WASHINGTON - A large new study found no sign that vitamin D lowers the overall risk of dying from cancer, injecting a note of caution to the latest vitamin craze.
The exception: People with more vitamin D in their blood did have a significantly lower risk of death from colorectal cancer, supporting earlier findings.
Getting enough of the vitamin the skin makes from ultraviolet rays is vital for strong bones.
But in recent years research saying it may be a powerful cancer fighter has sparked a push for people to get more than the recommended amounts, either through diet or sun exposure.
The first-of-a-kind government study released yesterday shows the issue is far from settled.
National Cancer Institute researchers analyzed vitamin D levels in almost 17,000 people as part of a national study that tracked their health. About a decade after enrolling, 536 of those people had died of cancer.
Whether people had low or high vitamin D levels played no role in their risk of dying from cancer in general, they reported yesterday in the Journal of the National Cancer Institute.
Then the researchers examined different types of cancer. There were 66 deaths from colorectal cancer, but people with high levels of vitamin D appeared 72 percent less likely to die of it than people with the lowest levels.
"While vitamin D may well have multiple benefits beyond bone, health professionals and the public should not, in a rush to judgment, assume that vitamin D is a magic bullet and consume high amounts," Johanna Dwyer, a dietary supplement specialist at the National Institutes of Health, cautioned in an accompanying editorial.
Indeed, there are numerous risk factors for colorectal cancer, including obesity and low physical activity, and it's unclear whether low vitamin D levels play an independent role or are just a marker for those other risks, she said.
Scientists have been interested in vitamin D's effects for decades, since noticing that cancer rates among similar groups of people were lower in sunny southern latitudes than in northern ones. A handful of studies since then have found that people given vitamin D supplements have less risk of developing certain cancers, but much of the evidence is circumstantial.
Experts are cautious because other vitamins and nutrient supplements once widely thought to prevent cancer didn't pan out when put to rigorous testing.
This study is the first to compare blood levels of vitamin D with cancer mortality, and "it's the best research we have on this topic," said Dr. Len Lichtenfeld of the American Cancer Society.
But it measured levels at just one point in participants' lives, when they can vary widely with dietary changes and the seasons.