Homocysteine, vitamins, and coronary artery disease

Transcription

1 Comprehensive review of the literature Brian V. Taylor, MD, MSC Gavin Y. Oudit, MD, MSC Michael Evans, MD, CCFP abstract OBJECTIVE To summarize results of clinical trials investigating the role of homocysteine (thcy) as a risk factor for coronary artery disease (CAD) and the role of vitamin therapy (folic acid and vitamins B 6 and B 12 ) in primary and secondary prevention of CAD. QUALITY OF EVIDENCE MEDLINE was searched from January 1976 to January 1999 to locate cross-sectional, retrospective and prospective cohort studies and meta-analyses on CAD using the MeSH words homocysteine, folic acid, vitamins B 6 and B 12, and coronary artery or heart disease. MAIN MESSAGE Elevated thcy levels are prevalent; most retrospective and cross-sectional studies show an association with increased risk of CAD. Results from recent prospective studies are less consistent. Folic acid, alone or with vitamins B 6 and B 12, reduces thcy concentrations in the blood. Results from ongoing randomized controlled trials could determine the effect of vitamins B 6 and B 12 and folic acid supplementation on CAD-related morbidity and mortality and could indicate whether routine supplementation with these vitamins should be advocated. Before mass screening for thcy can be done, the thcy assay must be standardized. CONCLUSION The role of homocysteine and vitamins B 6 and B 12 in managing CAD is unclear. Routine screening is not recommended. résumé OBJECTIF Faire la synthèse des résultats d essais cliniques analysant le rôle de l homocystéine à titre de facteur de risque de coronaropathies et le rôle d une thérapie à base de vitamines (acide folique et vitamines B 6 et B 12 ) dans la prévention primaire et secondaire des coronaropathies. QUALITÉ DES DONNÉES Une recension a été effectuée dans MEDLINE de janvier 1976 à janvier1999 pour trouver des études transversales, de cohortes rétrospectives et prospectives ainsi que des méta-analyses sur les coronaropathies à l aide des termes MeSH en anglais pour homocystéine, acide folique, vitamines B 6 et B 12, cardiopathies ou coronaropathies. PRINCIPAL MESSAGE Des taux élevés d homocystéine sont fréquents; la plupart des études rétrospectives et transversales démontrent un lien entre ces derniers et un risque accru de coronaropathies. Les résultats d études prospectives récentes sont moins cohérents à ce chapitre. L acide folique, seule ou combinée à des vitamines B 6 et B 12, réduit les concentrations d homocystéine dans le sang. Les résultats d essais aléatoires contrôlés présentement en progression pourraient déterminer l effet d un supplément de vitamines B 6 et B 12 ainsi que d acide folique sur la morbidité et la mortalité associées aux coronaropathies. Ils pourraient aussi indiquer s il faudrait préconiser systématiquement l administration d un tel supplément vitaminique. Avant de pouvoir procéder à un dépistage généralisé de l homocystéine, il faudrait en normaliser les épreuves. CONCLUSION Le rôle de l homocystéine et de la vitamine B 6 et B 12 dans la prise en charge de la coronaropathie reste imprécis. Le dépistage systématique n est pas recommandé. This article has been peer reviewed. Cet article a fait l objet d une évaluation externe. Can Fam Physician 2000; Canadian Family Physician Le Médecin de famille canadien VOL 46: NOVEMBER NOVEMBRE 2000

2 onventional risk factors for coronary artery C disease (CAD) include smoking, hypertension, hyperlipidemia, diabetes, and a family history. Family physicians should be aware of the discovery of new, potentially reversible, risk factors for CAD. Elevated serum levels of the amino acid homocysteine (thcy) is one such factor. Because this is a topic of increasing discussion in our medical community, this paper will review the medical literature to determine criteria for and prevalence of hyperhomocysteinemia in the general population; how thcy serum levels are measured in the laboratory; how thcy is metabolized, causes for elevated levels, and putative atherogenic role; results from retrospective and case-control studies and prospective cohort studies that do or do not support thcy as an independent risk factor for CAD; the association between dietary intake and serum levels of folic acid, vitamin B 6 and B 12, and risk of CAD; and whether good evidence supports use of vitamin supplementation for primary or secondary prevention of CAD. Quality of evidence A MEDLINE search of English-language literature from January 1976 to January 1999 revealed 17 retrospective (case-control, cross-sectional) studies, 1-17 one meta-analysis (based on studies completed before 1995), 18 and eight prospective studies supporting thcy as a risk factor for CAD One of the supportive prospective studies, when extended by 2.5 years, however, no longer showed a statistically significant association between thcy and incidence of CAD. 27 More recently, results from four large prospective cohort studies also did not demonstrate a significant association between thcy concentration and increased risk of CAD Plasma vitamins B 6 and B 12 and folic acid are strong correlates of thcy; 12,16,18,32-34 the literature is not entirely consistent, but does suggest that folic acid and vitamin B 6 and B 12 concentrations in blood are associated positively with CAD occurrence. 12,18,27,32-44 Results from five prospective epidemiologic studies showed low dietary folic acid and vitamin B 6 and B 12 or low serum folic acid to be associated with increased incidence of CAD Twelve intervention studies showed notable decreases in thcy levels after administration of folic acid with or Dr Taylor is a resident and Dr Evans teaches in the Department of Family and Community Medicine, and Dr Oudit is a resident in the Department of Medicine, in the University Health Network at the University of Toronto in Ontario. without vitamins B 6 and B 12. 