Low birth weight, bone metabolism and fracture risk

As for other diseases of higher age, low birth weight was expected to be a risk factor for an altered bone metabolism and osteoporosis. On the first glance this expectation appears to be confirmed by animal data: rats with intrauterine growth restriction following maternal protein malnutrition show a reduction of bone mineral density going in line with a decrease in serum vitamin D concentrations. However, the situation is less clear in newborns with low birth weight: Some studies show a relation of birth weight and bone mineral density whereas others don't. The older the former low birth weight patients the fainter the effect seems to be. In fact young adults with idiopathic short stature have a low bone mineral density than the low birth weight group irrespective of whether they have experienced catch-up growth or not. As a consequence low birth weight is can not be identified as a relevant risk factor for hip fractures in menopausal women. Postmenopausal women with low birth weight even show higher vitamin D concentrations than normal birth weight individuals. In conclusion, there is no consistent long term effect of low birth weight on bone mineral density or hip fracture risk later in life. Whether methodological weaknesses in the studies performed so far are causal or whether postnatal factors such as physical activity and nutrition are of higher importance can only be speculated upon at present.