Seasonal effects of blood pressure in postmenopausal women

The aim of this study was to determine whether oral vitamin D3 at daily doses of 10 or 25 lg affected seasonal blood pressure measurements. Postmenopausal women (n = 305 recruited; mean age SD 63.8 2.2 years) took part in a double blind placebo-controlled trial (ISRCTN20328039) to receive either 10 or 25 lg vitamin D3 or placebo daily for 12 months. The baseline visit for all women was between January and March 2009. Seven study visits were completed at bimonthly intervals with a total of 265 women attending the final visit. At each study visit, three blood pressure measurements were taken using a digital blood pressure monitor (OMRON 705 CP-II). The first measurement was discarded and the mean of the final two measurements was used for this analysis. Differences between the treatment groups were tested by repeated measures ANOVA (SPSS version 18). Our preliminary analysis showed that the intervention had no effect on mean blood pressure measurements (Figure 1). However, overall there was a significant seasonal variation in mean systolic and diastolic blood pressure over the 12 month duration of the study (P < 0.001, repeated measures ANOVA). The mean reduction in systolic blood pressure from winter to summer was 6.5 mm Hg (Figure 1). Further analysis will involve mixed modelling in order to adjust for the possible confounding effects of body weight, weight change and physical activity. These data highlight the importance of taking season into account when designing studies where blood pressure is a key outcome measurement. Vitamin D does not appear to influence blood pressure and observational studies showing a link between vitamin D status and non-bone health outcomes may be a result of other mechanisms that are affected by season.