Despite its active role in bone formation and uptake of calcium in the body, vitamin D hasn’t been shown in randomized trials to offer effective treatment for acute or chronic conditions, such as cancer, diabetes, cardiovascular disease, and Parkinson’s, according to comprehensive meta-analyses performed by a team of French researchers.

The findings peg vitamin D, not as a cause of poor health, but a consequence of it. Such a relationship would dispel the notion that a vitamin D deficiency could result in the string of such disorders, and that it’s actually the disorders themselves that cause vitamin D levels to drop. For nearly half the U.S. adults who take some form of vitamin supplement at an overall cost of $600 million, the findings may cause them to rethink the way they achieve disease prevention.

Vitamin D has something of a storied past. It’s been found to treat the pains associated with diabetes, although a deficiency in the vitamin has been linked to a risk for childhood anemia and hepatitis B virus. Studies have also found that vitamin D does not, in fact, support women’s bone health after menopause or lower people’s blood pressure. Because the vitamin’s effects are so scattered, the National Institutes of Health recently set aside an undisclosed dollar amount (in the millions) to investigate its effects on diabetes over the long-term.

The present researchers believe their meta-analysis of 290 prospective observational studies and 172 randomized trials, which examined the effects of vitamin D levels on non-bone health outcomes up to December 2012, ultimately finds truth in the randomized trials.

"If the health benefits of high vitamin D concentrations shown by data from observational studies are not reproduced in randomized trials,” explained lead author Professor Philippe Autier, of the International Prevention Research Institute, in a statement, “then the relation between vitamin D status and disorders are probably the result of confounding or physiological events involved in these disorders.”

In other words, without testing for placebo effects or indirect effects from external factors, researchers have no way of knowing whether, say, vitamin D supplements diminished subjects’ depression or whether their hopeful attitude, as a result of the study itself, led them to expect an outcome. What’s more, even this expectation of an outcome can bias a study’s result — among both subject and experimenter. This is why double-blind studies are often considered the gold standard for clinical trials, as neither the experimenter nor the subject knows what is being tested; a separate third-party recruits them both.

A dearth of randomized trials, according to Autier, has resulted in the observance of low levels in the presence of certain disorders, yet he discounts the idea that it’s vitamin D doing the heavy lifting. For instance, he and his colleagues observed high vitamin D levels reduced the risk of cardiovascular events by up to 58 percent, diabetes by 38 percent, and colorectal cancer by 34 percent. However, these findings weren’t confirmed in randomized clinical trials. Among the studies meeting Autier’s criteria, none managed to replicate the reduced risks or improved treatments just from the vitamin alone.

"What this discrepancy suggests,” Autier concluded, “is that decreases in vitamin D levels are a marker of deteriorating health. Aging and inflammatory processes involved in disease occurrence and clinical course reduce vitamin D concentrations, which would explain why vitamin D deficiency is reported in a wide range of disorders."