People with low levels of vitamin D in their blood are twice as likely to die earlier than those with high levels of the sunshine vitamin.

The study - conducted by researchers at the University of California, San Diego School of Medicine - is based on data from 32 previous studies on the subject. Researchers looked at 25-hydroxyvitamin D, which is the primary form of vitamin D found in blood. A total of 566,583 people participated in the studies.

Low levels of vitamin D are often found to be associated with poor muscle function in adults and children. Related research had shown that high levels of vitamin D and calcium in diet extend life and that a deficiency of vitamin D in diet might lead to depression. The present study found a link between vitamin D deficiency and early death risk.

"Three years ago, the Institute of Medicine (IOM) concluded that having a too-low blood level of vitamin D was hazardous," said Cedric Garland, lead author of the study. "This study supports that conclusion, but goes one step further. The 20 nanograms per milliliter (ng/ml) blood level cutoff assumed from the IOM report was based solely on the association of low vitamin D with risk of bone disease. This new finding is based on the association of low vitamin D with risk of premature death from all causes, not just bone diseases."

Around two-thirds of the U.S. population has vitamin D level of less than 30 ng/ml. According to Garland, 30 ng/ml or above reduces early death risk.

Salmon, tuna, beef liver and egg yolks contain small amounts of vitamin D, according to the Office of Dietary Supplements.

"It's always wise to consult your physician when changing your intake of vitamin D and to have your blood level of 25-hydroxyvitamin D checked annually. Daily intakes above 4,000 IU per day may be appropriate for some patients under medical supervision," said Heather Hofflich, DO, professor in the UC San Diego School of Medicine's Department of Medicine, in a news release.

The study is published in the American Journal of Public Health and was funded by UC San Diego Department of Family and Preventive Medicine.