Vitamin D: Beyond Bone Health

Most people think of vitamin D simply as a regulator of calcium homeostasis and bone mineralization. Over the past decade, however, it has become clear that the active or hormonal form of this vitamin also plays a significant role in xenobiotic metabolism, in the immune system, and in cellular proliferation and differentiation. As a result, adequate vitamin D status is viewed as important for reducing the risk of many malignancies and autoimmune diseases. Now, a recent study suggests that vitamin D is also involved in protecting macular health.

Study Design and Methods

The objective of the study was to evaluate the association between levels of vitamin D (25-hydroxyvitamin D) in serum and prevalence of both early and advanced age-related macular degeneration (AMD) among participants of the 3rd National Health and Nutrition Examination Survey (NHANES III). NHANES III, which is conducted by the Centers for Disease Control, is a representative sampling of the general population. Of 7,752 people (> 40 years) analyzed from this survey, 11% had AMD as determined by non-mydriatic fundus photographs. Questionnaires assessed dietary and supplemental intake, and blood samples were taken to determine vitamin D serum levels.

Protective Effect Seen for Higher Serum D

When participants were split into five groups based on serum vitamin D levels, those in the highest quintile had a 40% lower risk for early AMD compared to those in the lowest quintile (see table). An inverse association between AMD and greater vitamin D-fortified milk consumption was observed. The researchers also found a significantly reduced prevalence of AMD among consistent vitamin D supplement users who did not consume milk daily.

Anti-Inflammatory Role is Likely Connection

A growing body of epidemiologic evidence suggests that inflammation underlies AMD pathology, and the authors of this study speculate that vitamin D may reduce AMD risk by its anti-inflammatory actions. Studies report that vitamin D decreases proliferation of T helper cells, T cytotoxic cells, and natural killer cells, while enhancing T suppressor cell activity. Vitamin D also decreases the production of pro-inflammatory interleukins such as IL-6 and 8. In addition, a recent study has shown that vitamin D intake reduces C-reactive protein, a marker of systemic inflammation.

Reference

1. Reference: Parekh N, et al. Association between vitamin D and age-related macular degeneration in the Third National Health and Nutrition Examination Survey. Arch of Ophthalmol 125:661-69, 2007.