Vitamin D and hypertension:
Racial/ethnic disparities

By Tasleyma Sattar, D.O., Kimberly Fulda, Dr.P.H., Kun Don Yi, Ph.D., and Roberto Cardarelli, D.O., M.P.H.
Primary Care Research Center, Department of Family Medicine
University of North Texas Health Science Center

The benefits of adequate vitamin D levels are well documented including the regulation of calcium and parathyroid hormone levels (Holick, 2006) and a potential impact on cancer risk (Lieberman DA, 2003). Despite the physiological importance and efforts at food fortification, the number of persons with deficient or insufficient vitamin D serum levels is estimated at approximately one billion worldwide (Holick, 2006). There are many factors that affect vitamin D levels including age, weight, skin pigmentation, gender, sunlight exposure, inadequate dietary intake, smoking, and medications. Differences in vitamin D deficiency or insufficiency unequally impacts certain socioeconomic, age, and racial/ethnic groups (Nesby-O’Dell S, 2002 and Holick, 2006). In addition to these disparities, racial/ethnic minorities are also at increased risk of developing hypertension and subsequent comorbid consequences. The exact causes of hypertension disparities continue to be investigated as new emerging factors and theories are developed in the research community. One promising factor includes serum vitamin D and its relationship with hypertension. However, no research has assessed whether this relationship differs by race or ethnicity. We propose that due to the diverse causes of vitamin D deficiency (some of which are more prevalent in certain racial/ethnic groups), hypertension disparity observed among racial/ethnic groups can be partially explained by the presence of vitamin D deficiency.

Methods

Study design: Cross-sectional study.

Subjects and setting: Existing plasma samples collected from participants of the North Texas Healthy Heart Study (NTHH) were used to retrospectively determine levels of vitamin D, specifically 25(OH)D. These levels were compared to clinical blood pressure measures. In addition, physical, demographic, and behavioral measures from the NTHH were utilized in the analyses.

The NTHH includes a convenience sample of 571 Caucasians, African Americans, and Hispanics/Latinos recruited from 12 participating sites of the North Texas Primary Care Practice-Based Research Network (NorTex) from April 2006 to May 2008.

Study participants in the NTHH study were African American, Caucasian, and Hispanic men and women 45 years of age and older. Plasma samples were stored on 428 of the 571 NTHH participants. Collection of the samples was approved by the University of North Texas Health Science Center and JPS Health Network Institutional Review Boards, and participants signed a separate written informed consent to have their samples stored and used for future research studies. Informed consent and surveys were administered in Spanish for Spanish speaking individuals. Of these samples, race/ethnicity was recorded for 424 participants. Due to costs of performing the assays, only African American and Caucasian subjects were utilized for this pilot study. The 230 samples were randomly selected from those available. Table 1 provides the racial/ethnic distribution of these samples.

Table 1. Racial/ethnic distribution of samples for proposed study

Inclusion/exclusion criteria: Participants were eligible for the study if they were over the age of 44, self-identified as non-Hispanic Caucasian, non-Hispanic African American, or Hispanic/Latino, and had no history of self-reported cardiovascular disease (coronary artery disease, peripheral arterial disease, history of myocardial infarction or stroke, or congestive heart failure), renal failure, or liver failure.

Results

Blood pressure and vitamin D levels were obtained for all 230 participants and were included in the analysis. Vitamin D levels were dichotomized into ≤20.99 (deficient) and ≥21 (sufficient). Overall, 69.4 percent (160) of participants had deficient vitamin D levels, and 30.4 percent (70) had sufficient levels. Forty-three percent (99) of all participants had blood pressure readings <140/90 (no hypertension) and 57 percent (131) were ≥140/90 (hypertension).

African Americans had a higher percentage of vitamin D deficiency at 82.6 percent versus Caucasians at 56.5 percent. African Americans had a higher percentage of hypertension than Caucasians, 60.0 percent versus 53.9 percent, respectively.

Data were analyzed using simple logistic regression to determine whether vitamin D levels were associated with hypertension. Hypertension was used as the dependent variable, and vitamin D as the independent variable. The odds ratio (OR) was found to be 1.380, but did not reach significance (p=0.264, 95 percent CI 0.785, 2.429). In addition, when analyzing for race/ethnicity, the OR was 1.761 (95 percent CI 0.836, 3.710) for Caucasians, and 0.773 (95 percent CI 0.283, 2.114) for African Americans.

Discussion

This pilot study provides preliminary data regarding the relationship between hypertension and vitamin D deficiency among Caucasians and African Americans. No association between vitamin D deficiency and hypertension was observed in the current study. There was a definite difference in hypertension among Caucasians and African Americans, as expected, and an even greater difference in vitamin D deficiency. However, the data indicate that African Americans, who have a greater incidence of vitamin D deficiency and hypertension, may actually have an inverse relationship between the two variables, although the relationship did not reach significance. The researchers believe that a greater number of participants are needed to better define this relationship. In the Caucasian population, a direct relationship was seen between hypertension and vitamin D deficiency, which is in keeping with previous studies in this group. Again, the data did not reach statistical significance, but the researchers believe this is due to the low number of participants in the study.

Strengths: This study will add to the body of knowledge of the health effects of vitamin D deficiency or insufficiency and will also guide further research into eliminating these health disparities. Future treatment recommendations for hypertension could potentially be targeted to specific ethnic groups to achieve optimal blood pressure control, and vitamin D level determination in patients could become a standard of care, especially in racial/ethnic minorities. If indeed African Americans have an inverse relationship between hypertension and vitamin D deficiency, then current clinical recommendations on vitamin D supplementation may have to be revisited.

In addition, this pilot study provides valuable information to estimate the appropriate sample size for future research studies. It is seen that the sample size of 115 participants per group was not a large enough number.

Limitations: The small number of samples for each racial/ethnic group severely limited the ability to determine a statistically significant relationship between the variables. Future studies will need to contain a larger sample size.

Conclusion

Although the data obtained were not statistically significant, it was seen that a general direct relationship exists between hypertension and vitamin D deficiency in Caucasians, whereas the relationship is inverse in African Americans. Whether this was due to the small sample size of this pilot study remains to be seen in future, larger scale, studies. If this relationship is indeed true, clinical recommendations on the supplementation of vitamin D could be significantly impacted.

References

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