Vitamin D may play a role in muscle strength, yet little is known about how this is affected by emigrating from low to high latitudes. This longitudinal study examined the impact of seasonal vitamin D status on muscle strength among people from sunny climate countries (45°N to 45°S to the equator) visiting Scotland (57°N). Ethical approval was obtained for this study. A total of 66 healthy adults (73% Asians (96% Southeast Asians), 15% Africans and 12% Middle Easterns) aged 19-41 years took part; 58% were seen within 3 months of arriving in Scotland. Participants attended visits every 3 months for fifteen months. At each visit, fasted blood samples were collected for analysis of serum total 25-hydroxyvitamin D (25(OH)D) by dual tandem-mass spectrometry. Maximal voluntary contraction (MVC) was measured using a Takei digital grip dynamometer (both arms) and a Biodex dynamometer (right knee extension). Skin type (I-VI) was determined using a CM-2600d spectrophotometer (inner arm). Dietary vitamin D intake and sunlight exposure were assessed by questionnaires. Mean(SD) baseline 25(OH)D was lower in Middle Easterns (16.9(6.8)) nmol/L, p=0.010) compared with Asians (31.1(13.1)) nmol/L and Africans (32.4(10.9)) nmol/L. Mixed model analysis showed that days spent in Scotland was associated with significantly reduced 25OHD (beta=-0.01). Taking vitamin D supplements (beta=10.3), holidays abroad (beta=2.6), higher percentage body surface exposed to sunlight (beta=2.9) and skin type (beta=2.3) were significant positive predictors of 25OHD. There was no association between vitamin D status and mean handgrip strength, although handgrip strength increased over time (first visit=30 kg, fifth visit=32 kg), possibly due to improved technique. For knee strength, the model predicted that for each 10 nmol/L increase in 25OHD, peak torque increased by 2 Nm (p=0.025). Whilst vitamin D status was low among the ethnic minorities residing in northeast Scotland, this condition has a small effect on muscle strength.
Disclosure: The authors declared no competing interests. This work was supported by a PhD studentship from the Government of Malaysia.