Abstract: Seasonal variations in blood pressure have been consistently reported. However, uncertainty remains about the size of the seasonal effect in different regions, and about factors that explain the differences observed across and within populations. Using data from a national panel study, we investigated seasonal variations in blood pressure in the South African adult population, and whether these variations differed across socioeconomic strata.
We estimated age-specific seasonal effects on blood pressure using a multilevel structural equation model, with repeated measurements nested within subjects. Effect modification by socioeconomic status was assessed by repeating the analyses in the subpopulations defined by levels of education, household income per capita, and type of housing.
In men and women, season had a statistically significant effect on blood pressure, with higher levels in winter and lower levels in summer. For systolic blood pressure, the magnitude of the seasonal effect was 4.25/4.21 mmHg (women/men) and was higher in the older age groups. For diastolic blood pressure, the effect size was 4.00/4.01 mmHg, with no evident age trend. Seasonal effects were higher among subjects in the lowest socioeconomic classes than in the highest, with differences between 2.4 and 7.7 mmHg, depending on gender, whether systolic or diastolic blood pressure, and socioeconomic status indicator.
In the South African adult population, blood pressure shows seasonal variation modified by age and socioeconomic status. These variations have epidemiological, clinical, and public health implications, including the prospect of population level intervention to reduce elevated risk of cold weather cardiovascular morbidity.
From the Division of Epidemiology and Biostatistics (AC); and Centre for Environmental and Occupational Health Research, School of Public Health and Family Medicine, University of Cape Town, Cape Town, South Africa (RE).
Correspondence: Annibale Cois, Division of Epidemiology and Biostatistics, School of Public Health and Family Medicine, University of Cape Town, Anzio Road, Observatory 7925, Cape Town, South Africa (e-mail: Annibale.Cois@uct.ac.za).
Abbreviations: BMI = body mass index, DBP = diastolic blood pressure, NIDS = National Income Dynamics Study, SBP = systolic blood pressure.
The original study was funded by the Presidency of the Republic of South Africa.
The authors have no conflicts of interest to disclose.
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Received May 11, 2015
Received in revised form July 20, 2015
Accepted July 24, 2015