9,45-55 Several prospective, double-blinded studies examining the effect of vitamin supplementation on morbidity and mortality of patients with CAD are currently under way. 56 Main findings Prevalence of hyperhomocysteinemia. Normal thcy levels range from 5 to 15 µmol/l in fasting patients. 40,43 Hyperhomocysteinemia is defined as any value above the 95th percentile or more than two standard deviations above the mean values obtained from healthy, fasting control subjects. Elevated thcy levels are classified as moderate (15 to 30 µmol/l), intermediate (>30 to 100 µmol/l), or severe (>100 µmol/l) based on fasting blood samples. Using this definition of hyperhomocysteinemia, its prevalence in the general population is 5%. 57 Between 13% and 47% of people with symptomatic atherosclerotic disease, however, have been reported to have hyperhomocysteinemia. 55 Measurement of plasma thcy levels. About 80% of total thcy in blood is protein bound by disulfide linkage. 58 Free thcy is variable, but thcy frozen immediately or centrifuged within 2 hours of collection (thcy levels may increase secondary to the export of homocysteine from red and white blood cells) remains constant. 59 Studies have not definitively established thcy reference standards by age and sex, although these variables are known to influence thcy levels, with higher thcy concentrations in men and older people. 60 Among patients with vitamin B 12 deficiency, thcy levels are generally higher than 23 µmol/l; confirmation might require serum methylmalonic acid assay. 61 A single measurement of thcy after fasting appears to be the most cost-effective method and has recently been offered in Canada at a cost of about $35 per assay. Testing for thcy should always be done after fasting; efforts are under way to standardize testing methods. How homocysteine works. Homocysteine is an amino acid formed as a result of metabolism of sulfurous methionine supplied from dietary proteins (eg, yellow and green leafy vegetables, grains, poultry, and meats). A simplification of thcy metabolism 62 is summarized in Figure 1. Several disease states and medications have been shown to elevate thcy levels, and this elevation could lead to CAD if undetected (Table 1). Nutrition: Researchers have suggested that up to two thirds of all hyperhomocysteinemia is attributable to VOL 46: NOVEMBER NOVEMBRE 2000 Canadian Family Physician Le Médecin de famille canadien 2237

3 Table 1. Causes of hyperhomocysteinemia NUTRITION Deficiencies in folic acid, vitamin B 6, and vitamin B 12 DISEASE STATES Chronic renal failure Hypothyroidism Malignancies (all, especially breast, ovarian, pancreatic) Pernicious anemia Systemic lupus erythematosus DRUGS Cigarettes Antimetabolites (eg, methotrexate) Anticonvulsants (eg, phenytoin, carbamazepine) Cholestyramine Nicotinic acid Nitrous oxide Thiazide diuretics GENETIC DEFICIENCIES Cystathionine β-synthase Methionine synthase Methylenetetrahydrofolate reductase deficiencies of one or more B vitamins. 36 Homocysteine levels can be markedly elevated with deficiencies of the essential cofactor vitamin B 12 or the cosubstrate folic acid. 28,63,64 An inverse relationship has been found to exist with plasma vitamins B 6 and B 12 and folic acid in relation to plasma thcy levels in normal subjects. 36 Disease states: Markedly elevated thcy levels have been reported in association with acute lymphoblastic leukemia; they decrease dramatically after chemotherapy. 65,66 Moderately elevated thcy levels have been found in patients with breast, ovarian, and pancreatic cancer. 65 In patients with chronic renal failure, plasma thcy levels can be two to four times normal, but they decrease with dialysis. 8,20 A positive correlation exists between fasting thcy and serum creatinine levels. Increases in thcy levels during chronic renal failure might be due to impaired metabolism rather than excretion. 20 The observed acceleration of atherosclerosis during end-stage renal disease could be due to elevated thcy concentrations. Hyperhomocysteinemia has been found in hypothyroid patients, suggesting an explanation for the observed higher incidence of vascular disease in hypothyroid patients. Hyperhomocysteinemia has also been reported in patients with pernicious anemia. Elevations in plasma thcy levels are useful in diagnosing this disorder. 61 Drugs: Several drugs increase plasma thcy levels: methotrexate, phenytoin, carbamazepine, and nitrous oxide. Methotrexate depletes folic acid, the cosubstrate for methionine synthase, and causes an increase in thcy levels. 67,68 Both phenytoin and carbamazepine interfere with folic acid metabolism, causing mild hyperhomocysteinemia. 67,68 Nitrous oxide inactivates vitamin B 12 - dependent methionine synthase and is known to elevate thcy levels. 68 Smoking is associated with a proportional increase in plasma thcy levels, particularly in older men who smoke 20 or more cigarettes a day. 69 Genetic causes: Cystathionine β-synthase deficiency is the most common genetic cause of hyperhomcysteinemia. Heterozygotes for this disorder have thcy levels in the range of 20 to 40µmol/L. Congenital hyperhomocystinuria, the homozygous form of the disease, can be associated with fasting levels of thcy of up to 400 µmol/l, 70 but this form of cystathionine β-synthase deficiency is rare (one in births). 