You've heard the old saying, "If a little is good, more must be better." But it isn't always true. Not when it comes to taking an annual megadose of Vitamin D once a year, at any rate. Research seems to indicate that not only does it not help to take large doses, it can actually make things worse.
Older women who took an annual dose of Vitamin D were found to have a higher rate of falls and fractures.
"The study included 2,256 community-dwelling women who were older than 70 at the start of the study and who were considered to be at high risk of fractures. Women were randomly assigned to receive either an annual dose of 500,000 IUs of vitamin D or a placebo for three to five years. Both the vitamin D and the placebo consisted of 10 tablets to be taken in a single day."
One theory is that when given such high dosage of Vitamin D, the body may try to lower its vitamin levels. Dr. Michael Perskin compares it to attempting to take care of our need for protein by eating a year's worth in one day.
http://health.msn.com/health-topics/articlepage.aspx?cp-documentid=100258031