Homocysteine is a naturally occurring amino-acid which derives from dietary protein and which, under conditions of nutrient adequacy, is broken down to harmless methionine through a process of methylation.
However, if methylation cannot take place, due to a lack, or absence, of certain B-vitamins, and certain other nutrients, then blood levels of homocysteine can rise, resulting in a condition called, hyperhomocysteinaemia , which may in turn cause damage to the inner lining of blood vessels ( the vascular endothelium) with subsequent inflammation in the wall of the artery, resulting in plaque formation, and the potential for thrombi formation and vessel occlusion with end-organ damage.
Architectural change to the lining of an arterial wall with an unstable plaque sets the stage for cardiovascular disease and increased rate of mortality, with the coronary arteries susceptible targets for resultant blockage.
Cardiovascular disease is Ireland’s number one killer, with 2,665 cardiovascular related deaths being recorded in the first 3 months of 2006.
Coronary heart disease remains a major public health concern throughout the western world, and traditional identifiable risk factors, such as hypertension, cigarette smoking, obesity, diabetes mellitus and hyperlipidaemia, have received widespread attention with concerted efforts being made at various levels to implement risk reduction strategies for related cardiovascular disease.
More recently, renewed interest in the possible deleterious effects of raised plasma levels of homocysteine (tHcy) have prompted a review of earlier literature, and several meta-analyses have been conducted against the background of ongoing trials for purposes of clarifying spurious interpretations and moving towards a consensus for implementing or withholding appropriate therapeutic strategies for risk reduction.
The literature abounds with extensive trial data and interpretations, often provoking controversy and dissention in the debate about conclusive “evidence”.
Conflicting views and contradictory interpretations about the significance of hyperhomocysteinaemia continue, and in the absence of accepted clinical guidelines clinicians have been challenged to ignore the oft-perceived bias of opinion shapers, and formulate their own policies on the basis of evidence they themselves judge to be biologically plausible and potentially useful as a basis for therapeutic strategy within the clinical setting.
Much of the current documented evidence is supportive of earlier proposals, and strongly suggestive of the view, that tHcy level determinations in clinical practice have an important and underestimated role to play in respect of identifying high risk patients, and clinicians are challenged to be more proactive in selecting patients for tHcy status assessment.
The practice of general population screening has received attention, and while not currently supported by many investigators, is considered by others (Stephen Barrett) to be beneficial in respect of identifying risk and reducing morbidity and mortality.
The decision to pursue such a policy is an option for primary care physicians and a subject for future debate.
Homocysteine is a thiol—containing amino acid intermediate that is formed during the metabolism of methionine, an essential amino acid, present in a protein diet.
The observation, by Kilmer McCully in 1969, that children and young adults with inborn errors of tHcy metabolism developed anteriosclerosis raised the possibility that milder elevations of tHcy might predispose individuals to atherosclerotic vascular disease.
McCullly’s theories, while plausible, did not receive universal acceptance, and he was punished with dismissal from his academic post for his non-conformist views. Nevertheless, to this day be continues to focus his research on the complex interplay of hyperhomocysteinaemia and vascular endothelium dysfunction.
Now, 37 years later, the accumulated evidence points clearly to hyperhomocysteinaemia as an independent risk factor for arteriosclerotic outcome, with numerous epidemiological studies confirming the prevalence of raised tHcy in patients with stroke, myocardial infarction, peripheral vascular disease, and venous thrombosis.
Prospective studies have largely supported (with a few exceptions) these findings and the outcomes of continuing trials such as Vitatops are awaited.
However, a recent meta-analysis report from Tulane University, New Orleans (13 Dec 2006) concludes little or no benefit from Folate in cardiovascular disease.
This report contradicts the findings of Wald et al., but is not without its own shortcomings.
CAUSAL OR CASUAL ?
The strong associations between tHcy and morbidity and mortality, in the presence of atherogenesis has prompted debate about causality, with some researchers holding the view that homocysteine is merely a surrogate marker for endotheial dysfunction, rather than an independent causal agent, as proposed by McCully.
Mechanisms of atherogenesis under the direct influence of tHcy are well researched and described by investigators such as Hofman, Loscalzo, Werstuck, and others, and the complex interplay of tHcy and hypercholesterolaemia in plaque formation has been documented by Zhou et al.
The authors of VISP challenged the validity of the “homocysteine hypothesis” of atherothrombotic vascular disease, yet believe it to be viable. (The initial negative results of VISP, contradicting the homocysteine hypothesis, have been dismissed on grounds of withholding important and unreported data, and thus challenged by Graham Hanley.)
Australians, Graham Hanley and John Eikelboom, believe that the tHcy hypothesis of atherothrombotic vascular disease in general, and stroke in particular, remains viable.
