CHICAGO – Dr. Richard L. Gallo put on his Myth Busters hat at a recent dermatology meeting to debunk – and in some cases uphold – some of the most popular ideas about vitamin D.
"This subject is nothing new," noted Dr. Gallo, recounting a bit of vitamin D yore. "In 1936, Schlitz beer urged customers to drink the beverage because it contained 100 units of vitamin D, and could ward off colds and flu. So even back then, they were on to something."
But, Dr. Gallo questioned, is the idea that vitamin D can strengthen the immune system a reality – or a myth? And how about other claims touted in the public press, that sunlight is the best source of vitamin D, that the vitamin strengthen bones and protects against cancer.
"Unfortunately, vitamin D information has become something of a shell game, with positions that overstate the strength of the evidence. As dermatologists, for example, we know the carcinogenic potential of sunlight, but there are now opposing groups that advocate health by increasing vitamin D through sun exposure."
Myth No. 1: Fifteen minutes per day of sunlight provides adequate amounts of vitamin D.
"In a test tube, ultraviolet B is the optimal spectrum for converting 25-hydroxy D into vitamin D in the human body," Dr. Gallo said. "But randomized studies on this vary in results."
One frequently cited study examined the issue in Denmark. "Northern latitudes are very useful for studies like this because of the high intensity of the sun during the summer, and the low intensity in winter," said Dr. Gallo, chief of dermatology and professor of medicine and pediatrics at the University of California San Diego. "In this study, the 25-hydroxy D in the population varied dramatically with change in sunlight exposure, and tended to lag about 1 month behind the sunlight levels."
But the study also found that 53% of the subjects who sought sun exposure were still suboptimal in their vitamin D levels. "So sun-seeking behavior in one of the most intense sun-exposed areas of the world is not sufficient to cover optimal vitamin D in a population." (Photochem. Photobiol. 2009;85:1,480-4).
A 2009 study looked at sunlight exposure and vitamin D in twins (PLoS One 2010 5(7):e11555 [doi: 10.1371/journal.pone.0011555]). More than 200 twins were evaluated for the seasonal impact of genetic factors on serum 25-hydroxy vitamin D concentrations. "This showed very wide distributions in levels during the different seasons, and concluded that more than 50% of the variation in summer levels was not due to sun or diet, but to genetic influences independent of skin pigment," Dr. Gallo said.
"So, Myth No. 1 – busted," he concluded.
Myth No. 2: Vitamin D improves bone health.
Prospective cohort studies such as the National Health and Nutrition Examination Survey show that hip fracture is reduced by more than one-third in patients with adequate vitamin D levels (more than 60 nmol/L). "However, we still have a lot to learn. Data from a recent 3-year study of 2,000 perimenopasual women concluded that if vitamin D were given as a single annual dose of 500,000 IU, the women had a 15% increased risk of falls and a 26% increase in the risk of fractures." (JAMA 2010;303:1815-22).
"As far as Myth No. 2 goes, I'd say it's true, but we don't understand everything yet."
Myth No. 3: Vitamin D protects against cancer.
"This has been quite a popular theme in the press for years now, but there are no great mechanistic explanations as to why it may be true," Dr Gallo said. "There are a number of randomized controlled trials, but the data are inconsistent."
A 2009 Agency for Healthcare Research and Quality review examined more than 170 studies and reviews for several health outcomes and vitamin D. "Only one study really showed a level of significance [for cancer reduction]. The others showed inconsistent data on cancer and some showed a slight trend toward an increased risk for colon cancer." (Evid. Rep. Technol. Assess. (Full Rep.) 2009;183:1-420).
"Myth No. 3 is still a plausible possibility, but no benefit has been clearly demonstrated."
Myth No. 4: Vitamin D improves immune function.
"We have excellent mechanistic data to support this claim, including a number of observational studies and a few randomized controlled trials," Dr. Gallo said.