EAL

HTN: Vitamins (2015)

Citation:

Fiscella K, Winters P, Tancredi D, Franks P. Racial disparity in blood pressure: is vitamin D a factor? J Gen Intern Med 2011;26(10):1105-11.

PubMed ID: 21509604

Study Design:

Cross-Sectional Study

Class:

D - Click here for explanation of classification scheme.

Quality Rating:

POSITIVE: See Quality Criteria Checklist below.

Research Purpose:

To evaluate the potential contribution of low serum vitamin D levels to racial differences in BP using data from a national US sample.

Inclusion Criteria:

Non-Hispanic black
Non-Hispanic white
Aged 20 years and older
BP and serum Vitamin D levels available.

Exclusion Criteria:

Pregnancy
Early renal disease (GFR less than 60ml per minute or urine albumin to creatinine ratio higher than 30mg per g).

Description of Study Protocol:

Recruitment

Nationally representative NHANES 2001 to 2006 data set.

Design

Cross-sectional study.

Blinding Used

Implied with measurements.

Statistical Analysis

Multiple linear regression analyses to examine black/white differences in SBP, adjusting for socioeconomic and social characteristics, health and health care, behavioral risk factors and biomarkers, and compared differences with and without adjustment for 25 (OH)D
Sensitivity analyses assessed contribution of non-linear components of continuous covariates, examined serum 25(OH)D as a continuous variable and excluded those taking anti-hypertensive medication
Sub-analysis of those with hypertension (BP higher than 140/90)
Models that included interactions between 25(OH)D and race were examined.

Data Collection Summary:

Timing of Measurements

Cross-sectional analysis; one measurement.

Dependent Variables

Systolic BP (SBP): Mean of second and third values.

Independent Variables

Serum vitamin D [25(OH)D]
Race/ethnicity.

Control Variables

Demographics:
- Age
- Sex
- Race.
Socioeconomic and social characteristics:
- Education
- Income
- Marital status.
Health and health care:
- Self-rated health
- Regular course of care
- Number of anti-hypertensive medications.
Behavioral risk factors
- Smoking and smoking history
- Exposure to smoke
- BMI
- Physical activity
- Alcohol intake
- Calcium intake
- Sodium intake
- Potassium intake
- Hemoglobin A1C
- C-reactive protein
- Serum albumin.

Description of Actual Data Sample:

Initial N: N=9,373
Attrition (final N): N=7,140 included in the analysis with 1,984 African American participants and 5,156 white participants

Age

Non-Hispanic African American: 41.8 years (SE 0.4 years)
Non-Hispanic white: 45.6 (SE 0.3 years).

Ethnicity

Non-Hispanic African American: N=1,984 (47.4% male) (12.2% of sample after weighting)
Non-Hispanic white: N=5,156 (51% male).

Other Relevant Demographics

	Non-Hispanic African American (%)	Non-Hispanic White (%)
SES and social poverty (%)
Less than 100%	21.8	7.9
100% to 149%	13.9	7.6
150% to 199%	12.1	8.3
200% to 299%	17.4	15.3
300% or more	34.8	60.9
Education
Less than high school	26.0	10.8
High school or equivalent	24.8	26.9
More than high school	49.2	62.3

Anthropometrics

	Non-Hispanic African American (%)	Non-Hispanic White (%)
BMI (kg/m²)
Less than 20	4.6	5.3
20 to less than 25	25.8	33.7
25 to less than 30	28.3	30.8
30 or more	41.3	30.2
Vitamin D (ng per ml)
First quintile (two to 15)	60.7	11.0
Second quintile (16 to 20)	20.6	17.8
Third quintile (21 to 24)	10.0	19.4
Fourth quintile (25 to 30)	6.4	26.3
Fifth quintile (31 to 80)	2.3	25.5

Location

United States.

Summary of Results:

Key Findings

In a fully adjusted model, participants in lowest quintile of serum 25(OH)D (two to 15ng per ml) had a mean SBP that was 2.64mm Hg (95% CI: 2.58 to 2.70) higher than those in the highest quintile (31 to 80ng per ml)
In analyses of those with hypertension, only (N=1,726), the lowest quintile, was associated with statistically significant 6mm Hg higher SBP (significance level not reported). Adjustment for vitamin D status reduced racial differences in mean SBP from a statistically significant 3.2mm Hg to a non-statistically significant 0.7mm Hg
In the final model, 25(OH)D accounted for 26% (95% CI: 7% to 46%) of residual difference in BP between African Americans and whites
Excluding participants taking BP medications, the addition of 25(OH)D reduced the effect of race by 39% (95% CI: 14% to 65%).