71 About 50% of patients with untreated hyperhomocystinuria will have a thromboembolic event before age A thermolabile variant of 5, 10-methylenetetrahydrofolate reductase, caused by a point mutation (C677T) in the coding region for the folate binding site, has been found in 38% of French Canadians and 5% to 15% of the general Canadian population. This variant of the 5, 10-methylenetetrahydrofolate reductase gene is not a significant independent risk factor for CAD Those who are homozygous for this mutation, however, might be at increased risk of vascular disease. 75 Toxicity theory: It has been postulated that thcy increases risk of CAD through direct toxicity to endothelial cells, increased coagulability, elevated triglyceride levels, and oxygen free radical production, which leads to lower endothelial reactivity with stimulation of smooth muscle cell proliferation (Figure 1). 58,68,76-78 Cross-sectional and retrospective case-control studies. Some evidence indicates thcy is an independent risk factor for CAD. A meta-analysis based on 27 observational studies from 1976 to 1995 estimated that 10% of the United States population s CAD risk could be attributed to elevated thcy levels. 18 The 2238 Canadian Family Physician Le Médecin de famille canadien VOL 46: NOVEMBER NOVEMBRE 2000

4 Figure 1. Homocysteine metabolism and putative atherogenic mechanism Methionine Folic acid Vitamin B 12 Homocysteine Vitamin B 6 Cystathionine Auto-oxidation Low-density lipoprotein oxidation Smooth muscle cell proliferation Elevated triglyceride levels Increased coagulability Development of coronary artery disease? odds ratio (OR) for CAD of a 5-µmol/L thcy increment was 1.6 (95% confidence interval [CI] 1.4 to 1.7) for men and 1.8 (95% CI 1.3 to 1.9) for women. The authors concluded that a 5-µmol/L increase in thcy raised CAD risk by as much as a 0.5-µmol/L increase in cholesterol. 18 The summary estimate for homocysteine as a risk factor for CAD was determined in this meta-analysis to be 1.7 (95% CI 1.5 to 1.9). 18 Since this meta-analysis, three more retrospective studies have provided evidence to support elevated thcy levels increasing risk of CAD (Table ). Other evidence suggests thcy is not an independent risk factor. Two smaller studies (one crosssectional, one case-control) 79,80 did not show increased risk of CAD with elevated thcy levels. These results were later dismissed due to inadequate sample size and erroneous methods of measuring thcy levels. Prospective studies. Some evidence indicates thcy is an independent risk factor for CAD. Eight prospective cohort studies have reported statistically significant positive associations between elevated thcy levels and risk of CAD One study reported a relative risk of 1.41 (95% CI 1.16 to 1.71) for every 4-µmol/L increase in serum thcy. 20 The study by Nygard et al 23 used patients with angiographically proven heart disease who were followed for a median of 4.6 years. This cohort showed a strong, graded association between thcy concentration and overall mortality. 23 This association was strongest for thcy levels above 15 µmol/l, with an adjusted mortality OR of 1.6 for patients with thcy concentrations of 15 µmol/l compared with patients with levels of 10 µmol/l. After 4 years, only 3.8% of patients with thcy <9 µmol/l had died compared with 24.7% of those with thcy >15 µmol/l (mortality OR 4.5; 95% CI 1.22 to 16.6) for patients with the highest thcy levels compared with the lowest. Similarly, after 5 years of follow up the Physician s Health Study reported an adjusted relative risk of fatal or non-fatal MI of 3.4 (CI 1.3 to 8.8; P=.01) 19 (Table ,28 ). Other evidence suggests thcy is not an independent risk factor. In contrast to the results discussed above, recent studies have failed to demonstrate a significant association between thcy concentration and incidence of CAD (Table ). Five reports of four large cohorts followed prospectively do not support a cause-and-effect association between elevated thcy levels and risk of CAD In particular, when the Physician s Health Study was extended by 2.5 years, results no longer showed a statistically significant association between thcy levels and incidence of CAD. 27 Similarly, the Multiple Risk Factor Intervention Trial 29 and the Atherosclerosis Risk in Communities (ARIC) Study 30 did not find a significant association between elevated thcy levels and fatal or non-fatal CAD. Three of the five studies showed a trend toward increased risk of CAD with elevations in thcy. 27,28,30 A significant association might not have been reached because the three studies did not compare events (ie, fatal or non-fatal myocardial infarctions [MIs]) in people at extreme ends of thcy plasma concentration as the studies that supported thcy as a risk factor for CAD did. One of these studies suggests, however, that thcy is a consequence, not a cause, of CAD. 28 Among patients with CAD, elevated thcy levels strongly predict poor outcome, 77 suggesting that thcy reflects the severity of CAD and perhaps the risk of thrombosis. Among smokers, using an adjusted doseresponse relationship, elevated levels of thcy are not associated with increased mortality. 69 Dietary intake and serum levels of folic acid and vitamins B 6 and B 12. Although not entirely consistent, most retrospective cross-sectional and prospective cohort studies show a positive and dosedependent association between serum levels or dietary intake of folic acid and vitamins B 6 and B 12 and risk of CAD. 12,18,27,32-34,36-44 Changes in thcy levels might simply be an epiphenomenon and might, in fact, have no direct effect on risk of developing CAD. A recent prospective epidemiologic study among women enrolled in the Nurse s Health Study followed for VOL 46: NOVEMBER NOVEMBRE 2000 Canadian Family Physician Le Médecin de famille canadien 2239

