Preventive health care, 2000 update: screening and management of hyperhomocysteinemia for the prevention of coronary artery disease events

Gillian L. Booth,* Elaine E.L. Wang,† with the Canadian Task Force on Preventive Health Care‡

CMAJ 2000;163(1):21-9

• Abstract
• Introduction
• Methods
• Screening tests for hyperhomocysteinemia
• Association between homocysteine and risk of CAD events
• Association between genetic predisposition to hyperhomocysteinemia and CAD
• Association between serum folate, vitamin B₆ or vitamin B₁₂ levels and risk of CAD events
• Effect of vitamin therapy
• Prevention of CAD
• Recommendations
• Research agenda
• References

Objective: To establish guidelines for the screening and treatment of hyperhomocysteinemia in the investigation and management of coronary artery disease (CAD).

Options: Measurement of plasma total homocysteine (tHcy) levels in the fasting state or 4–6 hours after oral methionine load; vitamin supplementation with folic acid and vitamins B₆ and B₁₂; adherence to the recommended daily allowance of dietary sources of folate and vitamins B₆ and B₁₂.

Outcomes: This article reviews the available evidence on the association between plasma tHcy levels and CAD and the effect of lowering tHcy levels through vitamin supplementation or dietary intake.

Evidence: MEDLINE was searched for relevant English-language articles published from January 1966 to June 1999; also reviewed were additional articles identified from the bibliographies.

Benefits, harms and costs: Cardiovascular disease is the leading cause of death in Canada. Homocysteine, generated in the metabolism of methionine, may have a role in the development of cardiovascular disease. The prevalence of hyperhomocysteinemia in the general population is between 5% and 10% and may be as high as 30%–40% in the elderly population. If population-based studies are correct, tHcy may be responsible for up to 10% of CAD events and thus may represent an important and potentially modifiable risk factor for cardiovascular disease. Laboratory testing for tHcy is currently restricted to research centres, and costs range from $30 to $50 per person. Newer, less costly techniques have been developed and should become readily available with time.

Values: The strength of evidence was evaluated using the methods of the Canadian Task Force on Preventive Health Care.

Recommendations: Although there is insufficient evidence to recommend the screening or management of hyperhomocysteinemia at present (grade C recommendation), adherence to recommended daily allowance of dietary sources of folate and vitamins B₁₂ and B₆ should be encouraged. If elevated tHcy levels are discovered, vitamin deficiency should be ruled out to allow specific treatment and prevention of complications, such as neurological sequelae due to vitamin B₁₂ deficiency. Experts in the field advocate treatment of elevated tHcy levels in high-risk people, such as those with a personal or family history of premature atherosclerosis or a predisposition to develop hyperhomocysteinemia. Definitive guidelines for the management of hyperhomocysteinemia await the completion of randomized trials to establish the effect of vitamin supplementation on CAD events.

Validation: The findings of this analysis were reviewed through an iterative process by the members of the Canadian Task Force on Preventive Health Care.

Sponsors: The Canadian Task Force on Preventive Health Care is funded through a partnership between the Provincial and Territorial Ministries of Health and Health Canada.

[Contents]

The prevalence of hyperhomocysteinemia in the general population is between 5% and 10%, according to a threshold set at the 90th or 95th percentile (about 15 µmol/L).⁷ However, rates may be as high as 30%–40% in the elderly population.¹⁰ If the results from population-based studies are correct, then up to 10% of events due to coronary artery disease (CAD) may be attributable to elevated plasma homocysteine levels.¹¹ Thus, homocysteine may represent an important and potentially modifiable risk factor for cardiovascular disease.

The purpose of this review was to evaluate the quality of evidence pertaining to homocysteine and CAD events and to make recommendations regarding the screening and management of hyperhomocysteinemia.

[Contents]

A computerized search of MEDLINE for English-language articles published between January 1966 and June 1999 was conducted using the MeSH (medical subject heading) terms "homocysteine," "hyperhomocysteinemia," "methionine," "coronary disease," "arteriosclerosis," "myocardial ischemia," "folic acid," "vitamin B₁₂," "vitamin B₆," and "pyridoxine" in various combinations. Relevant articles were also identified through a manual review of references. Where possible, the highest level of evidence was sought; hence, abstracts, cross-sectional studies, case reports and case series were not included. Studies concerning other types of vascular disease were also excluded.

