Vitamin D status is well recognised as an essential pro-hormone for optimum bone health and adequate status during wintertime can be particularly challenging for ethnic groups living in high latitudes⁽Reference Darling¹⁾. Low concentrations of 25-hydroxyvitamin (25(OH)D) are associated with a significant increase in parathyroid hormone (PTH). Therefore, the aim of the present study was to investigates the effect of vitamin D₃ supplementation on 25(OH)D status and PTH concentrations in Brazilian women living in Southern England.

In a double-blind randomized control trial (RCT), 56 women (age 35·75 ± 9·027 years) were supplemented with daily 15 μg vitamin D₃ or placebo, for 12 weeks during the wintertime.

At baseline, mean 25(OH)D concentrations were 36·06 ± 14·97 nmol/l, and 25% and 57·1% of women had 25(OH)D concentrations <25 nmol/l and <50 nmol/l, respectively. As shown in the Table below, mean 25(OH)D concentrations increased significantly in the supplemented group (+17·06 ± 17 nmol/l; p = 0·002), achieving a mean value above the inadequacy cut-off of ≥50 nmol/l⁽²⁾. As expected the increase in the placebo group was not statistically significant (p = 0·087). Post-intervention no participants had concentrations <25 nmol/l whereas 20·8% of the placebo group had concentrations below this threshold. Vitamin D intake from food was extremely low in both groups, but interestingly the placebo group had significantly higher intakes than the supplemented group post-intervention (3·46 ± 2·37 and 2·12 ± 1·18 respectively, p = 0·017). An Analysis of Covariance (ANCOVA) was carried out with post-intervention 25(OH)D as the dependent variable, the dichotomy of treatment group as the independent variable and baseline 25(OH)D as a covariate. There was a significant difference in post-intervention 25(OH)D concentrations between placebo and supplement, after having controlled for the baseline 25(OH)D [F(1,46), p = 0·002, partial eta squared = 0·2]. Both Intention-To-Treat (ITT), here presented, and Per-Protocol populations showed similar results for effectiveness of supplementation compared to placebo. Plasma PTH increased significantly in the placebo group (1·01 ± 2·2 pmol/l; p = 0·04) whilst vitamin D supplementation prevented this seasonal increase (baseline and post-intervention: 5·48 ± 2·53 and 5·98 ± 2·21 pmol/l; p = 0·253), although only in the ITT analysis.

¹Values are means and SDs; ² ANCOVA. ³ Statistical analysis: ^a Independent t-test; ^b Mann-Whitney; ^c Paired t-test: ^d p = 0·000; ^e p = 0·04; ^f p > 0·05.

The results suggest that vitamin D supplementation of 15 µg/d maintained mean 25OHD concentrations above the inadequacy threshold (<50 nmol/l)⁽²⁾ and prevented vitamin D deficiency⁽³⁾ (<25 nmol/l) during winter months for Brazilian women living in England. Considering the limited food sources rich in vitamin D and the very low intake observed, supplementation during wintertime was shown to be an effective strategy to help this ethnic group achieve adequate vitamin D status when sunlight exposure is limited. The findings for PTH and the potential impact on bone health warrants further investigation.

This study is funded by the Science Without Borders Scheme (CNPq).