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Research Briefs: Vitamin D, AREDS Antioxidants & Citrus Flavonoids

Introducing EduFacts Research Briefs

While EduFacts will continue to highlight a newly published study or review article on nutrition and ocular health, it will now occasionally present Research Briefs: short summaries of several studies from various journals.

Vitamin D Again Linked to Reduced AMD Risk

Researchers from the University of Wisconsin and elsewhere examined the relationship between serum vitamin D and the prevalence of early AMD among 3000+ postmenopausal women (1). Higher serum vitamin D concentrations (> 38 nmol/L) were found to be significantly associated with a 48% decreased odds of early AMD and a 57% decreased risk of pigmentary abnormalities in women under 75 yrs. For those over 75, the association was of borderline significance.

In terms of vitamin D intake from supplements and foods, those consuming the highest amount (on average about 600 IU) daily, had a 59% lower risk of AMD compared to those obtaining the least (about 300 IU on average). This is the 2nd study to report a relationship between vitamin D and AMD. Because vitamin D has known anti-inflammatory and immune modulating actions, the authors speculate this nutrient may suppress destructive inflammation that occurs at the RPE-choroid interface in early stage disease.

AREDS, Ocular Blood Flow & Endothelial Function

Oxidative stress is a contributor to ocular diseases such as diabetic retinopathy, cataract, AMD, uveitis, and possibly glaucoma. The presence of increased oxygen radicals in vascular tissue is believed to cause endothelial dysfunction, leading to decreased dilation, more inflammatory cell and platelet adhesion, and increased coagulation activity. The retina is especially vulnerable to oxidative stress because of its high oxygen consumption, its high concentration of poly-unsaturated fatty acids, and its direct exposure to light.

Using an experimental protocol, clinicians from the Medical University of Vienna showed that systemic inflammation diminishes the retinal vascular response to hyperoxia. (The protocol uses an endotoxin to reduce the response of retinal blood flow under conditions of excess oxygen). The reduced response is due to endothelial dysfunction caused by oxidative stress. In a double-blind, placebo-controlled parallel study (2), 21 healthy volunteers were given the AREDS formula or a placebo for two week periods. At the beginning and end of each period, retinal blood flow and retinal blood flow reactivity to hyperoxia was assessed.

The investigators found that the AREDS antioxidants restored retinal vascular reactivity. According to the authors, this was likely due to oxygen radical scavenging by the antioxidants. The results suggest that this model might be a good approach to screening antioxidants for their relative beneficial effects in the retina.

Flavonoids, Inflammation and Blood Pressure

Researchers studied the effects of orange juice (OJ) and its major flavonoid, hesperidin, on blood pressure and blood vessel function in overweight but otherwise healthy men (3). About 1/3 of the men had elevated blood pressure but were taking no anti-hypertensive medications. During 3 month-long periods, the men drank either 17 oz of OJ daily, a control drink with 300 mg hesperidin capsules (the same amount as the OJ contained), or a control drink plus a placebo capsule. At the end of both the OJ and hesperidin test periods, diastolic pressure was significantly lower than after the placebo period. Studies suggest that even a decrease of just 3-4 points in diastolic pressure could result in a 20% reduction in the incidence of coronary heart disease. Hypertension also appears to raise the risk of retinopathy in non-diabetics (4).

The researchers also tested a variety of measures of blood vessel activity, including the ability to dilate. Both OJ and hesperidin showed positive effects compared to placebo. This finding is consistent with recent data linking higher intake of citrus flavonoids with lower markers of inflammation and endothelial dysfunction in participants of the Nurses Health Study (5).

References

1. Millen AE, et al. Vitamin D status and early ARMD in postmenopausal women. Arch Ophthalmol 129:481-89, 2011.
2. Pemp B, et al. Effects of antioxidants (AREDS medication) on ocular blood flow and endothelial function in an endotoxin-induced model of oxidative stress in humans. Invest Opthalmol Vis Sci 51:2-6, 2010.
3. Morand, C et al. Hesperidin contributes to the vascular protective effects of orange juice: a randomized crossover study in healthy volunteers. Am J Clin Nutr 93:73–80, 2011.
4. Ojaimi E, et al. Retonopathy signs in people without diabetes. Ophthalmol 118:656-62, 2011.
5. Landberg R, et al. Selected dietary flavonoids are associated with markers of inflammation and enendothelial dysfunction in US women. J Nutr 141:618-25, 2011.