5 Table 2. Retrospective studies supporting homocysteine as a risk factor for coronary artery disease (CAD): Summary risk estimate for CAD in references 1 to 17 is 1.7 (95% CI, 1.5 to 1.9) as determined by Boushey et al 18 STUDY CASES/CONTROLS SEX* AGE (Y) DESIGN MEAN TOTAL HOMOCYSTEINE LEVELS (µmol/l) CASES/CONTROLS (P FOR DIFFERENCE) MAIN FINDINGS Wilcken and Wilcken 1 25/22 Men < 50 Cross-sectional NA/NA Murphy-Chutorian 99/39 Men Cross-sectional 0.7/0.6 et al 2 Kang et al 3 241/202 < 69 Cross-sectional 5.5/4.3 (<.001) Israelsson et al 4 21/36 Men Malinow et al 5 64/92 35/167 99/259 Men Women 65/60 70/62 Population case control 16.4/13.5 Cross-sectional 13.1/11.3 (<.05) 12.9/10.2 (<.05) NA Genest et al 6 170/255 Men <60 Case control 13.7/10.9 (<.001) Ubbink et al 7 163/195 Men 55 Case control 16.2/13.4 Clarke et al 8 60/27 <55 Case control 18.7/13.4 Dudman et al 9 14/36 48/20 62/56 Women Men <52 Case control NA NA NA Pancharuniti 101/108 Men Population et al 10 case contol 13.5/11.9 (<.001) Wu et al /168 < 55 (men) <65 (women) Population case control 13.4/10.1 (<.001) Von Eckardstein 199/156 Men Case control 8.9/7.8 (<.001) et al 14 Hopkins et al / Case control 13.1/10.1 Men: OR 13.8 (CI ) Women: OR 12.8 (CI ) Dalery et al / Case control 11.8 (<.001)/ 8.6 (<.01) Men: P <.001 Women: P <.01 Robinson et al / Case control 14.6/10.6 (<.01) Men: OR 0.29 (CI ) Women: OR 3.5 (CI ) Malinow et al / Case control 16.7/ /14.7 Irish arm: OR 3.4 (CI ) French arm: OR 5.18 (CI ) Graham et al / Case control 11.3/9.7 RR 2.2 (CI ) CI confidence interval; NA not available; OR odds ratio; RR relative risk. *Unless otherwise designated, includes both men and women. Unless otherwise specified, mean age of participants. For thcy levels >19 µmol/l vs < 9 µmol/l Canadian Family Physician Le Médecin de famille canadien VOL 46: NOVEMBER NOVEMBRE 2000

6 Table 3. Prospective studies supporting homocysteine as a risk factor for coronary arter y disease (CAD) STUDY CASES/CONTROLS SEX* AGE (Y) MAIN OUTCOMES MEAN TOTAL HOMOCYSTEINE LEVELS (µmol/l) CASES/CONTROLS RR OR OR (95% CI) Stampfer et al /271 Men Fatal/non-fatal MI 11.1/10.5 (P <.05) 3.4 ( ) Arnesen et al / Fatal/non-fatal CAD 12.7/ ( ) Perry et al /118 Men Fatal/non-fatal stroke 13.7/ ( ) Petri et al Arterial thrombosis NA/NA 3.49 ( ) Nygard et al Men Women Fatal CAD 11.4/NA 10.5/NA 4.5 ( ) Wald et al /126 Men Fatal CAD >15.2/< ( ) Stehouwer et al /162 Men MI NA/NA 1.81 ( ) Bots et al /553 >55 MI 17.3/NA 2.43 ( ) CI confidence interval; MI myocardial infarction; NA not available; OR odds ratio; RR relative risk. *Unless otherwise designated, includes both men and women. Unless otherwise specified, mean age of participants. Results no longer showed significant association of total homocysteine with incidence of CAD when was study extended by 2.5 years. 28 Table 4. Prospective studies not supporting homocysteine as a risk factor for coronary arter y disease (CAD) STUDY CASES/CONTROLS SEX* AGE (Y) MAIN OUTCOMES MEAN TOTAL HOMOCYSTEINE LEVELS (µmol/l) CASES/CONTROLS RR OR OR (95% CI) Alfthan et al 28 92/141 99/128 Men Women Fatal/non-fatal MI 9.8/ / ( ) 1.22 ( ) Chasan-Taber 333/333 Men Fatal/non-fatal MI 5.0/ ( ) et al 27 Evans et al / /286 Men Non-fatal MI, CAD, death 12.6/ / ( ) Folsom et al / All CAD events 8.86/ ( ) Verhoef et al /427 Men New angina/cabg 10.9/ ( ) CABG coronary artery bypass grafting; CI confidence interval; MI myocardial infarction; OR odds ratio; RR relative risk. *Unless otherwise designated, includes both men and women. Non-fatal MI cases. Deaths from coronary heart disease (CHD). Composed of 146 definite or probable MIs, 19 silent MIs, 30 definite fatal CHDs, and 37 revascularization procedures. VOL 46: NOVEMBER NOVEMBRE 2000 Canadian Family Physician Le Médecin de famille canadien 2241

7 14 years examined the relation between intake of folic acid and vitamin B 6 and risk of CAD. 81 A large portion of this study group had insufficient intake of vitamin B 6 and folic acid to prevent CAD, in agreement with the Framingham Heart Study. 41 Women with the lowest intake of folic acid and vitamin B 6 had the greatest risk of mortality and MI. 81 In the large prospective ARIC study, only plasma vitamin B 6 was associated with incidence of CAD after accounting for other risk factors. 30 The relative risk (RR) for highest versus lowest quintile of vitamin B 6 was 0.28 (95% CI 0.1 to 0.7). 30 Interestingly, arterial lesions have been seen in animals raised on vitamin B 6 deficient diets. 82,83 A summary of the main prospective studies supporting the role of folic acid and vitamins B 6 and B 12 in risk of CAD 44,63,81,84 are shown in Table 5. 44,63,81,84,85 Vitamin supplementation for preventing CAD. Several studies have shown substantial decreases in thcy levels when patients add folic acid, with or without vitamins B 6 and B 12, to their diets. 9,45-53 According to the Nurse s Health Study, risk of CAD was reduced among women who regularly used multiple vitamins (RR 0.76; 95% CI 0.65 to 0.90). 81 Maximum benefit was found among women in the highest quintile of both folic acid and vitamin B 6 intake (RR 0.57; 95% CI 0.40 to 0.82). This finding is consistent with the independent effects of these vitamins in lowering thcy levels, suggesting that maximum benefit can be obtained with optimal levels of both vitamins. The association between dietary folic acid intake and thcy levels appears to reach a plateau beyond which thcy levels remain stable. Existence of a plateau is supported by studies showing that thcy levels did not decrease any more after 6 weeks of treatment with 1000 µg of folic acid, 0.4 mg of vitamin B 12, and 12.2 mg of vitamin B 6 when twice these amounts were given. 