly population: effect on hemoglobin and several serum nutritional indicators. J Am Geriatr Soc 1986;34:800–6.
8. Russell RM, Krasinski SD, Samloff IM, Jacob RA, Hartz SC, Brovender SR. Folic acid malabsorption in atrophic gastritis. Possible compensation by bacterial folate synthesis. Gastroenterology 1986;91:1476–82.
9. Russell RM, Dhar GJ, Dutta SK, Rosenberg IH. Influence of intraluminal pH on folate absorption: studies in control subjects and in patients with pancreatic insufficiency. J Lab Clin Med 1979;93:428–36.
10. Selhub J, Jacques PF, Wilson PWF, Rush D, Rosenberg IH. Vitamin status and intake as primary determinants of homocysteinemia in an elderly population. JAMA 1993;270:2693–8.
11. Goodwin JS, Goodwin JM, Garry PJ. Association between nutritional status and cognitive functioning in a healthy elderly population. JAMA 1983;249:2917–21.
12. Karnaze DS, Carmel R. Low serum cobalamin levels in primary degenerative dementia. Arch Intern Med 1987;147:429–31.
13. Lindenbaum J, Healton EB, Savage DG, et al. Neuropsychiatric disorders caused by cobalamin deficiency in the absence of anemia or macrocytosis. N Engl J Med 1988;318:1720–8.
14. Renvall MJ, Spindler AA, Ramsdell JW, Paskvan M. Nutritional status of free-living Alzheimer's patients. Am J Med Sci 1989;298:20–7.
15. Tucker DM, Penland JG, Sandstead HH, Milne DB, Heck DG, Klevay LM. Nutrition status and brain function in aging. Am J Clin Nutr 1990;52:93–102.
16. Nijst TQ, Wevers RA, Schoonderwaldt HC, Hommes OR, Haan AFJ. Vitamin B12 and folate concentrations in serum and cerbrospinal fluid of neurological patients with special reference to multiple sclerosis and dementia. J Neurol Neurosurg Psychiatry 1990;53:951–4.
17. Bell IR, Edman JS, Selhub J, et al. Plasma homocysteine in vascular disease and in nonvascular dementia of depressed elderly people. Acta Psychiatr Scand 1992;86:386–90.
18. Levitt AJ, Karlinsky H. Folate, vitamin B12 and cognitive impairment in patients with Alzheimer's disease. Acta Psychiatr Scand 1992;86:301–5.
19. Kristensen MO, Gulmann NC, Christensen JEJ, Ostergaard K, Rasmussen K. Serum cobalamin and methylmalonic acid in Alzheimer dementia. Acta Neurol Scand 1993;87:475–81.
20. Crystal HA, Ortof E, Frishman WH, Gruber A, Hershman D, Aronson M. 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;42:933–6.
21. Nilsson K, Gustafson L, Faldt R, et al. Hyperhomocysteinaemia—a common finding in psychogeriatric population. Eur J Clin Invest 1996;26:853–9.
22. Ortega RM, Manas LR, Andres P, et al. Functional and psychic deterioration in elderly people may be aggravated by folate deficiency. J Nutr 1996;126:1992–6.
23. Dror Y, Stern F, Nemesh L, Hart J, Grinblat J. Estimation of vitamin needsóriboflavin, vitamin B-6 and ascorbic acidóaccording to blood parameters and functional, cognitive and emotional indices in a selected well-established group of elderly in a home for the aged in Israel. J Am Coll Nutr 1996;15:481–8.
24. Riggs KM, Spiro A III, Tucker K, Rush 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;63:306–14.
25. La Rue A, Koehler KM, Wayne SJ, Chiulli SJ, Haaland KY, Garry PJ. Nutritional status and cognitive functioning in a normally aging sample: a 6-y reassessment. Am J Clin Nutr 1997;65:20–9.
26. Joosten E, Lesaffre E, Riezler R, et al. Is metabolic evidence for vitamin B12 and folate deficiency more frequent in elderly patients with Alzheimer's disease? J Gerontol A Biol Sci Med Sci 1997;52:M76–9.
