Longitudinal changes in serum 25-hydroxyvitamin D in older people

Cross-sectional studies have suggested that serum 25-hydroxyvitamin D (25OH D) levels decline with aging. We have examined this putative decline in a l ongitudinal study using participants in the New Mexico Aging Process Study. 25OHD levels were measured in participants in whom serum samples were avai lable between 1980 to 1982 and 1989 to 1994 (37 men and 99 women). The avai lable data for these visits included age, gender, and the date the sample w as obtained. Questionnaires assessing physical activity and vitamin D intak e were administered at the visits. A seasonal variation (r = .25, P < .05) in 25OHD was demonstrated in the whole group of subjects. In 25 subjects wh o were not receiving vitamin D supplementation at either time and had sampl es obtained in the same season, both serum 25OHD (P < .05) and physical act ivity (P < .05) decreased over a mean period of 11.4 years. In 23 subjects who had samples obtained in the same season but used vitamin D supplements at both times, there was no change in serum 25OHD. Mean summer 25OHD levels did not change with the duration of study. On the other hand, the mean ser um 25OHD declined with the duration of study when measured from winter to w inter or spring to spring. Multiple regression analysis demonstrated that t he month, activity level, vitamin D supplementation, and gender (P < .001) were independent determinants of serum 25OHD levels. This study confirms th at aging is associated with a reduction in serum 25OHD, and suggests that t his decrease is a reflection of reduced sun exposure rather than aging per se. The reduction in serum 25OHD was the result of decreasing winter and sp ring 25OHD serum concentrations. It is clear that vitamin D supplementation can prevent the age-related decline in 25OHD levels. This is a US governme nt work. There are no restrictions on its use.