Hyperhomocysteinemia and thrombosis: acquired conditions

Hyperhomocysteinemia is a condition which, in the absence of kidney disease, indicates a disrupted sulfur amino acid metabolism, either because of vitamin (folate, B12 and B6) deficiency or a genetic defect. Epidemiological evidence suggests that mild hyperhomocysteinemia is associated with increased risk of arteriosclerotic disease and stroke. The relationship between hyperhomocysteinemia and thrombosis has been investigated in 10 studies involving a total of 1200 patients and 1200 controls. Eight of these studies demonstrated positive association with odds ratios that ranged from 2 to 13. This association was enhanced by including a methionine loading test. There is some evidence which suggests that hyperhomocysteinemia and APC resistance have a synergistic effect on the onset of thrombotic disease. Studies on the mechanism that underlies the relationship between thrombosis and hyperhomocysteinemia used non-physiologically high levels of homocysteine, rendering the data doubtful as to their patho-physiological relevance.     

Homocysteine: a new risk factor for atherosclerosis

The accumulating evidence for the role of homocysteine as a risk factor for atherosclerosis is persuasive. A high plasma homocysteine concentration induces pathologic changes in the arterial wall and thus is strongly associated with an increased risk of atherosclerosis, manifested as cardiovascular, cerebrovascular and peripheral vascular events. Studies are being conducted to determine whether lowering homocysteine levels prevents occlusive events. At present, testing for elevated homocysteine concentrations should be considered in patients with premature atherosclerosis or a strong family history of atherosclerosis, since hyperhomocysteinemia is a common risk factor in these patients. Treatment of hyperhomocysteinemia is straightforward and associated with minimal risk. This disorder is usually correctable with vitamin supplements containing folic acid.     

High plasma homocysteine: a risk factor for vascular disease in the elderly

Homocysteine is an intermediate compound formed during the metabolism of methionine. Several studies have shown that plasma homocysteine concentrations rise with age. Overt or borderline deficiencies of folate, vitamin B12 or B6 and possibly age-related kidney dysfunction are the major causes of homocysteine elevation in the elderly population. Multiple case-control and prospective studies have shown that a high plasma homocysteine concentration is an independent risk factor for cardiovascular diseases; this association persists in the elderly. Supplementation with folic acid either alone or with vitamins B12 and B6 can lower plasma homocysteine. Intervention studies to assess the effects (if any) of such treatment on prognosis are now in progress in patients with vascular disease.     

Treatment of hyperhomocysteinemia in renal transplant recipients. A randomized, placebo-controlled trial

BACKGROUND: Stable renal transplant recipients have an excess prevalence of hyperhomocysteinemia, which is a risk factor for arteriosclerosis. OBJECTIVE: To determine the effect of treatment with 1) vitamin B6 or 2) folic acid plus vitamin B12 on fasting and post-methionine-loading plasma total homocysteine levels in renal transplant recipients. DESIGN: Block-randomized, placebo-controlled, 2 x 2 factorial study. SETTING: University-affiliated transplantation program. PATIENTS: 29 clinically stable renal transplant recipients. INTERVENTION: Patients were randomly assigned to one of four regimens: placebo (n = 8); vitamin B6, 50 mg/d (n = 7); folic acid, 5 mg/d, and vitamin B12, 0.4 mg/d (n = 7); or vitamin B6, 50 mg/d, folic acid, 5 mg/d, and vitamin B12, 0.4 mg/d (n = 7). MEASUREMENTS: Fasting and 2-hour post-methionine-loading plasma total homocysteine levels. RESULTS: Vitamin B6 treatment resulted in a 22.1% reduction in geometric-mean post-methionine-loading increases in plasma total homocysteine levels (P = 0.042), and folic acid plus vitamin B12 treatment caused a 26.2% reduction in geometric-mean fasting plasma total homocysteine levels (P = 0.027). These results occurred after adjustment for age; sex; and pretreatment levels of total homocysteine, B vitamins, and creatinine. CONCLUSIONS: Vitamin B6 should be added to the combination of folic acid and vitamin B12 for effective reduction of both post-methionine-loading and fasting plasma total homocysteine levels in renal transplant recipients.     

