BOSTON, Sept 12, 2005 /PRNewswire via COMTEX/ -- Researchers from Harvard have uncovered what seems at first to be an unlikely link: excess body fat may elevate one's risk for age-related eye diseases such as macular degeneration (AMD) and cataracts. In a study of 261 individuals with early-stage AMD, higher adiposity -- especially around the waist and abdomen -- predicted increased risk for progressing to advanced forms of AMD.(1)
AMD is the leading cause of blindness in older adults today, accounting for 54 percent of all blindness in white Americans and 45 percent of visual disability in the general population. It is characterized by a gradual loss of central vision. Because there is currently no effective medical treatment, researchers have sought to identify dietary and lifestyle factors that can prevent the condition.
Previously known modifiable risk factors for AMD include low fruit and vegetable intake, smoking and exposure to ultraviolet light, all of which are related to antioxidant status. The new link to obesity adds another modifiable risk factor to this list.
One theory to explain it is this: adipose tissue is a storage site for up to 80 percent of the carotenoids consumed in the diet, which are largely fat-soluble compounds. Two of the carotenoids that are essential for eye health and believed to be protective against AMD are lutein and zeaxanthin (pronounced loo'-teen and zee-ah-zan'-thin), found in yellow and leafy green vegetables such as corn and spinach.
Normally, these pigments are preferentially gobbled up by the retina of the eye, where they function as potent antioxidants to protect the macula (center of the retina) against free radicals generated by light. However, if the body has too much adipose tissue, too much lutein and zeaxanthin remain there in storage and do not get to the eye where they are needed.
The amount of lutein and zeaxanthin in the retina is measured as "macular pigment density." Higher macular pigment density has been correlated with reduced risk for AMD. However, higher body mass has been correlated with lower macular pigment in earlier studies.
For example, in an analysis of 680 people in 2002, both men and women with higher body fat tended to have lower macular pigment density. In those with a body mass index (BMI) greater than 29 -- indicative of obesity -- macular pigment was reduced by more than a fifth.(2) In another preliminary study, a significant inverse relationship was seen between percent of body fat and macular pigment density in 100 healthy volunteers.(3)
The investigators suggested that a higher amount of body fat provides a larger "sink" for lutein and zeaxanthin deposition, making them less available for the retina. Because of their relative lack of macular pigment, people with excess body fat may be at higher risk of developing AMD. If they already have early stage AMD, they are at higher risk of progressing to late stage AMD. There is also some evidence that higher body fat may elevate risk for cataracts.
A second theory involves inflammation. Adipose tissue is a very active tissue metabolically and produces an overload of pro-inflammatory cytokines. Markers of inflammation in the blood were observed to increase in parallel to increasing body weight in the third National Health and Nutrition Survey (NHANES-III) of more than 16,000 adults.
Elevations in markers of inflammation have been associated with the presence of intermediate and advanced AMD, and inflammation also influences the processes involved in cataract formation. Chronic, systemic inflammation was associated with both higher BMI and increased risk for cataracts in the Physicians' Health Study.
A final explanation may be simply that diets of overweight or obese people are low in fruits and vegetables, so they lack adequate supplies of lutein and zeaxanthin, along with the other eye-protecting antioxidants -- vitamin C, vitamin E and zinc. In at least two studies, individuals with higher BMI and abdominal obesity had lower intakes of fruits and vegetables. However in a third study, the 18 percent difference in macular pigment density found between individuals with low versus high BMI could not be explained by diet alone.
While healthy weight should be maintained for many reasons, individuals who are overweight may want to reduce their risk for AMD and cataract by getting extra lutein and zeaxanthin in the diet, in multivitamin supplements or in supplements specially formulated to support eye health.
Roche Vitamins Inc. founded VNIS in 1979 as a source of accurate and credible vitamin and carotenoid information for health professionals, educators, and communicators. The VNIS is now supported by DSM Nutritional Products, Inc. We continue to monitor and disseminate vitamin and carotenoid research, sponsor professional symposia on current topics, and generate materials to educate professionals about the role of these nutrients in health.
1. Seddon JM, Cote J, Davis N, Rosner B, 2003. Progression of Age
Related Macular Degeneration: Association with Body Mass Index,
Waist Circumference and Waist-Hip Ratio. Arch Ophthalmol
2. Hammond BR Jr, Ciulla TA, Snodderly DM, 2002. Macular pigment
density is reduced in obese subjects. Invest Ophthalmol Vis Sci
3. O'Donovan OM, Kavanagh H, Nolan J, Harrison M, Beatty S. Percentage
body fat and macular optical density. Association for Research in
Vision and Ophthalmology Annual Meeting 2004. April 25-29, Fort
Lauderdale FL (abstract B604)