(NEW YORK) -- For years, low levels of vitamin D have been viewed as a potential cause of diseases ranging from cancer to diabetes. But now, an extensive review of hundreds of studies reveals it's actually the other way around: Low levels of vitamin D are more likely a consequence, not a cause, of illness. Many Americans began taking vitamin D supplements after research found low levels of D in a number of illnesses, including heart disease, autoimmune diseases, diabetes, cancer and Parkinson's disease.
Now, in a review of almost 500 studies, researchers found conflicting results about what vitamin D can do. Observational studies, which looked back at what people ate or the types of supplements they took, showed a connection between higher vitamin D levels in the body and better health. But in studies where the vitamin was given as a treatment to combat a particular ailment, it had no effect. The only exception was a decreased death risk among older adults who were given vitamin D supplements -- in particular, older women.
"The discrepancy between observational and intervention studies suggests that low [vitamin D] is a marker of ill health," wrote review authors led by Philippe Autier, at the International Prevention Research Institute in Lyon, France.
Vitamin D is nicknamed the “sunshine vitamin” because the body produces it when exposed to the sun. Vitamin D is also found in certain foods, including egg yolks and fatty fish.
The review was published online this week in The Lancet Diabetes & Endocrinology.
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