27. Clarke R, Smith AD, Jobst KA, Refsum H, Sutton L, Ueland PM. Folate, vitamin B12 and serum total homocysteine in confirmed Alzheimer's disease. Arch Neurol 1998;55:1449–55.
28. D'Angelo A, Selhub J. Homocysteine and thrombotic disease. Blood 1997;90:1–11.
29. Bellamy MF, McDowell IF. Putative mechanisms for vascular damage by homocysteine. J Inherit Metab Dis 1997;20:307–15.
30. Deifen JB, Beek EJ, Orlebeke JF, Berg H. Vitamin B-6 supplementation in elderly men: effects on mood, memory, performance and mental effort. Psychopharmacology 1992;109:489–96.
31. Martin DC, Francis J, Protetch J, Huff FJ. Time dependency of cognitive recovery with cobalamin replacement: report of a pilot study. J Am Geriatr Soc 1992;40:168–72.
32. Strittmatter WJ, Gagnon C, Axelrod J. Beta adrenergic stimulation of protein carboxymethylation and amylase secretion in rat parotid gland. J Pharmacol Exp Ther 1978;207:419–24.
33. Axelrod J. Methylation reactions in the formation and metabolism of catecholamines and other biogenic amines. Pharmacol Rev 1966;18:95–113.
34. Flynn DD, Kloog Y, Potter LT, Axelrod J. Enzymatic methylation of the membrane-bound nicotinic acetylcholine receptor. J Biol Chem 1982;257:9513–7.
35. Axelrod J, Hirata F. Phospholipid methylation and membrane function. Ann N Y Acad Sci 1981;373:51–3.
36. Enk D, Hougaard K, Hippe E. Reversible dementia and neuropathy associated with folate deficiency 16 years after partial gastrectomy. Scand J Haematol 1980;25:63–6.
37. Rosenberg IH, Miller JW. Nutritional factors in physical and cognitive functions of elderly people. Am J Clin Nutr 1992;55:1237S–43S.
38. Crellin R, Bottiglieri T, Reynolds EH. Folates and psychiatric disorders. Drugs. 1993;45:623–36.
39. Weir DG, Keating S, Molloy A, et al. Methylation deficiency causes vitamin B12 associated neuropathy in the pig. J Neurochem 1988; 51:1949–52.
40. Bottiglieri T, Hyland K, Reynolds EH. The clinical potential of ademetionine (Sadenosylmethionine) in neurological disorders. Drugs 1994;48:137–52.
41. Agnoli A, Andreoli VM, Casacchia M, Maffei F, Fazio C. Result and possible developments of the clinical use of S-adenosyl-L-methionine (SAMe) in psychiatry. Monogr Gesamtgeb Psychiatr Psychiatry Ser 1978;18:170–82.
42. Lipinski JF, Cohen BM, Frankenberg F, et al. Open trial of S-adenosylmethionine for treatment of depression. Am J Psychiatry 1984;141:448–50.
43. Carney MWP, Toone BK, Reynolds EH. S-adenosylmethionine and affective disorder. Am J Med 1987;83:104–6.
44. Rosenbaum JF, Fava M, Falk WE, et al. An open-label pilot study of oral S-adenosyl-l-methionine in major depression: interim results. Psychopharmacol Bull 1988;24:189–94.
45. Janicak PG, Lipinski J, Davis JM, et al. S-adenosylmethionine in depression. A literature review and preliminary report. Ala J Med Sci 1988;25:306–13.
46. Bressa GM. S-adenosyl-l-methionine (SAMe) as antidepressant: meta-analysis of clinical studies. Acta Neurol Scand Suppl 1994;154:7–14.
47. Selhub J, Miller JW. The pathogenesis of homocysteinemia: interruption of the coordinate regulation by S-adenosylmethionine of the remethylation and transsulfuration of homocysteine. Am J Clin Nutr 1992;55:131–8.
48. Boushey CJ, Beresford SAA, Omenn GS, Motulsky AG. A quantitative assessment of plasma homocysteine as a risk factor for vascular disease. JAMA 1995;274:1049–57.