THURSDAY, Dec. 5, 2013 (HealthDay News) -- Low levels of vitamin D have been implicated as a potential cause of diseases ranging from cancer to diabetes. Now an extensive review suggests it's really the other way around: Low levels of the "sunshine vitamin" are more likely a consequence -- not a cause -- of illness.
In their review of almost 500 studies, the researchers found conflicting results. Observational studies, which looked back at what people ate or the kinds of supplements they took, showed a link between higher vitamin D levels in the body and better health.
But, in studies where vitamin D was given as an intervention (treatment) to help prevent a particular ailment, it had no effect. The one exception was a decreased death risk in older adults, particularly older women, who were given vitamin D supplements.
"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 known to play a key role in bone health. Low levels of vitamin D have been found in a number of conditions, including heart disease, autoimmune diseases, diabetes, cancer and Parkinson's disease. These findings may explain why so many Americans are currently taking vitamin D supplements.
It's nicknamed the sunshine vitamin because the body produces vitamin D when exposed to the sun (if someone isn't wearing sunscreen). It's also found in some foods, such as egg yolks and fatty fish, and in foods that have been fortified with vitamin D, such as milk.
The current review, published online Dec. 6 in The Lancet Diabetes & Endocrinology, looked at 290 observational studies. In these studies, blood samples to measure vitamin D levels were taken many years before the outcome of the study occurred. The review also included results of 172 randomized clinical trials of vitamin D. In randomized trials, some people receive a therapy while others do not.
The observational studies showed a potential benefit from vitamin D. For example, vitamin D was associated with a 58 percent reduced risk of cardiovascular events, a 38 percent decreased risk of diabetes and a 34 percent decreased risk of colon cancer in these studies.
But, when the researchers looked to the randomized clinical trials that used vitamin D as a treatment, they failed to find any effect on disease occurrence or severity from raising vitamin D levels.
However, vitamin D did reduce the risk of dying from any cause in older people taking 800 international units a day, according to the review.
Dr. Shaun Jayakar, an internal medicine and geriatric specialist from St. John Hospital and Medical Center in Detroit, said the findings in elderly people "are likely due to a reduction in falls and fractures. Supplementing with vitamin D would lead to stronger bones, which would reduce falls and factures."
Because the majority of interventional trials failed to find any benefit from vitamin D, the review's authors conclude that low vitamin D levels don't lead to ill health, rather they're caused by ill health.
They theorize that inflammation that occurs in many illnesses may be what depletes vitamin D levels.
Dr. Robert Graham, an internist from Lenox Hill Hospital in New York City, said, "This comprehensive review did a really good job at trying to tease out the effects of different study designs, and the findings will be controversial."
He said there are currently five, large ongoing interventional trials that will help to better define vitamin D's role in disease. However, the results of those studies won't be available for a number of years. Until then, he recommended, "Try to achieve homeostasis [equilibrium]. You don't want to get to a low level of vitamin D."
The Institute of Medicine recommends 600 international units of vitamin D for adults, and 800 international units for people over 70.
Both Graham and Jayakar agreed that those are reasonable supplement levels. Jayakar said that for most people, vitamin D supplements are harmless, but added that "it's a pocketbook issue. Almost 50 percent of the population is taking vitamin D supplements. That's a lot of money for something that likely has no benefit," he said.
Jayakar added that this review's findings suggest that low vitamin D levels could be used as a marker -- a sign -- of disease in younger people. "If someone isn't feeling well and they have low vitamin D, maybe we should use that to start searching to see if something else is going wrong," he said.
Read more about vitamin D from the U.S. National Library of Medicine.