A study reported in the American Journal of Clinical Nutrition found that the current Daily Recommended amounts of Vitamin D are way too low. Dr. Vieth recommends 800 to 1,000 IUs of Vitamin D per day. He also reported some studies having administered doses of 100,000 IU per day for brief periods with no negative effect. One study even injected 300,000 IUs and discovered that they reduced the incidence of osteoporosis fractures by 25 percent.
Vitamin D also helps calcium be absorbed into your system. Without D the calcium will simply pass through your system. So, if you're not taking 800 to 1,000 IUs of Vitamin D per day, make sure to increase your dosage. I personally take 800 IUs per day.