Physical performance and 25-hydroxyvitamin D: a cross-sectional study of pregnant Swedish and Somali immigrant women and new mothers.

BACKGROUND

Severe vitamin D deficiency can impair muscle strength. The study aims were to examine physical performance in the hands and upper legs, and analyze plasma 25-hydroxyvitamin D (25(OH)D) concentrations in women with presumably low (veiled, Somali-born) and high levels (unveiled, Swedish-born).

METHODS

Women (n=123, 58% Swedish) enrolled at a Swedish antenatal clinic, latitude 60° N, were recruited. Plasma 25(OH) D was analyzed, measured as nmol/L, then categorized as <10 = undetectable, 10-24, 25-49, 50-74 or >75. Muscle strength was tested: maximal hand grip strength (in Newtons, N), and upper leg performance (categorized as able/unable to perform squatting, standing on one leg, standing from a chair, and lifting their hips). Social and anthropometric data were collected. Non-parametric statistics tested the data for differences in their ability to perform the tests across 25(OH)D categories. Undetectable values (<10 nmol/L) were replaced with '9' in the linear correlation statistics. A final main effect model for grip strength (in N) was calculated using stepwise linear regression for independent variables: country of birth, 25(OH)D levels, age, height, weight, physical activity, lactation status, parity, and gestational age.

RESULTS

Somali participants (35%) had 25(OH)D levels of <10 nmol/L, and 90% had <25 nmol/L; 10% of Swedish participants had <25 nmol/L of 25(OH)D, and 54% had <50 nmol/L. Somali women had a relatively weak grip strength compared with Swedish women: median 202 N (inter-quartile range 167-246) vs. median 316 N (inter-quartile range 278-359), respectively. Somali women were also weak in upper leg performance: 73% were unable to squat, 29% unable to stand on one leg, and 21% could not lift their hips (not significant across 25(OH)D categories); most Swedish women could perform these tests. In the final model, grip strength (N) was significantly associated with 25(OH)D levels (B 0.94, p=0.013) together with Somali birth (B -63.9, p<0.001), age (B 2.5, p=0.02) and height (B 2.6, p=0.01).

CONCLUSIONS

Many Somali women had undetectable/severely low 25(OH)D concentrations and pronounced hand and upper leg weakness; grip strength was strongly associated with 25(OH)D. Maternity health care personnel should be aware of this increased frequency and manage care accordingly.