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In an earlier issue the relationship between macular pigment density, dietary carotenoids, and age-related macular degeneration (AMD) was explored. The studies summarized in that issue demonstrated the following relationships: 1) increased pigment density with increased dietary intake of lutein and zeaxanthin; 2) reduced risk of AMD with higher serum levels of the retinal carotenoids lutein and zeaxanthin; and 3) reduced risk of AMD with increased dietary intake of these carotenoids. There is evidence that increasing macular pigment MP density may help prevent AMD. There is wide inter-patient variation in macular pigment density. However, not much research has been carried out describing the variation among different groups.
In a study by Hammond and colleagues (1), results indicated a higher macular pigment density among males compared with females (38% higher density) despite similar plasma levels and dietary intake of carotenoids. This is consistent with epidemiological findings that women may be at greater risk of exudative AMD (at least at older ages). Furthermore the authors noted that in this study (1) the relationship between dietary intake of fat and the effect on MP density differed between men and women; among women the relationship was negative (higher dietary fat = lower MP density) but among men it was positive. The relationship between individual genetics, gender, dietary patterns, and MP density is murky. The relationship between dietary and plasma levels of carotenoids is clearly moderated by individual and probably gender-related genetic factors. These factors, along with dietary practices would impact on the variation in MP density and would be consistent with the familial history risk factor for AMD. Hammond, Fuld and Curran-Celentano published an interesting twin study (2) in 1995 to evaluate the importance of genetic factors in MP density.
Twin with Higher MP: Mean
Twin with lower MP: Mean
Dietary fiber - g/day
Oleic acid - g/day
Linoleic acid -g/day
Iron - g/day
Total fat - g/day
They enrolled 10 pairs of younger (19-22 years) monozygotic twins into the study. There were 6 male pairs and 4 female pairs. The method of measuring MP density is described in the paper (2). Dietary intake of various nutrients was determined using a 100-item food intake questionnaire, which gathers data on frequency, and serving size. A computer program from the National Cancer Institute was used to determine the nutrient composition of the food items ingested. Serum carotenoid levels were measured 7 times over a 3-month period.
The main finding was that half (5/10) of the monozygotic twin pairs had statistically significant differences in MP density despite the identical genetics. The twins showing differences in MP density had certain systematic differences in their diets. In each of the 5 pairs, the twin with higher MP density had higher dietary intake of fat, iron, linoleic and oleic acids, fiber and total calories. A composite score for these variables showed a statistically significant positive relationship with MP density. The authors cited studies supporting a possible hypothesis that these nutrients influence carotenoid metabolism and thus macular pigment levels. The authors concluded from this twin-study controlling genetic make-up, that a good deal of the variation in MP density is caused by external factors. They concluded that dietary factors contributed, although the relationships between dietary levels, uptake, plasma levels and the ultimate effect on MP density are complex.
Dietary and supplement intake of specific nutrients probably affects MP density, which, in turn, affects the risk of AMD.
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