Beginning in the late 1990s, medical researchers began to suspect that high levels of homocysteine (a substance produced when the body breaks down the amino acid methionine) may accelerate atherosclerosis, the primary cause of heart attacks, strokes, and intermittent claudication. During a brief period, it was widely proclaimed that homocysteine was an even more important risk factor for heart disease than cholesterol. However, it currently appears that reducing homocysteine provides minimal benefits, if any.

Most of the supporting evidence for a homocysteine–atherosclerosis connection comes from observational studies that found an association between high levels of homocysteine and increased atherosclerosis. Observational studies, however, do not show cause and effect. It is quite possible that unknown underlying factors increase homocysteine levels and also accelerate atherosclerosis, rather than that high homocysteine causes accelerated atherosclerosis. Only intervention trials (studies where people are actually given a treatment) can show whether a treatment is effective.

Several massive studies of this type were initiated in response to the observational data. The results of 5 such trials have now been reported, involving a total of more than 18,000 men and women.20,23-25,49,50 In these studies, high doses of supplementary vitamin B6, vitamin B12, and folate were used to lower homocysteine levels. None of these studies found significant benefit for preventing stroke, heart attack, or heart-related death.

A smaller study failed to find that these same homocysteine-lowering vitamins preserved mental function in people with loss of mental function caused by atherosclerosis in the brain.26 Another study failed to find that lowering homocysteine with B-vitamins can improve mental functionin seniors.36On one of the few positive notes, one substantial trial found that use of these homocysteine-lowering nutrients helped prevent restenosis (recurrent vessel clogging) after angioplasty.27

Besides atherosclerosis, correlations have also been found between high homocysteine levels and numerous other diseases, including Alzheimer’s disease, osteoporosis, complications of pregnancy, deep venous thrombosis, and pulmonary embolism.3-9,28-30Again, however, most of the supporting evidence for a connection comes from observational studies; the results of double-blind studies are less encouraging. For example, one very substantial double-blind, placebo-controlled study failed to find that reducing homocysteine levels can help prevent recurrent deep venous thrombosis or pulmonary embolism.37Data gathered in another study also failed to show benefit for preventing these two problems.42Four double studies failed to find benefit for Alzheimer’s disease or other forms of dementia.26,47,48,51In addition, double-blind studies of B vitamins for reducing osteoporosis risk have not yet produced convincing evidence of benefit.44-46

On a positive note, a double-blind, placebo-controlled study of 728 Danish seniors with high homocysteine and relatively low folate intake found that use of folate supplements slowed the progression of age-related hearing loss.39Folate may also improve mental function in seniors with high homocysteine.40

Another study found that in people who had already had a stroke and were partially paralyzed, supplementation with vitamin B 12and folate reduced the risk of falls leading to hip fractures.21 Participants were elderly Japanese with high levels of homocysteine and low levels of folate and B 12. It is not clear how the treatment produced this benefit: it might have reduced the tendency for recurrent strokes, strengthened bones, improved balance, or produced benefit by some other means. Another study used blood tests to look at effects on bone, and failed to find that reducing homocysteine levels had any effect.41

People with diabetes or inflammatory bowel disease ( Crohn’s disease or ulcerative colitis) and those undergoing kidney dialysis may be at higher than normal risk for elevated homocysteine levels. A simple blood test can determine homocysteine levels. Both conventional and alternative practitioners use the natural substances described in the next section to treat elevated homocysteine.

Three nutrients act together to help the body reduce homocysteine levels: vitamin B6, vitamin B12, and folate.10-15 Many Americans are at least marginally deficient in vitamin B 6.16 Vitamin B 12deficiency occurs primarily in seniors as well as people who take drugs that suppress stomach acid.17Folate deficiency is thought to have become fairly uncommon in the US due to the enrichment of grains that began in the late 1990s.18However, it appears that the dose of folate required to achieve maximum homocysteine reduction is 800 mcg daily, higher than the usual nutritional recommendations.15,31,38

Nonetheless, as noted above, studies utilizing high doses of these vitamins for lowering homocysteine and therefore preventing cardiovascular disease have generally failed to find benefit. For more information on these nutrients, including dosage and safety issues, see the full Vitamin B6, Vitamin B12, and Folate articles.