Urbanisation and the associated nutrition transition have been linked with the rapid and recent rise in osteoporotic fragility fracture incidence in many countries⁽ Reference Ballane, Cauley and Luckey ^{1 )}. Predictions indicate that hip fracture incidence will increase 6-fold in Africa and Asia by 2050, partially attributed to demographic transition and population ageing⁽ Reference Cooper, Campion and Melton ^{2 )}. Differences in areal bone mineral density (aBMD) between rural and urban locations indicate that urban regions of high income countries (HIC) have lower aBMD and a higher incidence of hip fracture⁽ Reference Brennan, Pasco and Urquhart ^{3 )}. The few studies conducted in low and middle income countries (LMIC) provide inconsistent results; in contrast to HIC, most have found higher aBMD in urban populations⁽ Reference Matsuzaki, Pant and Kulkarni ^{4 )}.

In order to investigate the impact of migrating to an urban environment, we have conducted detailed studies of bone phenotype and factors affecting bone health in two groups of pre-menopausal Gambian women: urban migrant (n = 58) and rural (n = 81). Both groups spent their formative years in the same rural setting, urban women were known to have migrated when aged ≥16 years. Bone phenotype (bone mineral content (BMC); bone area (BA); areal bone mineral density (aBMD), and size-adjusted BMC (height, weight and BA) of the whole-body, lumbar spine and hip) was measured by dual energy x-ray absorptiometry (DXA) with further characterisation of bone phenotype by peripheral quantitative CT (pQCT). Data were also collected on anthropometry, body composition, food and nutrient intakes, physical activity, socio-demographic characteristics, vitamin D status and 24hr urinary mineral outputs (Na, K, P and Ca).

Mean age and height of rural and urban migrant groups were not significantly different (p > 0·05). Urban migrant women were significantly heavier (p < 0·01). Significant differences in BMC and aBMD were found between groups at all skeletal sites, with urban women having higher BMC and aBMD; BA was not significantly different. The greatest difference in BMC was found at the lumbar spine (8·5 % ± SE 3·0, p < 0·01). After adjusting for size, the differences between urban and rural spine BMC remained significant (6·2 % ± SE 2·1, p < 0·01). These results indicate that rural-to-urban migration is associated with higher BMC, with differences mostly attenuated by adjusting for body size, particularly weight. In this African population, higher SA-BMC may affect future fracture risk.

LS: lumbar spine, TB: total body, TH: total hip; ^a median ^b IQR (25^th and 75^th)