NEW YORK — People with low serum levels of vitamin D have an increased risk of developing hypertension, based on the results from two prospective studies, one of which included more than 38,000 men.
On the basis on both analyses, “we conclude that serum levels of 25-hydroxy vitamin D may be an independent risk factor for incident hypertension,” Dr. John P. Forman said at the annual meeting of the American Society for Hypertension.
The findings also suggested that people could probably substantially reduce their risk by boosting their sun exposure through an extra 30–60 minutes spent outdoors daily, said Dr. Forman, a nephrologist at Brigham and Women's Hospital in Boston.
“Diet contributes relatively little to vi-tamin D levels, compared with sun exposure,” said Dr. Forman. A typical multivitamin contains 200–400 IU of vitamin D. In contrast, an extra 30–60 minutes of sun exposure to the face and arms can generate 4,000 IU of vitamin D, although this relationship varies based on latitude and time of year. (Endogenous vitamin D production is minimal for people in the northern latitudes of the United States during the late fall and winter.)
Vitamin D deficiency has been hypothesized to cause hypertension based on epidemiologic studies that have shown that the further a population lives from the equator the higher the prevalence of hypertension. And mechanistic links exist: Active vitamin D suppresses renin expression in the juxtaglomerular apparatus, and vitamin D also inhibits the growth of vascular smooth muscle cells.
Both analyses used data collected from men enrolled in the Health Professionals Follow-up Study, which began in 1986. The first analysis focused on serum vitamin D levels measured from single blood samples drawn from 621 men who were normotensive at the time of their blood draw. These men had served as controls for a prior study that examined the link between vitamin D levels and the incidence of certain cancers.
In a multivariate analysis that controlled for a variety of demographic and clinical factors, including age, race, family history of hypertension, smoking, and alcohol intake, men with vitamin D deficiency (defined as a serum level of less than 15 ng/mL) had a statistically significant, 5.6-fold increased risk of developing hypertension over the subsequent 4 years, compared with men whose baseline vitamin D level was 15 ng/mL or greater. During 8 years of follow-up, men with a baseline deficiency had a threefold increased risk of developing hypertension, although this difference was not statistically significant.
This analysis was limited by the relatively small number of men involved and because the reference vitamin D level was based on a single blood draw. Therefore, Dr. Forman and his associates ran a second analysis, based on data collected on all 38,388 men in the overall study.
Blood samples were not available for all of these men, but other available information allowed the researchers to estimate the subjects' serum level of vitamin D at baseline, using factors such as residence location, body mass index, and reported level of physical activity. The estimated vitamin D levels at baseline were correlated with the incidence of hypertension during the follow-up period, with adjustment for demographic and clinical variables.
The analysis showed that men in the lowest decile for vitamin D had a 2.3-fold increased risk of new-onset hypertension, compared with men in the highest decile.
The relationship between vitamin D and hypertension was biphasic. The relative risk gradually rose from the decile with the highest vitamin D level to a risk that was about 60% increased for the eighth decile. The risk then increased sharply for the two deciles with the lowest vitamin D levels.