Deregulation of homocysteine metabolism and consequences for the vascular system

The link between vascular disease and elevated homocysteine levels has been recognized for more than 30 years, and association with moderately elevated levels has been suspected for 20 years. Homocysteine is a sulfhydryl-containing amino acid that is formed by the demethylation of methionine. It is normally catalysed to cystathionine by cystathionine beta-synthase a pyridoxal phosphate-dependent enzyme. Homocysteine is also remethylated to methionine by methionine synthase, a vitamin B12 dependent enzyme and by methylenetetrahydrofolate reductase. Environmental factors such as folate, or vitamin B12, or vitamin B6 deficiencies and genetic defects such as cystathionine beta-synthase or abnormality of methylene-tetrahydrofolate reductase or some vitamin B12 metabolism defects may contribute to increasing plasma homocysteine levels. Normal fasting levels of homocysteine lie within the range 6-16 mumol/l. Apart from differences in assay methods, age, sex and nutritional status may affect the plasma levels. Though it is now well known that homocysteine is an independent risk factor for premature vascular disease, the pathogenesis of homocysteine-induced vascular damage is, for the most part, unknown. It may be multifactorial, including direct homocysteine damage to the endothelium, an enhanced low-density lipoprotein peroxidation, an increase of platelet thromboxane A2, or a decrease of protein C activation.     

Antibacterial activity of bovine lactoferrin and its peptides against enterohaemorrhagic Escherichia coli O157:H7

OBJECTIVES: To assess the long term effects of small increases in dietary folic acid on the concentration of plasma homocysteine, an independent risk factor for occlusive vascular disease, in a general population. DESIGN: A randomized double-blind placebo-controlled intervention study. SUBJECTS: One hundred and nineteen healthy volunteers, whose intake of fortified or supplemental folic acid was low, were recruited by letter from the patient register of a large inner-city group general practice. METHODS: Volunteers were randomized to receive unfortified cereals, or cereals fortified with 200 microg of folic acid per portion, with or without other vitamins. Blood samples were taken presupplement and at 4, 8 and 24 weeks on treatment and analysed for plasma homocysteine, cysteine and vitamin B12 and serum and red cell folate. Ninety-four subjects completed the study providing blood samples on all four occasions. RESULTS: There were no significant changes in any measured parameter in those eating unfortified cereals. Overall, folic acid fortification of cereals led to significant increases (P < 0.001) in serum folate (66%), and red cell folate (24%), and a decrease in plasma homocysteine (10%; P < 0.001). There were no changes in vitamin B12 or cysteine. The homocysteine decrease persisted until the end of the study and was primarily seen in those who initially had the highest plasma homocysteine or the lowest serum folate. CONCLUSIONS: If homocysteine is found to be a causative risk factor in occlusive vascular disease, food fortification with physiological levels of folic acid should have a significant impact on the prevalence of the disease in the general population.     

An Arabidopsis mutant missing one acid phosphatase isoform

BACKGROUND: A high plasma homocysteine concentration is a risk factor for atherosclerosis and thrombosis, which are major causes of morbidity and mortality in heart transplant patients. High homocysteine concentrations may be caused by lower folate and vitamin B6 levels. We hypothesized that these patients might have high homocysteine concentrations and low levels of folate and vitamin B6, which could contribute to the development of vascular complications. METHODS: Total fasting plasma homocysteine was measured in 189 cardiac transplant recipients and in healthy controls, as were concentrations of folate, vitamin B12, vitamin B6, and creatinine. RESULTS: Homocysteine concentrations were higher in recipients than controls (19.1+/-13.0 vs. 11.0+/-3.0 micromol/L, P<0.01), and hyperhomocysteinemia (>90th percentile for controls, 14.6 micromol/L) was seen in 68% of recipients (P<0.01). Folate and vitamin B6 concentrations were lower (5.9+/-4.2 vs. 7.9+/-4.2 pmol/L and 40+/-25 vs. 84+/-77 nmol/L, respectively; P<0.01 for both). Folate and vitamin B6 deficiencies were seen in 10.8% and 17.91% of recipients, respectively (P<0.01). Hyperhomocysteinemia was more frequent in patients with vascular complications after transplantation than in those without (79.2% vs. 63.8%, P<0.05). CONCLUSIONS: Elevated plasma homocysteine and deficiencies of folate and vitamin B6 are common in transplant recipients. A high homocysteine concentration was more common in patients with vascular complications. Prospective studies are now required to evaluate the role of these abnormalities as risk factors for the atherothrombotic complications of transplantation.     

