Elevated homocysteine linked with macular degeneration risk
In the largest study of its kind to date, researchers at the Massachusetts Eye and Ear Infirmary and Devers Eye Institute in Portland, Oregon discovered that having a high level of homocysteine is associated with an increased risk of developing age-related macular degeneration (AMD). Homocysteine is an amino acid that, when elevated in the blood, is a biomarker for cardiovascular disease. The findings were published in the January issue of the American Journal of Ophthalmology.
Massachusetts Eye and Ear Infirmary director of epidemiology Johanna M. Seddon, MD and colleagues measured fasting plasma homocysteine in 934 participants in the Age-Related Eye Disease Study (AREDS). Five hundred and forty seven men and women diagnosed with age-related macular degeneration and 387 subjects without the disease were included in the current analysis.
The team found that median homocysteine levels were higher among those with advanced macular degeneration than in those without the disease or with mild AMD. Additionally, homocysteine levels over 12 micromoles per liter, which are considered to be high, were associated with increased macular degeneration risk. The authors propose several reasons for homocysteine's influence on increasing the risk of macular degeneration, including oxidative injury to endothelial cells, increased peroxidation of low-density lipoprotein and altered blood clotting mechanisms.
Dr Seddon summarized, "We found that elevated homocysteine in the blood may be another biomarker for increased risk of AMD. Homocysteine can be reduced by dietary intake of vitamins B6, B12, and folate, so the relationship between this amino acid and AMD deserves further study."
A report published in the December 28, 2005 issue of the Journal of the American Medical Association concluded that individuals whose diets contain high amounts of the antioxidants beta-carotene, vitamin C, vitamin E and zinc have a significantly lower risk of developing age-related macular degeneration (AMD) than people whose diets contain lower levels of the nutrients. Age-related macular degeneration is the most common cause of permanent blindness in the developed world. An earlier study found that supplements containing 5 to 13 times the recommended daily allowance of these nutrients slowed the progression of the disease.
Oxidative stress that reduces blood flow to the eye and increases the level of free radicals is a contributing factor to both wet and dry macular degeneration. This occurs when naturally occurring antioxidants are present in decreased concentrations. Diminished levels of glutathione occur during aging, which makes the lens nucleus susceptible to oxidative stress-induced clouding. Decreased vitamin C, normally highly concentrated in the aqueous humor and corneal epithelium, is less effective in helping absorb ultraviolet radiation and preventing cataracts than when present in high concentration. Deficiencies in L-carnosine and vitamin E also mitigate oxidative stress and free-radical damage.
Glutathione and vitamin C are antioxidants found in high concentrations in the healthy eye and in diminished quantities in AMD patients. Vitamin C aids in glutathione synthesis in the eye. When it is combined with cysteine, an amino acid antioxidant, cysteine remains stable in aqueous solutions and is a precursor to glutathione synthesis. Vitamin C is important because it absorbs ultraviolet radiation and prevents cataracts.
The largest and most important study on the relationship of nutritional supplements and AMD is the Age-Related Eye Disease Study (AREDS). AREDS was the first large study to show a benefit of antioxidant and zinc supplementation on the progression of AMD and associated vision loss. Thousands of patients were followed for over six years. AREDS revealed significant improvements for patients with AMD and recommended antioxidants plus zinc (with copper) for most patients with AMD, except for advanced cases in both eyes. The AREDS recipe consists of the following daily: Vitamin A (beta-carotene), vitamin C, vitamin E, zinc and copper.
Many B-complex products contain the same amount of each B vitamin. On the other hand, studies have shown that pantothenic acid (vitamin B5) has an ideal daily dose of over 700 mg. Complete B-complex contains potent, but safe, and different amounts of the B-vitamins.
The carotenoids zeaxanthin and lutein, found in the macular region of the eye, help shield eyes from sunrays, computer screens and other harmful forms of light that over time can cause photo-oxidative damage to the eyes. These fat-soluble antioxidants are uniquely able to absorb the most damaging portions of the light spectrum, helping to protect the lens, retina, and macula. In fact, zeaxanthin and lutein have been called "conditionally essential nutrients" because of their critical protective functions in the eye.
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