81 The minimum daily dose of folic acid that has maximal efficacy in decreasing plasma thcy levels is about 0.4 mg. 55,86,87 A recent meta-analysis summarizing the effects of vitamin therapy on thcy levels showed that folic acid alone at 0.5 to 5.0 mg/d was accompanied by a 25% reduction in thcy levels (CI 23% to 28%). 87 Table 5. Prospective studies of folate, vitamins B 6 and B 12, and cardiovascular risk STUDY AGE (Y) PARTICIPANTS DESIGN MEAN FOLLOW UP (Y) MAIN RESULTS de Bree et al men and women who died of CAD Prospective cohort nested case-control 15 RR 1.69 for lowest vs highest quarters of folate level Verhoef et al men after MI Prospective cohort nested case-control 7.5 RR 1.4 (CI ) for lowest vs highest fifths of folate level; RR 1.5 (CI ) for lowest vs highest fifths of vitamin B 6 level Morrison et al men and women with fatal and non-fatal CAD Prospective cohort nested case-control 3.3 RR 0.66 (CI ) for highest vs lowest fifths of folate level; RR 0.28 (CI ) for highest vs lowest fifths of vitamin B 6 level; RR 0.81 (CI ) for highest vs lowest fifths of B 12 level Giles et al men and women with ischemic stroke Prospective cohort nested case-control 13 RR 1.37 (CI ) for serum folate level <9.2µmol/L vs >9.2 µmol/l Rimm et al women with non-fatal MI and CAD death Prospective cohort nested case-control 14 RR 0.69 (CI ) for highest vs lowest fifths of folate level; RR 0.67 (CI ) for highest vs lowest fifths of vitamin B 6 level CAD coronary artery disease; CI confidence interval; MI myocardial infarction; RR relative risk Canadian Family Physician Le Médecin de famille canadien VOL 46: NOVEMBER NOVEMBRE 2000

8 Normalization of plasma thcy levels occurred 2 to 6 weeks after initiation of folic acid therapy regardless of the reason for the elevated thcy. Addition of 0.5 mg/d of vitamin B 12 to folic acid was associated with an additional 7% drop in thcy levels, 87 but addition of 16.5 mg/d of vitamin B 6 did not further lower them. The authors acknowledge, however, that these results might be due to their exclusion of studies using one version of the thcy assay, the results of which might depend more on vitamin B Canada has recently followed the lead of the United States Food and Drug Administration and has begun supplementing cereal and flour products with 140 µg of folic acid per 100 g of flour. It remains to be seen whether fortifying cereal or flour products with folic acid has any effect on thcy levels and ultimately CAD risk in the general population. A rare but potentially serious side effect of folic acid supplementation is subacute combined degeneration of the spinal cord in people with subclinical vitamin B 12 deficiency. To avoid this complication, physicians should rule out vitamin B 12 deficiency before therapy; 400 to 1000 µg/d of vitamin B 12 is suggested. The doses of vitamin B 6 used to treat mildly or moderately elevated total plasma thcy levels are too low (25 to 50 mg/d) to induce sensory neuropathy. Before vitamin therapy can be approved for primary and secondary prevention of CAD, results from ongoing randomized clinical trials evaluating the effect of vitamin therapy on patient morbidity and mortality must be analyzed. The trials looking specifically at MI as an end point include the Cambridge Heart Antioxidant Study (CHAOS-2), the Norwegian Study of Homocysteine Lowering with B-Vitamins in Myocardial Infarction (NORVIT), and the Study of the Effectiveness of Additional Reductions in Cholesterol and Homocysteine (SEARCH). 56 Conclusion Most retrospective studies support an association between elevated thcy levels and increased risk of CAD, but data from prospective studies are less consistent. Results from large randomized controlled trials are needed to better clarify the relationships among thcy, folic acid, B vitamins, and risk of CAD. Adding 0.5 mg/d of folic acid alone can reduce basal thcy levels by about 25%; adding 0.5 mg/d of vitamin B 12 to the folic acid further reduces levels by 7%. Most of the literature supports combining vitamin B 6, about 25 to 50 mg/d, with folic acid and vitamin B 12. A large randomized controlled trial with clear end points in terms of reducing CAD morbidity and mortality Key points Elevated homocysteine levels appear to be more common in people with symptomatic atherosclerotic disease. Homocysteine levels have an inverse relationship with plasma vitamins B 6 and B 12 and folic acid. Evidence exists both for and against homocysteine as an independent risk factor for CAD. More recent evidence is less compelling. Several studies show that supplementation with folic acid and vitamins B 6 and B 12 reduces serum homocysteine levels; many randomized controlled trials are currently studying their effect on morbidity and mortality. Until better evidence is available, mass screening for elevated homocysteine levels and recommending vitamin supplements is still experimental. Points de repère Des taux élevés d homocystéine semblent plus fréquents chez les personnes atteintes d athérosclérose symptomatique. Des taux élevés d homocystéine ont un rapport inversement proportionnel aux taux de vitamines B 6 et B 12 et d acide folique dans le plasma. Il existe à la fois des données favorables et défavorables au fait de considérer l homocystéine comme un facteur de risque indépendant de souffrir de coronaropathies. Les plus récentes données probantes sont moins convaincantes. Quelques études font valoir qu un