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
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
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
However, vitamin D did reduce the risk of dying from any cause
in older people taking 800 international units a day, according to
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
causedby 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
The Institute of Medicine recommends 600 international units of
vitamin D for adults, and 800 international units for people over
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.