Hyperhomocysteinemia and venous thrombosis: a meta-analysis

Hyperhomocysteinemia is an established risk factor for atherosclerosis and vascular disease. Until the early nineties the relationship with venous thrombosis was controversial. At this moment ten case-control studies on venous thrombosis are published. We performed a metaanalysis of these reports. We performed a MEDLINE-search from 1984 through June 1997 on the keywords "homocysteine" or "hyperhomocysteinemia" and "venous thrombosis", which yielded ten eligible case-control studies. We found a pooled estimate of the odds ratio of 2.5 (95% CI 1.8-3.5) for a fasting plasma homocysteine concentration above the 95th percentile or mean plus two standard deviations calculated from the distribution of the respective control groups. For the post-methionine increase in homocysteine concentration we found a pooled estimate of 2.6 (95% CI 1.6-4.4). These data from case-control studies support hyperhomocysteinemia as a risk factor for venous thrombosis. Further research should focus on the pathophysiology of this relationship and on the clinical effects of reducing homocysteine levels by vitamin supplementation.     

Homocysteine and coronary heart disease. Is slight or moderate homocysteinemia related to increased risk of coronary heart disease?

Homocystinuria, a rare inherited disturbance of amino acid metabolism is associated with severe atherosclerosis and thromboembolism already in childhood. The incidence of the homozygous disease of cystathionine beta synthase is estimated to be 1:200,000, that of the heterozygous form 1:300. There are, however, numerous other causes of a mild to moderate homocysteinemia, for example, a deficiency of the cofactors vitamin B6, B12 and folic acid. The question as to whether a mild to moderate elevation of homocysteine in the plasma is also associated with an increased risk of CAD has been investigated in a number of studies in recent years. In summary, however, the presently available data do not provide a basis of proof that a mild to moderate homocysteinemia is an independent risk factor for CAD. Our present knowledge suggests that only when the family history is clearly positive does it appear reasonable to undertake a diagnostic search for possible homocysteinemia.     

Genetic factors in the development of atherosclerosis]

Atherosclerosis is a complex multifactorial disease of the arterial wall, dependent on genetic disposition and multiple other risk factors. There are probably several candidate genes, that determine the individual susceptibility of the vessel wall to develop atherosclerosis. In recent years, a growing number of gene polymorphisms, associated with an elevated risk of myocardial infarction, has been identified. These genes and gene clusters play a crucial role in lipid metabolism, regulation of blood pressure and clotting. In contrast to rare monogenetic diseases with severe clinical signs and symptoms (e.g. familial hypercholesterolemia), genetic polymorphisms are relatively frequent. Due to their frequency, there is a high probability that one individual carries several alleles predisposing to coronary heart disease. Genetic polymorphisms become clinically important by interacting with lifestyle, environmental factors or endogenous metabolic disorders. We have recently established an animal model of rabbits, which may prove useful in the search for new genes predisposing to or protecting from atherosclerosis. Rabbits with high atherosclerotic response (HAR) show more than 70% atherosclerotic involvement of the aorta when fed a high cholesterol diet. In contrast, rabbits with low atherosclerotic response (LAR) show less than 20% atherosclerosis in spite of comparably high plasma cholesterol levels. Preliminary studies indicate that macrophages of LAR rabbits have a high scavenger receptor activity and high apolipoprotein E expression and thus appear to be very efficient in uptake and elimination of modified lipoproteins. This may result in a more efficient removal of cholesterol from the arterial wall and thus protect the animals from developing atherosclerosis. Today we are only at the beginning of understanding the complexity of gene interaction in atherosclerosis. Further identification of genetic factors of atherosclerosis will no doubt lead to a more efficient and economic prevention of coronary heart disease in the future.     

The role of hyper-homocysteinemia in the etiology of some vascular diseases

About 5% of population have a highly, while other 15% a moderately elevated plasma homocysteine level. Hyperhomocysteinemia may be responsible about 10-20% of coronary artery, 40% of cerebrovascular and 60% of peripheral vascular diseases. There in an inverse relationship between folate, cobalamin and pyridoxine intake or blood level and plasma homocysteine level. In addition, the intake of these three B vitamins can reduce high plasma homocysteine level. Folate-folic acid seems to be the most important in homocysteine reduction due to the compensation of thermolabile methylenetetrahydrofolate reductase insufficiency, however, a milder impact of cobalamin any pyridoxine (mainly following a methionine load test) is also proved. There are possibilities to reduce risk associated with elevated homocysteine: e. g. dietary supplementation or food fortification. In Hungary bread enriched by folic acid, cobalamin and pyriodixine might reduce rate of vascular diseases due to hyperhomocysteinemia.     