The evidence was reviewed systematically using the methodology of the Canadian Task Force on Preventive Health Care. The task force, comprised of expert clinicians and methodologists from a variety of medical specialties, used a standardized evidence-based method (Appendix 1) for evaluating the effectiveness of this intervention. The final recommendations were arrived at unanimously by an expert panel and principal author. Feedback from 2 content experts was incorporated into a final draft of the manuscript before submission for publication. Procedures to achieve adequate documentation, consistency, comprehensiveness, objectivity and adherence to the task force's methodology were maintained at all stages during review development, the consensus process and production of the final manuscript. The full methodology has been described previously.¹²

[Contents]

Most assays measure levels of total homocysteine (tHcy) (protein-bound and complexed moieties)¹³ in the fasting state or 4–6 hours after an oral methionine load (0.1 mg/kg).⁷ High-pressure liquid chromatography, the most common method, has a coefficient of variation of 3%–11%.¹³ Samples must be placed immediately on ice to avoid spurious elevations in tHcy levels.⁷ Furthermore, levels are falsely low in the acute phase of illness such as myocardial infarction (MI).¹⁴ Current screening tests cost between $30 and $50; however, newer, less costly techniques for measuring tHcy levels have been developed¹⁵ and should become readily available with time.

Aside from genetic predisposition, a number of factors raise plasma tHcy levels, including increasing age, male sex and elevated serum creatinine levels.^5–7 Drugs such as antiepileptic agents, methotrexate and nitrous oxide⁶ and certain disease states such as psoriasis, acute lymphoblastic leukemia, breast cancer and hypothyroidism^5,6 also increase tHcy levels, likely through effects on vitamin status. Homocysteine is inversely correlated with serum vitamin B₆, vitamin B₁₂ and folate levels.¹⁰ Thus, in populations with a higher prevalence of vitamin B₁₂ deficiency, such as elderly people,¹⁶ the specificity of plasma tHcy as a cardiac risk factor may be reduced.

[Contents]

Retrospective studies

Over 30 case–control studies have compared tHcy levels between CAD patients and healthy control subjects.^17–46 Patients with CAD had significantly higher fasting plasma tHcy levels in 22 of 27 studies^18–39 (odds ratio [OR] 1.2–10.9 after adjustment for other cardiac risk factors). Levels after methionine load were also higher in patients with CAD in 8 of 9 studies^35–42 (adjusted OR 1.3–6.7).^35,43 Measurement of serum levels yielded similar results.^17,40 Moreover, 2 meta-analyses of retrospective data^11,47 confirmed these findings: the odds ratios of CAD associated with elevated plasma tHcy levels were 1.7 (95% confidence interval [CI] 1.5–1.9) and 6.14 (95% CI 2.74–13.73). The relation between tHcy levels and the number of occluded coronary vessels was not consistent.^17,30–32

It is possible that some other factor predisposes to both CAD and hyperhomocysteinemia. As anticipated, cardiac risk factors were more common among patients with CAD. Volunteer bias may exaggerate this difference because participants of clinical trials tend to be healthier and thus control subjects may have fewer risk factors than do people in the general population. Certain healthy behaviours, such as low caffeine intake⁴⁸ and multivitamin use,^10,35 are inversely associated with plasma tHcy levels. Homocysteine may be related to risk factors such as smoking,^35,37 hypertension,^32–35 dyslipidemia^27–29,35 and hyperglycemia;⁴⁹ however, it appears to have an independent effect^33–38 and may even interact with other factors to influence CAD risk.^19,35 In one study adjustment for plasma fibrinogen abolished the association between tHcy and CAD.¹⁷ The relation between tHcy and other unconventional risk factors is unknown. Thus, retrospective studies can show an association but not a causal relation.

Prospective studies

Eight nested case–control studies prospectively evaluated the relation between tHcy and the occurrence of a first major CAD event^47,50–55 or new-onset angina necessitating coronary artery bypass surgery.⁵⁶ Unfortunately, these studies had conflicting findings. MI and coronary death were associated with higher tHcy levels in only 4 of 7 studies,^47,50–52 and adjustment for prevalent CAD attenuated this relation in 1 study.⁵² In the MRFIT trial,⁵⁴ a minority of patients who had early CAD events had sufficient frozen plasma available to measure tHcy levels. Thus, in many cases, plasma tHcy may have been measured too far in advance. In contrast to major CAD events, the development of angina was not related to plasma tHcy.⁵⁶

Prospective cohort studies suggest that tHcy is a greater risk factor for major CAD events in patients with established CAD. In 2 studies^57,58 coronary events occurred more frequently in men with hyperhomocysteinemia than in men with normal tHcy levels; however, adjustment for prevalent CAD at baseline attenuated this association. In contrast, nonfasting tHcy levels were independently related to death due to cardiovascular disease (relative risk [RR] 1.52, 95% CI 1.16–1.98) and overall mortality (RR 1.54, 95% CI 1.31–1.82) in elderly men and women from the original Framingham cohort.⁵⁹ The most compelling evidence comes from Nygård and associates,⁶⁰ who prospectively followed 587 patients with significant stenosis on coronary angiography. A dose–response relation between baseline homocysteine levels and both death due to CAD and overall mortality was observed (Table 1). Similar findings were noted among patients with other forms of atherosclerosis.⁶¹

The evidence from the prospective studies suggests that homocysteine acts by promoting acute thromboembolic events. Few prospective studies have evaluated the role of homocysteine in the chronic progression of atherosclerosis. In the Shunt Occlusion Trial⁶² graft occlusion rates 1 year after coronary artery bypass surgery were not related to preoperative tHcy levels. However, atherosclerotic changes incurred by tHcy may require longer than 1 year to develop. In summary, the evidence strongly suggests that plasma tHcy is a risk factor for acute cardiac events in patients with underlying vascular disease.