Drs. Graham and O’Callaghan (Dublin) have concluded from an Irish survey that an increase in tHcy is associated with an increased risk of cardiovascular disease, and state that “this association may be causal—it is biologically plausible, fairly strong, graded, and an increase in tHcy precedes the onset of vascular disease.” (2004)
Subsequent research (AHJ 2004) has shown that rHcy “causes endothelial dysfunction and damage, accelerates thrombin formation, inhibits native thrombolysis, promotes lipid peroxidation through free radicle formation and induces vascular smooth muscle proliferation and monocyte chemotaxis”.
More recently Prof. Peter Jacobs (Tygerberg Hosp. South Africa) has drawn attention to the time-honoured classical triad of Virchow, in which dysfunctional intimal cells, impaired by tHcy, are perceived as a target favouring thrombocyte adhesion. He thus acknowledges the role that homocysteine may play in damaging vascular endothelium, setting the stage for inflammatory changes in the vessel wall.
Arguments for “cause” are plausible, strong, and becoming frequent, despite strong resistance from the camp that espouses reverse-causality.
Bostom and Culleton (Framingham 1999) found the arguments for reverse causality untenable in the light of pooled epidemiological evidence from all published observational studies.
However, the debate continues.
The Hordaland Homocysteine Study (HSS) in Norway investigated 18000 subjects from 1992-1999 and found that subjects with raised tHcy levels had increased risk of cardiovascular morbidity, cardiovascular and non-cardiovascular mortality, and are more likely to suffer from depression and from cognitive defect (elderly).
Among females, raised tHcy levels were associated with decreased bone mineral density and increased risk of osteroporosis. Women with raised tHcy levels were also shown to have an increased risk of pregnancy complications and adverse pregnancy outcome. And for most conditions there is a continuous concentration—response relation with no apparent threshold concentration.
Overall, the findings of HSS indicate that a raised tHcy level is associated with multiple clinical conditions, whereas a low tHcy level is associated with better physical and mental health.
Velset and Refsum (HSS) reported that a tHcy increment of 5 uM/L was associated with a 49% increase in all cause mortality and a 104% increase in non cancer, non-cardiovascular mortality.
They conclude that tHcy is a strong predictor of both cardiovascular and non-cardiovacular mortality in a general population of 65-72 year olds, a result which prompts the need to extend enquiry or tHcy effect beyond cardiovascular disease.
David Wald and colleagues (BMJ 25 Nov 2006) examined the evidence from 3 groups of study, showing from cohort studies that a 3umol/l decrease in serum homocystene lowers the risk of myocardial infarction by 15% and stroke by 24%, and in two meta-analyses of genetic polymorphism studies, moderately raised homocysteine concentrations occurring as a result of mutation in the methylenetetrahydrofolate reductase (MTHFR) gene as 25% reduction in risk for a 3 umol/L decrease in tHcy.
Wald dismissed with, sound argument, the conclusions of those authors who denied causality on the basis of publication bias and heterogeneity in their meta analysis.
Randomised controlled trials have been reported and others are in progress.
Despite the reported reduction in the risk of stroke in HOPE-2, no reduction in cardiovascular risk was evident in the first 2 years, a fact the Wald attributes to the inadequacy of a short term trial and is thus misleading in its conclusions.
Wald’s conclusion that tHcy is a cause of cardiovascular disease is rooted in evidence from different types of study, and his team endorses earlier recommendations to implement tHcy lowering strategies with the confidence of thereby reducing heart attacks and stroke, a timely and welcome (overdue?) strategy in the face of escalating vascular related disease worldwide.
The emergence of tHcy as a prevalent risk factor for artheroscleros vascular disease in the coronary, cerebral and peripheral vessels, and for arterial and venous thromboembolism has prompted investigations into the use of Folate, Cobalamin (B12) and Pyridoxine (B6), as well as Riboflavin (B2), Zinc and Trimethylglycine (TMG) as nutritional methods of reducing risk by controlling hyperhomocysteinaemia.
Folate deficiency is inversely related to elevated tHcy levels. In the Nutrition Canada Survey (JAMA 1996) involving over 5000 subjects a statistically significant association was found between serum Folate levels and risk of fatal coronary heart disease.
The Nurses Health Study, over a 14-year follow-up period showed similar results with a calculated risk reduction of 5.8% of coronary heart disease for every 100 mcg per day increase in Folate.
These figures correlate with the findings of Boushey and co-workers who estimated that 100 mcg per day of Folate would reduce tHcy by 6% and coronary heart disease by 5%.
(The recommended daily dosage of Folate for homocysteine reduction is between 800mcg and 3200 mcg, according to Life Extention researchers.)
The benefits of Folate are widely attested to, despite a few inconclusive trials, such as NORVIT in which reports of the adverse outcome of Folate are judged by Prof. Peter Jacobs to be a “premature comment of unsubstantiated preliminary finding.”
