Homocysteine is an intermediate metabolite of the methionine to cysteine pathway. Under normal circumstances homocysteine is synthesised back to methionine using the salvage pathway or is further metabolised to cysteine via the enzyme cystathionine beta-synthase. However, these two pathways rely on the vitamins folic acid and vitamin B12 (the salvage pathway) and vitamin B6 (the cysteine pathway), and low intakes of these vitamins are now thought to cause cellular accumulation of homocysteine. The accumulation of homocysteine in cells eventually causes it to spill over into plasma and high levels of homocysteine in plasma are now associated with a number of diseases including cardiovascular disease and dementia. Age-related macular degeneration is the leading cause of blindness and poor vision in older adults. Because of the way that homocysteine damages the vasculature in cardiovascular disease, it has been suggested that high levels of plasma homocysteine could aloo be a cause of macular degeneration.
Studies show that high levels of homocysteine and low levels of vitamin B12 are associated with age-related macular degeneration. In a recent study published in the American Journal of Clinical Nutrition1, Researchers investigated the associations between serum concentrations of homocysteine and age-related macular degeneration in subjects over 55 years of age. The results of the study showed that as serum levels of homocysteine increased, the incident of age-related macular degeneration also increased. In addition, those subjects with low serum levels of vitamin B12 (<185 pmol/L) also had a higher risk of developing age-related macular degeneration. A folate deficiency (<11 nmol/L) was associated with a 75 % increase in early and 89 % increase in late age-related macular degeneration. However, those who took supplements of vitamin B12 has a 47 % lower risk of age-related macular degeneration. This evidence supports the contention therefore that homocysteine is a causative factor in the development of age-related macular degeneration.
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