[Contents]

Homozygosity for a mutation in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene, involved in homocysteine metabolism, is found in 4%–14% of the general population² and is associated with elevated plasma tHcy^63–67 levels under conditions of impaired folate status.^63–66 The importance of this mutation in the development of CAD may depend on the population. Studies involving Japanese people have consistently shown a higher prevalence of the mutation among patients with CAD,^68–71 whereas only a minority of studies involving whites detected an association.^24,36 Moreover, a relation between the MTHFR genotype and the number of occluded vessels on coronary angiography was observed in Japanese patients⁶⁸ but not in whites.^72–74 No association between the mutation and other risk factors, including a family history of premature CAD, has been shown.^73–75

In one meta-analysis, involving almost 5000 patients from 8 studies, the homozygous mutation was associated with an increased risk of CAD (OR 1.22, 95% CI 1.01–1.47).⁶⁷ However, a larger meta-analysis of 23 studies failed to demonstrate an association between the MTHFR genotype and either CAD (OR 1.11, 95% CI 0.91–1.37) or any cardiovascular end point (OR 1.12, 95% CI 0.92–1.37).⁷⁶ A Japanese study found a declining prevalence of the MTHFR mutation with increasing age; this finding suggests that the mutation may lead to early deaths due to cardiovascular disease.⁷⁷ However, the mutation does not appear to be related to longevity in whites (OR 0.87, 95% CI 0.69–1.11).⁷⁸ In light of these observations, it has been suggested that other genetic abnormalities or risk factors may interact with the MTHFR mutation to increase cardiovascular risk.⁷⁹

[Contents]

Homocysteine may simply be a nonspecific marker of vitamin deficiency. Several studies identified an inverse relation between serum folate levels and CAD events on univariate analysis; however, adjustment for plasma tHcy levels obliterated this effect.^80–82 Serum vitamin B₁₂ levels have no relation to CAD; however, vitamin B₆ may be an independent risk factor. Robinson and associates⁸² observed an increased risk of CAD among patients with low vitamin B₆ levels (OR 1.84, 95% CI 1.39–2.42) that remained after adjustment for plasma tHcy levels. Similarly, homocysteine remained a significant predictor of CAD after controlling for serum vitamin levels.^60,82

[Contents]

Several randomized controlled trials have evaluated the effect of multivitamin supplementation on fasting tHcy levels. All treatment regimens, including a combination of folic acid (1–5 mg), vitamin B₆ (5–50 mg) and vitamin B₁₂ (0.02–1 mg), lowered fasting plasma tHcy levels by 20% to 50%.^83–88 Treatment response in patients with CAD was equivalent to that in healthy control subjects.²⁴ A meta-analysis⁸⁹ of 12 studies, involving a total of 1114 patients, revealed that folic acid (0.4–5 mg) lowered tHcy levels by 25% on average in people with or without vascular disease. Higher plasma tHcy levels and lower serum folate levels enhanced this effect. The addition of vitamin B₁₂, but not vitamin B₆, led to a further reduction in fasting tHcy levels of about 7% (95% CI 3%–10%). The effect of folic acid was maximal at 0.5 mg/d; however, 0.2 mg may be sufficient to normalize fasting tHcy levels in some patients.^90,91 Lower doses, such as 0.1 mg, appear to be ineffective.^91,92

Although vitamin B₆ was found to have little effect on fasting plasma tHcy levels, uncontrolled studies have illustrated its utility in the treatment of hyperhomocysteinemia after methionine load. Post-load tHcy levels were found to decline by 21%–42% following treatment with vitamin B₆ (50–250 mg).^93,94 In patients who respond poorly to vitamin B₆ (up to 25%⁹⁴), folic acid may be effective. In the majority of patients 6 weeks of treatment with single or combination therapy is sufficient to normalize tHcy levels.⁹⁵