supplément d acide folique et de vitamines B 6 et B 12 réduit les taux sériques d homocystéine; plusieurs essais aléatoires contrôlés étudient présentement l effet de ce supplément sur la morbidité et la mortalité. Avant d avoir en main de meilleures données probantes, le dépistage systématique des taux élevés d homocystéine et la recommandation d administrer des suppléments vitaminiques n en sont encore qu au stade expérimental. with vitamin therapy would provide the best evidence on which to base our preventive interventions. Until we know the effect of vitamin supplementation on primary and secondary prevention of CAD, we have no evidence upon which to recommend testing any patient s thcy levels. Also, before routine testing can be advocated, the assay must be standardized. Finally, the effect of vitamin therapy on clinical outcome must be of proven clinical benefit. In the meantime, family physicians are urged to continue to VOL 46: NOVEMBER NOVEMBRE 2000 Canadian Family Physician Le Médecin de famille canadien 2243

9 detect and actively manage traditional risk factors for CAD, including promotion of a well-balanced and nutritious diet. Correspondence to: Dr Michael Evans, Department of Family and Community Medicine, University of Toronto, 750 Dundas St W, Toronto, ON M6J 3S3; telephone (416) ; fax (416) ; michael.evans@utoronto.ca References 1. Wilcken DEL, Wilcken B. The pathogenesis of coronary artery disease: a possible role for methionine metabolism. J Clin Invest 1976;57: Murphy-Chutorian DR, Wexman MP, Grieco AJ, Heininger JA, Glassman E, Gaull GE, et al. Methionine intolerance. J Am Coll Cardiol 1985;6: Kang S, Wong PWK, Cook HY, Norusis M, Messer JV. Protein-bound homocyst(e)ine. J Clin Invest 1986;77: Israelsson B, Brattstrom LE, Hultberg BL. Homocysteine and myocardial infarction. Atherosclerosis 1988;71: Malinow MR, Sexton G, Averbuch M, Grossman M, Wilson D, Upson B. Homocyst(e)inemia in daily practice. Coron Artery Dis 1990;1: Genest JJ, McNamara JR, Salem DN, Wilson PWF, Scaefer EJ, Malinow MR. Plasma homocyst(e)ine levels in men with premature coronary artery disease. J Am Coll Cardiol 1990;16: Ubbink JB, Vermaak WJH, Bennet JM, Becker PJ, van Staden DA, Bissbort S. The prevalence of homocysteinemia and hypercholesterolemia in angiographically defined coronary artery disease. Klin Wochenschr 1991;69: Clarke R, Daly L, Robinson K, Naughten E, Cahalam S, Fowler B, et al. Hyperhomocysteinemia: an independent risk for vascular disease. N Engl J Med 1991;324: Dudman NPB, Wilcken DEL, Wang J, Lynch JF, Macey D, Lundberg P. Disordered methionine/homocysteine metabolism in premature vascular disease. Arterioscler Thromb 1993;13: Pancharuniti N, Lewis CA, Suaberlich HE, Perkins LL, Go RC, Alvarez JO, et al. Plasma homocysteine, folate, and vitamin B 12 concentrations and risk for early onset coronary artery disease. Am J Clin Nutr 1994;59: Wu LL, Wu J, Hunt SC, James BC, Vincent GM, Williams R, et al. Plasma homocysteine as a risk factor for early familial coronary artery disease. Clin Chem 1994;40: Graham IM, Daly LE, Refsum HM, Robinson K, Brattstrom LE, Ueland PM, et al. Plasma homocysteine as a risk factor for vascular disease. The European Concerted Action Project. JAMA 1997;277: Malinow MR, Ducimetiere P, Luc G, Evans AE, Arveiler D, Cambien F, et al. Plasma homocysteine levels and graded risk for myocardial infarction: findings in two populations at contrasting risk for coronary artery disease. Atherosclerosis 1996;126: Von Eckardstein A, Malinow MR, Upson B, Heinrich J, Schulte H, Schonfeld R, et al. Effects of age, lipoproteins, and hemostatic parameters on the role of hyperhomocysteinemia as a cardiovascular risk factor in men. Arterioscler Thromb 1994;14: Hopkins PN, Wu LL, Wu J, Hunt SC, James BC, Vincent GM. Higher plasma homocysteine and increased susceptibility to adverse effects of low folate in early familial coronary artery disease. Arterioscler Thromb Vasc Biol 1995;15: Dalery K, Lussier-Cacan S, Selhub J, Davignon J, Latour Y, Genest J Jr. Homocysteine in French Canadian subjects: relation with vitamins B 12, B 6, pyridoxal phosphate and folate. Am J Cardiol 1995;75: Robinson K, Mayer EL, Miller DP, Green R, van Lente F, Gupta A, et al. Hyperhomocysteinemia and low pyridoxal phosphate: common and independent reversible risk factors for coronary artery disease. Circulation 1995;92: Boushey CJ, Beresford SAA, Omenn GS, Motulsky AG. A quantitative assessment of plasma homocysteine as a risk factor for vascular disease. JAMA 1995;274: Stampfer MJ, Malinow MR, Willett WC, Newcomer LM, Upson B, Ullman D, et al. A prospective study of plasma homocysteine and risk of myocardial infarction in US physicians. JAMA 1992;268: Arnesen E, Refsum H, Bonaa KH, Ueland PM, Forder OH, Nordrehaug JE. Serum total homocysteine. Int J Epidemiol 1995;24: Wald NJ, Watt HC, Law MR, Weir DG, McPartin J, Scott JM. Homocysteine and ischemic heart disease: results of a prospective study with implications regarding prevention. Arch Intern Med 1998;158: Perry IJ, Refsum H, Morris RW, Ebrahim SB, Ueland PM, Shaper AG. Prospective study of serum total homocysteine concentration and risk of stroke in middle-aged British men. Lancet 1995;346: Nygard O, Nordrehaug JE, Refsum H, Ueland PM, Farstad M, Vollset SE. Plasma homocysteine levels and mortality in patients with coronary artery disease. N Engl J Med 1997;337: Stehouwer CD, Weijenberg MP, van den Berg, Jacobs C, Feskens EJ, Kromhout D. Serum homocysteine and risk of coronary artery disease and cerebrovascular disease in elderly men: a 10-year follow-up. Arterioscler Thromb Vasc Biol 1998;18: Petri M, Roubenoff