The need for
vitamin D to prevent
rickets was the drive for selection of lighter skin color in temperate climates. Anthropologists also know that as human populations developed more sedentary lifestyles, this coincided with a decline in bone quantity, quality, and fracture resistance. Since
osteoporosis occurs after the reproductive years, there is no way that natural selection could have adapted our biology to prevent it. However,
osteoporosis can be largely prevented by optimizing physical activity, and the
vitamin D-related factors of environment, and nutrition. The role of
vitamin D3 in
osteoporosis is conclusively established from a very simple meta-analysis of the four randomized, placebo-controlled clinical trials into the effect of 20 microg (800 IU) per day. These have all demonstrated that this dose prevents approximately 30% of hip or non-vertebral fractures compared to placebo, in adults older than 65 years. Intakes less than this have never been found effective. The lowest average serum
25-hydroxyvitamin D concentration in any study demonstrating
fracture reduction was 74 nmol/L. Thus,
25-hydroxyvitamin D levels in older adults should exceed this amount. The role of
vitamin D supplementation is to provide humans with the nutrient in an amount closer to our species'
biological norm. This amount of
vitamin D results in the optimal function of many aspects of health, including balance and muscle strength that lessen the risk of fracture beyond what is possible via the quality and quantity of bone itself.