Our results suggest that circulating 25[OH]D concentrations are inversely associated with systolic and diastolic blood pressure in Hispanic and African Americans, but the effect is weakened after adjusting for BMI. This finding supports those from a nationally representative study including Hispanic and African Americans.4
If the inverse association between 25[OH]D and blood pressure could be replicated using an observational longitudinal study design or a randomized clinical trial, it could have public health significance, as the potential influence of raising 25[OH]D levels to sufficiency could substantially lower blood pressure. For example, raising 25[OH]D levels of individuals in the lowest quintile of 25[OH]D in this study (~8 ng/mL) to levels of the highest quintile (~32 ng/mL, a sufficient level of vitamin D) could potentially lower systolic blood pressure by 2.3 mmHg and diastolic blood pressure by 1.5 mmHg (estimates based on models including BMI). As noted by Scragg et al.,4
a decrease in systolic blood pressure of this magnitude would be estimated to produce an approximate 10–15% decline in cardiovascular mortality. However, as stated above, additional studies with a longitudinal study design are needed to confirm the causative effect of 25[OH]D levels on blood pressure. Only one large prospective randomized study has looked at the effect of vitamin D supplementation on blood pressure thus far.15
No effect was detected in that study, possibly due to several factors previously described,16
including the possibility of unequal proportions of participants in the treatment and placebo groups on anti-hypertensive medication during follow-up and the dosage level of vitamin D being too low to have an effect on blood pressure. Additionally, the effect of vitamin D supplementation on 25[OH]D levels was not reported.
The interpretation of the association between 25[OH]D levels and blood pressure depends on whether it is appropriate to adjust for BMI. Scragg and colleagues described evidence suggesting that BMI may be an intermediate step in the vitamin D-blood pressure causal pathway and therefore it should not be adjusted for.4
The lower levels of 25[OH]D typically seen in individuals with high BMI17, 18
is thought to be due to the sequestering of vitamin D within fatty tissue.19
However, there is evidence suggesting that lower vitamin D levels could lead to higher BMI, which in turn could increase blood pressure. Low vitamin D levels may promote weight gain through the action of excess parathyroid hormone, which causes an influx of calcium into adipocytes.20
Furthermore, several small studies have shown that administration of α-calcidol (synthetic analogue of 1,25-dihydroxyvitamin D2
D], the more biologically active vitamin D metabolite) have resulted not only in lowered blood pressure, but also moderate weight loss.21–23
The exclusion of participants on anti-hypertensive medication could have introduced a bias in our results. However, results from analyses investigating an association of 25[OH]D levels with baseline hypertension (defined as having either systolic blood pressure ≥140 mmHg and/or diastolic blood pressure ≥90 mmHg or currently taking anti-hypertensive medication) supported the inverse directionality of the association of 25[OH]D with blood pressure, although the association was not significant (p>0.05, data not shown). An additional limitation is that a relatively small proportion of the participants included in the analyses had high blood pressure (72 out of 1334 [5%] had a systolic blood pressure ≥140 mmHg and 93 [7%] had a systolic blood pressure ≥90 mmHg). Consequently, even though there was a trend toward a lower mean 25[OH]D as systolic blood pressure increased (), the results were not significant for the group with systolic blood pressure ≥140 mmHg, which could be due to a small sample size in this group.
This study shows an inverse association between vitamin D status, measured by 25[OH]D, and blood pressure in Hispanic and African-American populations, after adjustment for several potentially confounding variables, although this association was not significant after adjustment for BMI. This finding could potentially have important public health implications, as increased vitamin D levels can be easily attained by vitamin D supplementation, or modest sun exposure. However, this type of intervention should be supported by the results of large, well conducted, randomized intervention trials of vitamin D supplementation.