R, Dallal GE, Nadeau MR, Selhub J, Rosenberg IH. Plasma homocysteine as a risk factor for atherothrombotic events in SLE. Lancet 1996;348: Bots ML, Launer LJ, Lindemans J, Hoes AW, Hofman A, Witteman JC, et al. Homocysteine and short-term risk of myocardial infarction and stroke in the elderly: the Rotterdam Study. Arch Intern Med 1998;159: Chasan-Taber L, Selhub J, Rosenberg IH, Malinow MR, Terry P, Tishler PV, et al. A prospective study of folate and vitamin B 6 and risk of myocardial infarction in US physicians. J Am Coll Nutr 1996;15: Alfthan A, Pekkanen J, Jauhiainen M, Pitkaniemi J, Karvonen M, Tuomilehto J, et al. Relation of serum homocysteine and lipoprotein(a) concentrations to atherosclerotic disease in a prospective Finnish population-based study. Atherosclerosis 1994;106: Evans RW, Shaten BJ, Hempel JD, Cutler JA, Kuller LH, for the MRFIT Research Group. Homocysteine and risk of cardiovascular disease in the Multiple Risk Factor Intervention Trial. Arterioscler Thromb Vasc Biol 1997;17: Folsom AR, Nieto FJ, McGovern PG, Tsai MY, Malinow MR, Eckfeldt JH, et al. Prospective study of coronary heart disease incidence in relation to fasting total homocysteine, related genetic polymorphisms, and B vitamins: the Atherosclerosis Risk in Communities (ARIC) Study. Circulation 1998;98: Verhoef P, Hennekens CH, Allen RH, Stabler SP, Willett WC, Stampfer MJ. Plasma total homocysteine and risk of angina pectoris with subsequent CABG. Am J Cardiol 1997;79: Verhoef P, Kok FJ, Kruyssen DACM, Schouten EG, Witteman JCM, Grobbee DE, et al. Plasma total homocysteine, B vitamins, and risk of coronary atherosclerosis. Arterioscler Thromb Vasc Biol 1997;17: Verhoef P, Stampfer MJ, Buring JE, Gaziano JM, Allen RH, Stabler SP, et al. Homocysteine metabolism and risk of myocardial infarction: relation with vitamins B 6, B 12 and folate. Am J Epidemiol 1996;143: Robinson K, Arheart K, Refsum H, Brattstrom L, Boers G, Ueland P, et al. Low circulating folate and vitamin B 6 concentrations: risk factors for stroke, peripheral vascular disease,. Circulation 1998;97: Brattstrom L, Lindgren A, Israelsson B, Malinow MR, Norrving B, Upson B, et al. Hyperhomocysteinaemia in stroke: prevalence, cause, and relationships to type of stroke and stroke risk factors. Eur J Clin Invest 1992;22: Selhub J, Jacques PF, Wislon PWF, Rush D, Rosenberg IH. Vitamin status and intake as primary determinants of homocysteinemia in an elderly population. JAMA 1993;270: Brattstrom L, Lindgren A, Israelsson B, Andersson A, Hutlberg B. Homocysteine and cysteine: determinants of plasma levels in middle-aged and elderly subjects. J Intern Med 1994;236: Kang S, Wong PWK, Norusis M. Homocysteinemia due to folate deficiency. Metabolism 1987;36: Lewis CA, Pancharuniti N, Sauberlich HE. Plasma folate adequacy as determined by homocysteine level. Ann N Y Acad Sci 1992;669: Ueland PM, Refsum H, Stabler SP, Malinow MR, Andersson A, Allen RH. Total homocysteine in plasma or serum: methods and clinical applications. Clin Chem 1993;39: Lindenbaum J, Rosenberg IH, Wislon PWF, Stabler SP, Allen RH. Prevalence of cobalamin deficiency in the Framingham elderly population. Am J Clin Nutr 1994;60: Andersson A, Brattstrom L, Israelsson B, Isaksson A, Hamfelt A, Hultberg B. Plasma homocysteine before and after methionine loading with regard to age, gender and menopausal staus. Eur J Clin Invest 1992;22: Jacobsen DW, Gatautis VJ, Green R, Robinson K, Savon SR, Secic M, et al. Rapid HPLC determination of total homocysteine and other thiols in serum and plasma: sex differences and correlation with cobalamin and folate concentrations in healthy subjects. Clin Chem 1994;40: Morrison HI, Schaubel D, Desmeules M, Wigle DT. Serum folate and risk of fatal coronary heart disease. JAMA 1996;275: Perry IJ, Refsum H, Morris RW, Ebrahim SB, Ueland PM, Shaper AG. Serum total homocysteine and coronary heart disease in middle-aged British men [abstract]. Heart 1996;75: Witteman BML, Hoes AW, Kuodstaalm PJ, Grobbee DE. Homocysteine and risk of cardiovascular disease in the elderly: the Rotterdam Study [abstract]. Can J Cardiol 1997;13(Suppl B):150B. 47. Brattstrom LE, Israelsson B, Jeppsson JO, Hultberg BL. Folic acid: an innocuous means to reduce plasma homocysteine. Scand J Clin Lab Invest 1988;48: Ubbink JB, Vermaak WJH, van der Merwe A, Becker PJ, Delport R, Potgieter HC. Vitamin requirements for the treatment of hyperhomocysteinemia in humans. J Nutr 1994;124: Canadian Family Physician Le Médecin de famille canadien VOL 46: NOVEMBER NOVEMBRE 2000

10 49. Brattstrom LE, Israelsson B, Norrving B, Bergquist D, Thorne J, Hultberg B, et al. Impaired homocysteine metabolism in early-onset cerebral and peripheral arterial occlusive disease. Atherosclerosis 1990;81: Brattstrom LE, Hultberg BL, Hardebo JE. Folic acid responsive postmenopausal homocysteinemia. Metabolism 1985;34: Franken DG, Boers GHJ, Blom HJ, Trijbels FJM, Kloppenborg PWC. Treatment of mild hyperhomocysteinemia in vascular disease patients. Arterioscler Thromb 1994;14: Ubbink JB, Vermaak WJH, van der Merwe A, Becker PJ. Vitamin B 12, vitamin B 6 and folate nutritional status in men with hyperhomocysteinemia. Am J Clin Nutr 1993;57: Wilcken DEL, Wilcken B, Dudman NPB, Tyrrell PA. Homocystinuria the effects of betaine in the treatment of patients not responsive to pyridoxine. N Engl J Med 1983;309: Ubbink JB, van der Merwe A, Vermaak WJH, Delport R. Hyperhomocysteinemia and the response to vitamin supplementation. Clin Invest 1993;71: Malinow MR, Duell PB, Hess DL, Anderson PH, Kruger WD, Phillipson BE, et al. Reduction of plasma folate homocysteine levels by breakfast cereal fortified with