TUCSON, ARIZ. — Customary vitamin D supplementation and springtime sun exposure in the southern United States are inadequate to protect elderly African American women from vitamin D deficiency and osteoporosis, data from a prospective cohort study show.
Not only were most of the women deficient in vitamin D despite 6 weeks of supplementation and nearly 10 hours a week of Texas vernal sunshine, but an unexpectedly high number were osteoporotic or osteopenic, Dr. Sally Weaver and associates reported in a poster at the annual meeting of the North American Primary Care Research Group.
Darker skin pigmentation and aging are known to decrease the body's ability to synthesize vitamin D. But it has generally been assumed that sun exposure in southern latitudes is either sufficient to provide adequate vitamin D levels or would reduce the need for supplementation.
The study included 44 African American women living in central Texas who were given 1,000 mg of calcium with 400 IU vitamin D tablets daily for 6 weeks, and told not to make any changes in diet or sun exposure. Clinical evaluations were made at baseline in April and 6 weeks later in June.
That time frame was chosen because, theoretically, 6 weeks is enough time for the body to replenish its supply of vitamin D if it is deficient, and because sun exposure typically increases in the spring, said Dr. Weaver, research director for the McLennan County Medical Education and Research Foundation, in Waco, Tex.
Overall, 36 (82%) of women returned for follow-up. Their average age was 76 years (range 70–88 years) and average BMI (kg/m
The investigators chose 32 ng/mL as the cutoff for “normal” vitamin D levels because there is some work that supports this value as the minimum needed for bone health, Dr. Weaver said. The ideal cutoff is under debate: Many levels use 20–25 ng/mL, but many researchers use higher values, such as 30–32 ng/mL, she said.
No matter which measure was used, the majority of women remained vitamin D deficient. After 6 weeks of supplementation, 23 women (52%) had vitamin D levels lower than 20 ng/mL, 29 (66%) had levels lower than 25 ng/mL, and 37 (84%) had levels lower than 32 ng/mL. Moreover, some women's levels were inexplicably lower after supplementation. Changes in vitamin D levels varied widely, from a loss of 10 ng/mL to a gain of 19 ng/mL over the course of the study. The lower the women's levels at baseline, the more likely they were to show an increase, she said.
Overall, 24 of the women (55%) were osteopenic and 11 (25%) were osteoporotic. The finding was surprising because greater weight is associated with stronger bones, she said. Only 13 (30%) of patients were on medication for the prevention or treatment of osteoporosis or osteopenia.
The average bone mineral density T score was −1.7, with a range of 1.7 to −4.0. Interestingly, T scores were not correlated with vitamin D levels. This could be because of the high number of osteopenic patients in the population, she said.
The amount of sun exposure was not correlated with vitamin D levels or T scores at the beginning or end of the study. “I believe that the combination of darker skin pigmentation and older age is preventing adequate sun conversion of vitamin D to an active form, even in a southern latitude, leading these patients to require much higher doses of oral supplements,” Dr. Weaver said.