Dr. Philippe Autier of the International Agency for Research on Cancer, in Lyon, France, and Sara Gandini of the European Institute of Oncology, in Milan, Italy, searched for randomized controlled trials of vitamin D supplements published before November 2006.

The researchers analyzed 18 separate trials that included 57,311 participants and evaluated doses of vitamin D ranging from 300 to 2,000 international units, with an average dose of 528 international units.

People who took vitamin D had a 7 percent lower risk of death than those who did not. In the nine trials that collected blood samples, those who took supplements had an average 1.4- to 5.2-fold higher blood level of vitamin D than those who did not.

"Mechanisms by which vitamin D supplementation would decrease all-cause mortality are not clear," the study said. "The intake of ordinary doses of vitamin D supplements seems to be associated with decreases in total mortality rates."

The findings are published in the Archives of Internal Medicine.