In a study involving young women folate from natural food sources or fortified foods had the same effect on plasma tHcy levels (12%–21% reduction) as equivalent doses of vitamin supplements.⁹⁶ However, in middle-aged men and women, breakfast cereal fortified with 666 µg of folic acid per day had a modest effect on fasting tHcy levels (10%–14% reduction),^97–99 regardless of whether vitamin B₆ or B₁₂ or other micronutrients were added.⁹⁸ Thus, current recommendations by the Food and Drug Administration in the United States to fortify cereal grain products with 140 µg of folic acid per 100g serving may be insufficient to normalize elevated homocysteine levels.⁹⁹

[Contents]

Whether lowering plasma homocysteine levels will prevent CAD events is unknown because of the absence of longer randomized controlled trials with appropriate end points. Some epidemiological data suggest that folate and vitamin B₆ intake may influence the occurrence of major CAD events. The Nurses' Health Study¹⁰⁰ found that women who consumed more than 400 µg of folate or 3 mg of vitamin B₆ per day had a lower risk of heart disease than those with lower intake levels (adjusted RR 0.69 for folate, 95% CI 0.55–0.87, and 0.67 for vitamin B₆, 95% CI 0.53–0.85). Furthermore, an uncontrolled study showed that areas of plaque in carotid arteries regressed in 38 patients with hyperhomocysteinemia who were given daily amounts of folic acid (2.5–5 mg), vitamin B₆ (25 mg) and vitamin B₁₂ (250 µg) over 4 years.¹⁰¹ Similarly, in another study patients with homocystinuria who received a minimum of folic acid (5 mg) and vitamin B₆ (100–200 mg) experienced fewer vascular events (RR 0.09, 95% CI 0.02–0.38) than patients who remained untreated.¹⁰² Although striking, findings from the latter study cannot be extrapolated to the general population because of patient differences and other methodological issues.

[Contents]

By the Canadian Task Force on Preventive Health Care

The task force's recommendations are summarized in Table 2. There is insufficient evidence to include or exclude screening of tHcy levels in any population (grade C recommendation). Screening may enable identification of patients at high risk for CAD so that other risk factors can be managed aggressively. However, laboratory testing for homocysteine is currently restricted to research centres. Moreover, testing is not yet covered by provincial health insurance, and therefore patients may be required to cover the cost.

Although folic acid effectively lowers plasma tHcy levels, there is insufficient evidence to suggest that its use would prevent CAD events (grade C recommendation). Adherence to the recommended daily allowance of dietary sources of folate and vitamins B₆ and B₁₂ (Table 3) may prevent hyperhomocysteinemia due to vitamin deficiency. Once elevated tHcy levels are discovered, vitamin deficiency should be ruled out to allow specific treatment and prevention of complications, such as neurological sequelae due to vitamin B₁₂ deficiency. Some authorities recommend limiting folic acid intake to 1 mg/d^103,104 or adding higher doses of vitamin B₁₂ (0.2–1 mg/d)¹⁰⁵ because of the theoretical risk of unmasking occult vitamin B₁₂ deficiency.¹⁰³

By other groups

Guidelines from the American Heart Association¹⁰⁴ state that it may be reasonable to screen tHcy levels in people who are at risk for hyperhomocysteinemia (e.g., those with renal failure) or in those who have a personal or family history of premature atherosclerosis. Several experts in the area concur^5,105,106 and suggest lowering fasting tHcy levels to less than 10 µmol/L.¹⁰⁵ If initial treatment with dietary sources are ineffective, then supplements or fortified foods containing at least 400 µg of folic acid, 2 mg of vitamin B₆ and 6 µg of vitamin B₁₂ can be used.

[Contents]

Large-scale randomized trials designed to assess the effect of folic acid therapy on cardiovascular events are underway¹⁰⁷ (Table 4). Thus, evidence on which to base recommendations for the treatment of hyperhomocysteinemia is forthcoming. The optimal vitamin dose and regimen, and the role of methionine-load testing in the diagnosis of this disorder need to be clarified. Although folic acid attained through food sources alone may be insufficient to normalize elevated tHcy levels, people with low folate intake are more susceptible to hyperhomocysteinemia. Most flour and cereal products consumed by Canadians are produced domestically, where folic acid fortification is optional. Review of fortification policies will be necessary as our knowledge regarding prevention and treatment of hyperhomocysteinemia advances.

We thank Dr. Jacques Genest, Institut de Recherches Cliniques de Montréal, Montreal, and Dr. Killian Robinson, Department of Cardiology, Cleveland Clinic Foundation, Cleveland, for reviewing a draft of this manuscript. The views expressed in this report are those of the authors and the Canadian Task Force on Preventive Health Care and do not necessarily reflect the positions of reviewers.

Competing interests: None declared.

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[Contents]

Reprint requests to: Canadian Task Force on Preventive Health Care, Parkwood Hospital, 801 Commissioners Rd. E, London ON N6C 5J1; ctf@ctfphc.org

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