I shared in an earlier post on how I replace Vitamin D. The target level is above 30ng/ml (or 75 nmol/L).
A paper examining stress fractures in athletes, it is suggested that active people may need 40ng/ml even.
I have no idea about the evidence justifying these seemingly arbitrary figures. But since toxicity is unlikely at these levels, and there may be benefits, I would have no qualms about replacement.
There appears to be a sweet spot/range to target serum Vitamin D3 level (21-30 ng/ml) for falls prevention in the elderly. This corresponds to 800 iu/day of D3 supplementation. More or less may be bad.
This flies in the face of mounting studies seemingly advocating higher serum Vit D targets to improve a widening range of diseases. We don't like it when our worldview gets threatened or complicated. So I'm inclined to explain the findings in another way.
One way is to postulate that higher Vitamin D levels improve muscular strength, sense of well-being, and hence physical activity, which in turn increases "opportunities" to fall. It's akin to the rat experiment whereby injecting steroids into the arthritic joints resulted in greater wear-&-tear osteoarthritis, arguably aggravated by increased activity afforded by the salutary pain relief.
Also, the investigators were unable to offer any logical hypothesis as to why, at what is now considered a normal non-toxic level (>30 ng/ml), it should predispose the elderly to falls.
Otherwise, taking the results at face value, one has to conclude that different diseases may have different minimum Vit D level requirements, while falls propensity in the elderly is lowest within a narrow therapeutic Vit D range. Awkward