Treatment of mild hyperhomocysteinemia in vascular disease patients

Mild hyperhomocysteinemia is recognized as a risk factor for premature arteriosclerotic disease. A few vitamins and other substances have been reported to reduce blood homocysteine levels, but normalization of elevated blood homocysteine concentrations with any of these substances has not been reported. Therefore, we screened 421 patients suffering from premature peripheral or cerebral occlusive arterial disease by oral methionine loading tests for the presence of mild hyperhomocysteinemia. Thirty-three percent of patients with peripheral and 20% of patients with cerebral occlusive arterial disease were identified with mild hyperhomocysteinemia (14% of the men, 34% of the premenopausal women, and 26% of the postmenopausal women). Mildly hyperhomocysteinemic patients were administered vitamin B6 250 mg daily. After 6 weeks methionine loading tests were again assessed to evaluate the effect of treatment. Patients with nonnormalized homocysteine concentrations were further treated with vitamin B6 250 mg daily and/or folic acid 5 mg daily and/or betaine 6 g daily, solely or in any combination. Vitamin B6 treatment normalized the afterload homocysteine concentration in 56% of the treated patients (71% of the men, 45% of the premenopausal women, and 88% of the postmenopausal women). Further treatment resulted in a normalization of homocysteine levels in 95% of the remaining cases. Thus, mild hyperhomocysteinemia, which is frequently encountered in patients with premature arteriosclerotic disease, can be reduced to normal in virtually all cases by safe and simple treatment with vitamin B6, folic acid, and betaine, each of which is involved in methionine metabolism.     

Prospective blinded study of the relationship between plasma homocysteine and progression of symptomatic peripheral arterial disease

PURPOSE: An elevated plasma homocysteine level is an established risk factor for atherosclerotic coronary heart disease (CHD), cerebrovascular disease (CVD), and lower extremity occlusive disease (LED). An elevated plasma homocysteine level can be reduced by therapy with folate and vitamins B6 and B12. An accurate evaluation of the role of vitamin therapy requires knowledge of the influence of plasma homocysteine levels on the progression of CHD, CVD, and LED. METHODS: The Homocysteine and Progression of Atherosclerosis Study is a blinded prospective study of the influence of homocysteine and of other atherosclerotic risk factors on the progression of disease in patients with symptomatic CVD, LED, or both. This study is set in a university hospital vascular surgery clinic and the General Clinical Research Center. Consecutive patients with stable symptomatic CVD or LED underwent baseline clinical, laboratory, and vascular laboratory testing for homocysteine and other risk factors and were examined every 6 months. The primary endpoints were ankle brachial pressure index, duplex scan-determined carotid stenosis, and death. The secondary endpoints were the clinical progressions of CHD, LED, and CVD. The hypothesis that was tested was whether the progression of symptomatic CVD or LED was more frequent or more rapid in patients with elevated plasma homocysteine levels. plasma homocysteine levels. RESULTS: After a mean follow-up period of 37 months (range, 1 to 78 months) for deaths from all causes (>14 micromol/L; elevated, 18.6%; normal, 9.4%; P = .022), deaths from cardiovascular disease (elevated, 12.5%; normal, 6.3%; P = .05) and the clinical progression of CHD (highest 20% of homocysteine levels, 80%; lowest 20% of homocysteine levels, 39%; P = .007) were significantly more frequent or more rapid by life-table analysis when the homocysteine levels were elevated. Multivariate Cox proportional hazards regression model showed a significant independent and increasing relationship between the plasma homocysteine levels and the time to death (relative risk for highest one third of homocysteine values, 1.6; 95% confidence interval [CI], 1.04 to 2.56; P = 029; and relative risk for highest one fifth of homocysteine values, 3.13; 95% CI, 1.69 to 6.64; P = .0001). After an adjustment for age, smoking, hypertension, diabetes, cholesterol, and the vascular laboratory progression of CVD or LED, each 1.0 micromol/L increase in the plasma homocysteine levels resulted in a 3.6% increase (95% CI, 0.0% to 6.6%; P = .06) in the risk of death (all causes) at 3 years and a 5.6% increase (95% CI, 2.2% to 8.5%; P = .003) in the risk of death from cardiovascular disease. CONCLUSION: We conclude that elevated plasma homocysteine levels are associated significantly with death, with death from cardiovascular disease, and with the progression of CHD in patients with symptomatic CVD or LED. These results strongly mandate clinical trials of homocysteine-lowering vitamin therapy in such patients.     

