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Volume
3:
No. 3, July 2006
LETTER TO THE EDITOR
Physical Activity and Incident Hypertension Among Blacks: No Relationship?
Suggested citation for this article: Duncan DT, Quarells RC, Din-Dzietham R, Arroyo C, Davis SK. Physical activity and incident hypertension among blacks: no relationship? [letter to the editor]. Prev Chronic Dis [serial online] 2006
Jul [date cited]. Available from: URL:
http://www.cdc.gov/pcd/issues/2006/
jul/05_0197.htm.
PEER REVIEWED
To the Editor:
Understanding the role (particularly the mechanisms of action) that physical
activity plays in the development of hypertension among blacks is important for risk reduction efforts and public policy actions,
especially because blacks bear the highest burden of hypertension, and cross-sectional studies
suggest that physical activity may be associated with reduced hypertension in blacks (1). We therefore sought to ascertain
this relationship between physical activity and incident hypertension among
blacks with a study design permitting causal inference.
We performed a qualitative, systematic review of cohort studies
examining the relationship between physical activity and incident hypertension
among blacks. We searched for articles in Medline from January 1966 to February 2005, examined all potentially relevant articles, and reviewed the reference lists of those articles.
Fourteen studies assessed
physical activity and incident hypertension in all populations. Most studies were conducted
among whites and were conducted in the United States; four studies were conducted
in countries other than the United States and did not include blacks, and four
other studies included blacks but one did not report the estimate of the
physical activity–hypertension association by race. Only three studies met our inclusion criteria.
Overall, cohort studies confirmed the positive
effects of physical activity on hypertension among whites.
Five studies found that among whites, physical activity reduced hypertension in univariate analyses but not after adjusting for other covariates,
such as age, body mass index, and alcohol intake. Univariate analyses of one
study found a relationship between physical activity and hypertension among
blacks (2); no such relationship was detected among blacks in multivariate
analyses of the National Health and Nutrition Examination Survey I Epidemiologic
Follow-up Study (2), Coronary Artery Risk Development in Young Adults study (3),
and Atherosclerosis Risk in Communities study (4).
However, the three studies have limitations that threaten
internal and external validity. One limitation is possible nondifferential misclassification of physical activity levels
due to poor physical activity ascertainment (2,4).
To illustrate, one study (4) used a questionnaire that was inappropriate for
blacks and women (5). Another limitation is the lack of power to examine the
physical activity–hypertension association among blacks because of a smaller
sample of blacks, which was highlighted when investigators presented stratified analyses
of race, sex, and physical activity levels (2). Other
limitations that violate internal validity include the following: 1) the unavailability of data on physical activity between baseline and follow-up (2,3)
(i.e., physical activity change possibly indicating greater risk than baseline physical
activity, particularly in long follow-ups); 2) self-reported bias (i.e., the
possible inaccuracy of self-reported physical activity [2,4] and undiagnosed hypertension
[2,3]); and 3) interviewer bias (i.e.,
the possible inaccuracy of physical activity data collected by interviewers) (3). Furthermore, blood pressure was not
measured
(hypertension was self-reported) at follow-up in one study (2). Self-report and interviewer
bias can result in nondifferential misclassification. The limitations hindering external validity are as follows: 1) one study included only blacks
and whites aged 45 to 64 years at baseline (4), a sample representative of a limited subgroup of the black population (and hypertension affects blacks at
younger ages
than whites),
and 2) most blacks in one study were from the South (4), which limits the generalizability
of its findings. Given these limitations and the limited number of studies on
blacks, we question whether there is no relationship between physical activity and incident hypertension in blacks based on cohort studies.
We would conclude from the published evidence that physically active blacks are not at a reduced risk for hypertension, but we recognize the numerous limitations of the research. Cohort studies did, however, report a biologically plausible inverse association of physical activity with hypertension risk. Widening the criteria of our review to include evidence
from cross-sectional studies and controlled clinical trials (6) suggests that there is probably a relationship. For
example, controlled clinical trials demonstrate that physical activity is
associated with blood pressure reduction among blacks (6). Moreover, a cohort
study that assessed blood pressure continuously among blacks (7) suggests that loss of information and power due to
categorization may be a major contributor to the lack of association
observed in cohort studies with categorical outcomes. All of these limitations
may explain the lack of association between physical activity and incident
hypertension among blacks.
Additional cohort studies and controlled clinical trials are needed to
examine physical activity and incident hypertension
among blacks.
Longitudinal mechanistic research designs that include the mediating variable of physical activity to hypertension are essential for
researchers to develop effective hypertension risk reduction interventions and
for policymakers to implement informed and effective policies. This may put us
closer to reaching the aims of Healthy People 2010 — reducing the proportion of adults with hypertension
and eliminating health disparities overall.
Dustin T. Duncan
Department of Psychology, Public Health Sciences Institute, Morehouse
College, and
Social Epidemiology Research Center
Morehouse School of Medicine
Atlanta, Ga
Mr Duncan is now with the Department of Society, Human Development and Health, Harvard School of Public Health, and the Center for Community-Based Research
Dana-Farber Cancer Institute
Boston, Mass
Rakale Collins Quarells, PhD, Rebecca Din-Dzietham, MD, PhD, MPH, Cassandra Arroyo, PhD, Sharon K. Davis,
PhD, MEd, MPA
Social Epidemiology Research Center
Morehouse School of Medicine
Atlanta, Ga
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