Author Conclusion:

In cross-sectional analyses, 25(OH)D explains one quarter of the African American-white disparity in SBP. Randomized controlled trials are required to determine whether vitamin D supplementation could reduce racial disparity in BP.

Funding Source:

Government:

NHLBI (1R01 HL081066-01A2)

Reviewer Comments:

Large, nationally representative sample. Relationship between vitamin D and BP in hypertensives was a sub-analysis.

Quality Criteria Checklist: Primary Research
Relevance Questions
	1.	Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies)	Yes
	2.	Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about?	Yes
	3.	Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice?	Yes
	4.	Is the intervention or procedure feasible? (NA for some epidemiological studies)	Yes

Validity Questions
1.	Was the research question clearly stated?		Yes
	1.1.	Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified?	Yes
	1.2.	Was (were) the outcome(s) [dependent variable(s)] clearly indicated?	Yes
	1.3.	Were the target population and setting specified?	Yes
2.	Was the selection of study subjects/patients free from bias?		Yes
	2.1.	Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study?	Yes
	2.2.	Were criteria applied equally to all study groups?	Yes
	2.3.	Were health, demographics, and other characteristics of subjects described?	Yes
	2.4.	Were the subjects/patients a representative sample of the relevant population?	Yes
3.	Were study groups comparable?		Yes
	3.1.	Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT)	N/A
	3.2.	Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline?	Yes
	3.3.	Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.)	N/A
	3.4.	If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis?	Yes
	3.5.	If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.)	N/A
	3.6.	If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")?	N/A
4.	Was method of handling withdrawals described?		Yes
	4.1.	Were follow-up methods described and the same for all groups?	N/A
	4.2.	Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.)	Yes
	4.3.	Were all enrolled subjects/patients (in the original sample) accounted for?	Yes
	4.4.	Were reasons for withdrawals similar across groups?	N/A
	4.5.	If diagnostic test, was decision to perform reference test not dependent on results of test under study?	N/A
5.	Was blinding used to prevent introduction of bias?		Yes
	5.1.	In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate?	N/A
	5.2.	Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.)	N/A
	5.3.	In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded?	Yes
	5.4.	In case control study, was case definition explicit and case ascertainment not influenced by exposure status?	N/A
	5.5.	In diagnostic study, were test results blinded to patient history and other test results?	N/A
6.	Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described?		Yes
	6.1.	In RCT or other intervention trial, were protocols described for all regimens studied?	N/A
	6.2.	In observational study, were interventions, study settings, and clinicians/provider described?	Yes
	6.3.	Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect?	N/A
	6.4.	Was the amount of exposure and, if relevant, subject/patient compliance measured?	Yes
	6.5.	Were co-interventions (e.g., ancillary treatments, other therapies) described?	Yes
	6.6.	Were extra or unplanned treatments described?	N/A
	6.7.	Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups?	Yes
	6.8.	In diagnostic study, were details of test administration and replication sufficient?	N/A
7.	Were outcomes clearly defined and the measurements valid and reliable?		Yes
	7.1.	Were primary and secondary endpoints described and relevant to the question?	Yes
	7.2.	Were nutrition measures appropriate to question and outcomes of concern?	Yes
	7.3.	Was the period of follow-up long enough for important outcome(s) to occur?	N/A
	7.4.	Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures?	Yes
	7.5.	Was the measurement of effect at an appropriate level of precision?	???
	7.6.	Were other factors accounted for (measured) that could affect outcomes?	Yes
	7.7.	Were the measurements conducted consistently across groups?	Yes
8.	Was the statistical analysis appropriate for the study design and type of outcome indicators?		Yes
	8.1.	Were statistical analyses adequately described and the results reported appropriately?	Yes
	8.2.	Were correct statistical tests used and assumptions of test not violated?	Yes
	8.3.	Were statistics reported with levels of significance and/or confidence intervals?	Yes
	8.4.	Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)?	N/A
	8.5.	Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)?	Yes
	8.6.	Was clinical significance as well as statistical significance reported?	Yes
	8.7.	If negative findings, was a power calculation reported to address type 2 error?	N/A
9.	Are conclusions supported by results with biases and limitations taken into consideration?		Yes
	9.1.	Is there a discussion of findings?	Yes
	9.2.	Are biases and study limitations identified and discussed?	Yes
10.	Is bias due to study's funding or sponsorship unlikely?		Yes
	10.1.	Were sources of funding and investigators' affiliations described?	Yes
	10.2.	Was the study free from apparent conflict of interest?	Yes