THE FOLATE CONNECTION:
Folate and B12 are required for the remethylation of homocysteine, via the MTHFR enzyme to S-adenosyl-methionine(SAME), the precursor to Tyrosine and Tryptophan,hormones vital for mental and physical wellbeing.
Benefits in respect of neural tube development in expectant mothers, and risk reduction for cardiovascular disease,as well as improved cognitive function in the elderly and improved renal function with Folate supplementation are widely reported.
The apparent negative results of VISP in terms of cardiovascular risk reduction with the use of Folate may be explained by the fact of high prevalence of vitamin B supplement use in the North American community tested, as well as widespread fortification with Folate of the grain supply.
Rather than negative, VISP results may be described as “equivalent”.
There exists, however, potential harm in the careless and indiscriminate use of Folate alone, and clinicians should be mindful of this risk.
Since Folate can reverse megaloblastic anaemia caused by Vit B12 deficiency, it will not halt the neurological degeneration arising from Vit B12 deficiency, permitting the condition to progress unabated and unrecognised.
This potential may be aggravated in the present climate of folic acid fortification initiatives, and increased vigilance is called for in the treatment of the elderly with Folic acid, since they are likely candidates for B12 deficiency as a consequence of malabsorption secondary to pernicious anaemia.
An added consideration is the lack of Folate effectiveness in reducing tHcy in a small percentage of the population who lack the mechanisms for converting dietary Folate to L-methylfolate, its biologically active form.
In these cases L-methylfolate should be used, with B6, B2, B12, Zinc, and TMG.
L-methylfolate is available as a supplement called Metafolin and as a patent presciption by Merck. Several combinations, however, of Folic Acid, Vit B12, B2, B6, Zinc and TMG are available from reliable manufacturers, and Solgar have produced Homocysteine Modulators, incorporating the essential substrates/cofactors in a single capsule.
PYRIDOXINE (VIT.B 6)
Pyridoxyl-5-phosphate (P’-5-P’) is the biologically active form of Pyridoxine (B6) and its use alone in trials has received equivocal reports.
(P’-5-P’) is Zinc dependent and is essential for the transsulpheration of homocysteine to cystathionine, and is available in its active form as a supplement.
Pyridoxine dosages of 25mg to 100 mg daily may be used to reduce tHcy. If the active form is used instead of Pyridoxine the dosage may be halved. Low pyridoxyl-5-phoshate has been shown to confer an independent risk for CAD.
Trimethylglycine (TMG) is a powerful methyl donor, and is the active form into which choline (tetra-methylglycine) is converted, if taken in the diet or as a supplement.
Methyl donors, and food rich in choline and TMG (betaine) assist in the methylation of homocysteine to produce SAME which is a precursor of the vital neuro transmitters, seratonin, melatonin and dimethyl tryptamine.
Measurement of tHcy requires few simple precautionary measures in the clinical setting.
A fasting sample collected in the morning from a subject in the supine position is recommended.
A large protein-rich meal may increase tHcy concentration by 10-15% after 6-8 hours, and supine positions are likely to produce 10% lower tHcy concentration than from a sitting position, because of reduced plasma albumin in the supine position.
To prevent ongoing release of tHcy from erythrocytes immediate centrifugation is required to remove blood cells. Serum obtained is stable for at least 4 days at room temperature and for several weeks refrigerated.
Procedural instructions for sample collecting and handling should be obtained from relevant laboratories timeously in order to facilitate transport of specimens to centres for testing. According to Helga Refsum, the quality of tHcy measurements has improved in recent years, but the problem of standardization remains unresolved.
The Reference intervals are calculated as 0.5th-97.5th percentile interval for presumed healthy individuals, but even herein lies controversy, with most labs offering an upper normal limit of 15 mM/L, a figure regarded by nutritional researchers as being too high.
Plasma tHcy concentrations are affected by ethnic differences, dietary habits, age, gender, pregnancy and general health status, as well as Folate fortification of foods. Distinguishing normal (and desirable) levels from abnormal levels is controversial.
The Framingham Heart Study determined 11.4 uM/L as a cut off level for cardiovascular risk. Researchers at Bergen University, Norway cite 9 uM/L as a safe level, and a large-scale survey published in Circulation (1995 Vol 92) reports levels of above 6.3 having a greater risk of cardiovascular disease.
Life Extension researchers have for a long time suggested a level of 7-8 as being the ideal level with maximum risk reduction.
The conclusion of Wald et al, supported by others, serves to alert clinicians to the dangers of hyperhomocysteinaemia and to the possibilities for reduction methods which are safe, simple, inexpensive and potentially effective in reducing morbidity and mortality.
These conclusions and recommendations could serve as an effective strategy for maintaining health and pursuing optimal wellness with cost saving benefits.
Dr. Neville. S. Wilson.
09 Feb., 2007.