Acute methionine load-induced hyperhomocysteinemia enhances platelet aggregation, thromboxane biosynthesis, and macrophage-derived tissue factor activity in rats

A moderate elevation of plasma homocysteine is a risk factor for atherosclerosis and arterial and veinous thrombosis. However, the mechanisms leading to vascular disorders are poorly understood because studies that have investigated the potential atherothrombogenicity of hyperhomocysteinemia in vivo are scarce. Using a rat model, we were the first to show that dietary folic acid deficiency, a major cause of basal hyperhomocysteinemia, is associated with enhanced macrophage-derived tissue factor and platelet activities. We proposed that an homocysteine-induced oxidative stress may account for this hypercoagulable state. To determine the true thrombogenicity of moderate hyperhomocysteinemia and better understand its etiology, we have carried out an acute methionine load in control and folate-deficient animals. When rats were fed the control diet, a transient fourfold increase in plasma homocysteine levels was observed 2 h after the methionine administration. As with prolonged dietary folic acid deficiency, this methionine load potentiated the platelet aggregation in response to thrombin and ADP as well as the thrombin-induced thromboxane synthesis. It also stimulated the basal and lipopolysaccharide-induced tissue factor activity of peritoneal macrophages. These prothrombotic effects were associated with an increased lipid peroxidation characterized by an elevation of plasma conjugated dienes, lipid hydroperoxides, and thiobarbituric acid-reactive substances. When rats were fed a folic acid-deficient diet, the methionine load did not cause any further increase in plasma homocysteine concentration, platelet activation, macrophage tissue factor-dependent coagulation, or lipoperoxidation. Altogether, our data showed that the prethrombotic state due to both the altered remethylation and transsulfuration pathways resulted from the moderate elevation of circulating homocysteine. We conclude that moderate hyperhomocysteinemia plays a role in the development of a thrombogenic state that might be mediated by the occurrence of oxidative stress.     

Adenylosuccinase deficiency: clinical and biochemical findings in 5 Czech patients

Polycythemia and hyperhomocysteinemia are risk factors for thrombosis. Since red blood cells actively metabolize methionine to homocysteine, we investigated whether or not patients with polycythemia have increased plasma levels of homocysteine, which might contribute to their increased thrombotic risk. In ten patients with polycythemia, the plasma homocysteine levels were measured before phlebotomy, three days after the procedure and 1-2 months later. The baseline mean plasma homocysteine levels in patients (9.7 +/- 1.6 mumol/L [+/-SD]) did not differ significantly from that found in 30 sex- and age-matched healthy controls (12.2 +/- 6.9). Despite a fall in the patients' mean [+/-SD] hematocrit from 0.50 +/- 0.02 at baseline to 0.47 +/- 0.03 three days after phlebotomy (significant at 95%) and to 0.48 +/- 0.02 after 1 to 2 months (not significant), the mean plasma homocysteine levels did not change significantly (9.9 +/- 2.3 mumol/L at 3 days and 9.7 +/- 2.1 mumol/L at 1-2 months). It is unlikely that high plasma homocysteine levels contribute to the increased thrombotic risk of polycythemic patients.     

Plasma homocysteine, a risk factor for cardiovascular disease, is lowered by physiological doses of folic acid

Elevated plasma homocysteine, an independent risk factor for cardiovascular disease (CVD) can be lowered by administration of pharmacological doses of folic acid. The effect of lower doses in apparently normal subjects is currently unknown but is highly relevant to the question of food fortification. Healthy male volunteers (n = 30) participated in a chronic intervention study (26 weeks). Folic acid supplements were administered daily at doses increasing from 100 micrograms (6 weeks), to 200 micrograms (6 weeks), to 400 micrograms (14 weeks). Fasting blood samples collected before, during and 10 weeks post intervention were analysed for plasma homocysteine, serum and red-cell folate levels. Results, expressed as tertiles of baseline plasma homocysteine concentration, showed significant (p < or = 0.001) homocysteine lowering in the top (10.90 +/- 0.83 mumol/l) and middle (9.11 +/- 0.49 mumol/l) tertiles only. In the low tertile, where the mean baseline homocysteine level was 7.07 +/- 0.84 mumol/l, no significant response was observed. Of the three folic acid doses, 200 micrograms appeared to be as effective as 400 micrograms, while 100 micrograms was clearly not optimal. There is thus a minimal level of plasma homocysteine below which folic acid has no further lowering effect, probably because an optimal folate status has been reached. A dose as low as 200 micrograms/day of folic acid is effective in lowering plasma homocysteine concentrations in apparently normal subjects. Any public health programme for lowering homocysteine levels, with the goal of diminishing CVD risk, should not be based on unnecessarily high doses of folic acid.     

