U.S. new study discovered that vitamin D does not lower 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 so-called sunshine vitamin - the skin makes it from ultraviolet rays - is vital for strong bones.
But vitamin D has made headlines in recent years because of research saying it may be a powerful cancer fighter, sparking a push for people to get more than currently recommended amounts, either through diet or sun exposure.
The first-of-a-kind U.S.government study released Tuesday shows the issue is far from settled.
National Cancer Institute researchers analyzed vitamin D levels measured 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 Tuesday in the Journal of the National Cancer Institute.
Then the researchers examined different types of cancer. There were just 66 deaths from colorectal cancer. Still, people with high levels of vitamin D appeared 72 percent less likely to die of colorectal cancer than people with the lowest vitamin D 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 if 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 between similar groups of people were lower in sunny southern latitudes than in northern ones. A handful of studies since then have found 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.
The NCI's study is the first to compare blood levels of vitamin D to cancer mortality, and "it's the best research we have on this topic," said Dr. Len Lichtenfeld of the American Cancer Society.
But a big weakness: It measured vitamin D at just one point in participants' lives, when levels can vary widely with dietary changes and especially the seasons.
Overall, most research "seems to be pointing in the direction that there is a role of vitamin D," Lichtenfeld said. Tuesday's study "puts a note of caution in there that says with all the explosion of information and advocacy on behalf of vitamin D, we need to be cautious. ... We really need some further studies that are well done to answer the question."