People with low levels of vitamin D in their blood are twice more likely to die prematurely than people with high levels of the vitamin, according to a new study.

The study was conducted by researchers from the University of California, San Diego School of Medicine. They analysed 32 studies that were previously conducted on vitamin D, blood levels and human mortality rates. All the studies looked at a specific variant of vitamin D called 25-hydroxyvitamin D, the primary form found in blood.

The study included residents of 14 countries, including the United States, and data from 566,583 participants. Blood samples were drawn from participants at an average age of 55 years. All participants were followed for an average of nine years.

"Three years ago, the Institute of Medicine (IOM) concluded that having a too-low blood level of vitamin D was hazardous," said Cedric Garland, DrPH, lead author of the study in a press statement. "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."

Researcher noted that the vitamin D level in the blood associated with premature deaths was 30 ng/ml. What's concerning is that two-thirds of the U.S. population has an estimated blood vitamin D level below 30 ng/ml.

Though many recent studies have highlighted the negative consequences of excessive vitamin D in the blood, the findings of this study suggest that the nutrient is safe when used in appropriate doses up to 4,000 International Units (IU) per day. Study authors recommend getting 25-hydroxyvitamin D levels checked annually. Also, anyone making changes in their Vitamin D consumption should first seek advice from their doctors.

Many of the previously touted benefits of vitamin D have been questioned lately. A study conducted in March this year noted that the vitamin is not effective in reducing depression risks, debunking previous beliefs. Another study found no concrete evidence to suggest that vitamin D supplements are effective in preventing accidental falls in older people.

In January this year, University of Auckland researchers released a controversial report suggesting healthy people may not benefit as much from Vitamin D as it doesn't have a significant effect on preventing heart attacks, stroke, cancer or bone fractures in such people.

In another report, the U.S. National Cancer Institute noted that its studies conducted on the effects of vitamin D on cancer may not be completely accurate because studying a person's blood vitamin level at a single point in time, as many studies do, may not give an accurate picture of his or her true levels. Moreover, many studies don't use high enough doses to see the benefits.

Despite this, various studies and health experts continue to vouch for the health benefits of vitamin D and how it can help in keeping serious health issues at bay. Very recently, Northwestern University researchers said that vitamin D can help reduce the risk of aggressive prostate cancer in men.

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