folic acid in patients with coronary heart disease. N Engl J Med 1998;338: Clarke R, Collins R. Can dietary supplements with folic acid or vitamin B 6 reduce cardiovascular risk? Design of clinical trials to test the homocysteine theory of vascular disease. J Cardiovasc Risk 1998;5: McCully KS. Homocysteine and vascular disease. Nat Med 1996;2: Mansoor MA, Svardal AM, Ueland PM. Determination of the in vivo redox stsus of cysteine, cysteinglycine, homocysteine and glutathione in human plasma. Anal Biochem 1992;200: Malinow MR, Axthelm MK, Merdith MJ, MacDonald NA, Upson BM. Synthesis and transsulfuration of homocysteine in blood. J Lab Clin Med 1994;123: Rasmussen K, Moller J, Lyngbak M, Pedersen AM, Dybkjaer L. Age- and genderspecific reference intervals for total homocysteine and methylmalonic acid in plasma before and after vitamin supplementation. Clin Chem 1996;42: Delva MD. Vitamin B 12 replacement. To B 12 or not to B 12? Can Fam Physician 1997;43: Finkelstein JD. Methionine metabolism in mammals. J Nutr Biochem 1990;1: Verhoef P, Hennekens CH, Malinow MR, Kok FJ, Willett WC, Stampfer MJ. A prospective study of plasma homocysteine and risk of ischemic stroke. Stroke 1994;25: Taylor LM, DeFrang RD, Harris EJ, Porter JM. The association of elevated plasma homocysteine with progression of symptomatic peripheral arterial disease. J Vasc Surg 1991;13: Mayer EL, Jacobsen DW, Robinson K. Homocysteine and coronary atherosclerosis. J Am Coll Cardiol 1996;27: Refsum H, Wesenberg F, Ueland PM. Plasma homocysteine in children with acute lymphoblastic leukemia: changes during a chemotherapeutic regimen including methotrexate. Cancer Res 1991;51: Ueland PM, Refsum H. Plasma homocysteine, a risk factor for vascular disease: plasma levels in health, disease and drug therapy. J Lab Clin Med 1989;114: Ueland PM, Refsum H, Brattstrom L. Plasma homocysteine and cardiovascular disease. In: Francis RB Jr, editor. Atherosclerotic cardiovascular disease, hemostasis and endothelial function. New York, NY: Marcel Dekker; p Taylor BV, Oudit GY, Kalman PG, Liu P. Clinical and pathophysiological effects of active and passive smoking on the cardiovascular system. Can J Cardiol 1998;14(9): Mudd SH, Levy HL, Skovby F. Disorders of transsulufration. In: Scriver CR, Beaudet AL, Sly WS, Valle D, editors. The metabolic and molecular bases of inherited disease. 7th ed. Vol. 1. New York, NY: McGraw-Hill; p Mudd SH, Sovby F, Levy HL, Pettigrew KD, Wilcken B, Pyeritz RE, et al. The natural history of homcystinuria due to cystathionine β-synthase deficiency. Am J Hum Genet 1985;37(2): Ma J, Stampfer MJ, Hennekens CH, Frosst P, Selhub J, Horsford J, et al. Methylenetetrahydrofolate reductase polymorphism, plasma folate, homocysteine and risk of myocardial infarction in US physicians. Circulation 1996;94: Van Bockxmeer FM, Mamotte CD, Vasikaran SD, Taylor RR. Methylenetetrahydrofolate reductase gene. Circulation 1997;95: Gallagher PM, Meleady R, Shields DC, Tan KS, McMaster D, Rozen R, et al. Homocysteine and risk of premature coronary artery disease: evidence for a common gene mutation. Circulation 1996;94: Deloughery TG, Evans A, Sadeghi A, McWilliams J, Henner WO, Taylor LM Jr, et al. Common mutation in methylenetetrahydrofolate reductase: correlation with homocysteine metabolism and late-onset vascular disease. Circulation 1996;94: Tsai JC, Perrella MA, Yoshizumi M, Hsieh CM, Haber E, Schlegel R, et al. Promotion of vascular smooth muscle cell growth by homocysteine: a link to atherosclerosis. Proc Natl Acad Sci USA 1994;91: Stamler JS, Osborne JA, Jaraki O, Rabboni LE, Mullins M, Singel D, et al. Adverse vascular effects of homocyteine are modulated by endothelium-derived relaxing factor and related oxides of nitrogen. J Clin Invest 1993;91: Tawakol A, Omland T, Gerhard M, Wu JT, Creager MA. Hyperhomocysteinemia is associated with impaired endothelium-dependent vasodilatation in humans. Circulation 1997;95: Wilcken DEL, Reddy SG, Gupta VJ. Homocysteinemia, ischemic heart disease, and the carrier state for homocystinuria. Metabolism 1983;32: Boers GHJ, Smals AGH, Trijbels FJM, Fowler B, Bakkeren JA, Schooderwaldt HC, et al. Heterozygosity for homocystinuria in premature peripheral and cerebral occlusive arterial disease. N Engl J Med 1985;313: Rimm EB, Willett WC, Hu FB, Sampson L, Colditz GA, Manson JE, et al. Folate and vitamin B 6 from diet and supplements in relation to risk of coronary artery disease among women. JAMA 1998;279: Rinehart JF, Greenberg LD. Arteriosclerotic lesions in pyridoxine-deficient monkeys. Am J Pathol 1949;25: Smolin LA, Crenshaw TD, Kurtycz D, Benevenga NJ. Homocysteine accumulation in pigs fed diets deficient in vitamin B 6 : relationship to atherosclerosis. J Nutr 1983;113: Giles WH, Kittner SJ, Anda RF, Croft JB, Casper ML. Serum folate and risk for ischemic stroke. First National Health and Nutrition Examination Survey epidemiologic follow-up study. Stroke 1995;26: DeBree A, van Dusseldop M, Brouwer IA, van het Hof KH, Steegers-Theunissen RP. Folate intake in Europe: recommended, actual and desired intake. Eur J Clin Nutr 1997;51: Markus HS, Ali N, Swaminathan R, Sankaralingam A, Molloy J, Powell J. A common polymorphism in the methylene tetrahydrofolate reductase gene, homocysteine, and ischemic cerebrovascular disease. Stroke 1997;28: Brattstrom L, Landgren F, Israelsson B, Lindgren A, Hultberg B, Andersson A. Lowering blood homocysteine with folic acid based supplements: meta-analysis of randomized trials. Homocysteine Lowering Trialist s Collaboration. BMJ 188;316: VOL 46: NOVEMBER NOVEMBRE 2000 Canadian Family Physician Le Médecin de famille canadien 2245