With the exception of calcium deficiency rickets in Nigeria, no satisfactory explanation has been found for the apparently low prevalence of osteoporosis in countries on low calcium intakes. On international comparisons on a larger scale, it is very difficult to separate genetic from environmental factors. Osteoporosis was largely a disease of affluent industrialized cultures. Hip fracture prevalence (and by implication osteoporosis) is consequently related to animal protein intake, but also, paradoxically, to calcium intake because of the strong correlation between calcium and protein intakes within and between societies. This could be explained if protein actually increased calcium requirement. 

Fracture risk has recently been shown to be a function of protein intake in North American women. There is also suggestive evidence that hip fracture rates depend on protein intake, national income, and latitude. Vitamin D deficiency in hip fracture patients in the developed world was established. Such fractures can be successfully prevented with small doses of vitamin D and calcium. It is therefore possible that hip fracture rates may be related to protein intake, vitamin D status, or both.