Chronic heart failure in the United States: a manifestation of coronary artery disease

BACKGROUND: Hyperhomocysteinemia occurs in renal failure and may increase the risk for cardiovascular disease, possibly by damaging the endothelium. Folic acid and betaine are required in two separate homocysteine conversion pathways and may therefore lower plasma homocysteine. OBJECTIVE: To study the therapeutic role of betaine and the effect on endothelial function of long-term homocysteine-lowering therapy with folic acid, in peritoneal dialysis (PD) patients. PATIENTS AND DESIGN: Thirty PD patients were randomized to a 12-week treatment with 5 mg folic acid and 4 g betaine daily, or to 5 mg folic acid alone daily. They were then rerandomized to treatment with 1 or 5 mg folic acid daily for 40 weeks. MEASUREMENTS: At baseline and after 52 weeks, endothelial function was assessed by determination of endothelium-dependent vasodilatation and biochemical markers. RESULTS: Plasma total homocysteine (tHcy) was elevated at baseline: 42.6 (5.8) micromol/L. Only 1 patient (3%) had a normal plasma homocysteine (i.e., < or = 15 micromol/L) before therapy. Normalization of plasma homocysteine occurred in 39% of the patients at 12 weeks. Betaine had no additional homocysteine-lowering effect. Plasma tHcy levels were similar during treatment with 1 or 5 mg folic acid daily. Endothelial function was impaired at baseline and had not improved after 52 weeks of treatment. CONCLUSIONS: Peritoneal dialysis patients have hyperhomocysteinemia, which can be normalized with folic acid alone in about 40% of patients. Betaine does not further lower plasma homocysteine. A maintenance dose of 1 or 5 mg folic acid daily results in equivalent plasma homocysteine levels. Long-term reduction in plasma homocysteine did not result in improvement of endothelial function as assessed by our methods.     

Vitamin nutrition status and homocysteine: an atherogenic risk factor

In an epidemiologic survey, a marginal status of folic acid, vitamin B12, and vitamin B6 was shown to be associated with hyperhomocysteinemia. In a case-control study, a low plasma folate concentration was associated with increased coronary heart disease risk. This phenomenon appears to be mediated by folate's effect on homocysteine metabolism. Both studies offer further perspectives on homocysteine as an atherogenic risk factor.     

Wavelength dependence of skin cancer induction by ultraviolet irradiation of albino hairless mice

BACKGROUND: Because of the inverse relation between dietary fish consumption and coronary heart disease and because of the importance of serum homocysteine as an independent risk factor for atherosclerosis, the effect of fish oil on serum homocysteine was studied in hyperlipemic men. METHODS: Fifteen men with either type IIa or IIb lipoproteinemia or hypertriglyceridemia were maintained on a controlled, balanced diet and given either fish oil or olive oil supplements, 12 g/d for 3 weeks, followed by a cross-over period of 3 weeks during which the olive oil or fish oil supplements were given in reverse order. Serum homocysteine was determined by liquid chromatography of acid hydrolyzates of whole serum. RESULTS: Fish oil was found to diminish serum homocysteine levels in 14 of 17 subjects (P < 0.01). Serum homocysteine was 48% +/- 33% less than control values in seven of nine patients and 36% +/- 22% less than values in seven of eight subjects who had first received olive oil. There was no effect of olive oil supplements on serum homocysteine, compared with control values, but olive oil produced an increase in serum homocysteine in those who had first received fish oil. Serum triglycerides and very low-density lipoprotein were decreased by fish oil in patients who were first given olive oil, in agreement with previous studies. There was no effect of either fish oil or olive oil on total cholesterol, apolipoprotein B, low-density lipoprotein, or high-density lipoprotein. CONCLUSIONS: The protection against coronary heart disease afforded by a diet rich in fish may be attributed to the lowering of serum homocysteine levels by the n-3 polyunsaturated fatty acids of fish oils.