Optimal vitamin d status for the prevention and treatment of osteoporosis

Click here.

Kind of a technical report, but I'll cut here the bottomline:

Vitamin D deficiency causes muscle weakness, increasing the risk of falls and fractures, and should be aggressively treated with pharmacological doses of vitamin D. Vitamin D sufficiency can be sustained by sensible sun exposure or ingesting at least 800-1000IU of vitamin D(3) daily. Patients being treated for osteoporosis should be adequately supplemented with calcium and vitamin D to maximise the benefit of treatment.

Note that the article mentions Vitamin D3, not D2. Vitamin D2 is the kind you'll usually find in cheap supplements.

If you want it for free, there's always sunshine. ^_^