Vitamin supplements have been both heralded and hyped over the years, only to ultimately fall from grace once research proves them to be little more than placebos in our quest for longer life or better health.
But at least one substance may have true merit - vitamin D.
Long considered just a supplement consumed with calcium for bone health, this humble vitamin may have untapped potential in fighting or preventing disease, suggests an explosion of new research. Not only has it shown promise in reducing the risk of, among other things, diabetes, pancreatic cancer, breast cancer and cardiovascular disease, but it also seems to improve infertility, weight control and memory.
Two advocacy groups have sprung up in the United States to promote the substance. Food industry executives are exploring ways to fortify more products. And Pub Med, an international database of medical literature, shows that 2,274 studies referencing the vitamin have been published - just this year.
"Vitamin D is one hot topic," says Dr. Connie Weaver, a professor of foods and nutrition at Purdue University.
The last time guidelines were issued on the vitamin was in 1997, long before scientific information suggested people are getting too little. Currently, the recommended daily intake is 200 international units to 600 international units a day with an upper limit of 2,000 IU per day.
Some researchers are advocating at least 600 IU per day, with an upper limit of 10,000 IU. Giving impetus to this push are the facts that many people seem to be deficient and that the nutrient appears to play a role in many other conditions.
Other scientists say it's too soon to urge everyone to take supplements. An influential report released in June by the Agency for Healthcare Research and Quality found little conclusive evidence to support increasing the recommended amounts.
"I think there is a consensus that we might benefit from higher vitamin D levels," says James C. Fleet, a professor of foods and nutrition at Purdue University and a long-time researcher on the vitamin and prostate cancer. "But the committee is going to ask whether there is existing scientific evidence that is strong enough to make a change."
Vitamin D has long been known to be critical to bone and muscle health by improving calcium absorption in the intestines and the way calcium is regulated in bones. More recent research shows that receptors for it are found in almost every organ and tissue system in the body, suggesting that deficiencies may impact many types of cell functions.
The skin makes vitamin D when exposed to sunlight, but not everyone spends the five minutes a day or so outside that is necessary for synthesis - and many more people wear sunscreen to prevent skin cancer.
Most people age 50 and older aren't meeting the current recommendations, Weaver says. The vitamin is found in relatively few dietary sources - some fortified foods, such as milk and some cereals, and naturally only in some fatty fish. Three cups of milk per day provides only 300 IU.
"The largest source is sunshine, but not everyone can depend on that," Weaver says. "The elderly, dark-skinned people, higher-latitude dwellers all have trouble getting enough from sun."
In darker-skinned people, melatonin in the skin blocks absorption of the UV rays needed to make the vitamin; older people don't appear to synthesize it from the sun as well as younger people.
Some scientists argue that levels of 40 ng/ml to 60 ng/ml would be far better for disease prevention. That would require intakes much higher than the current 200 IU to 600 IU per day.
The July issue of the Annals of Epidemiology, devoted to vitamin D research, links the vitamin to lower risks of cancers of the breast, colon, ovary and prostate. Animal and lab studies also demonstrate its importance in many of the cellular mechanisms that control cancer, such as cell growth, cell death, inflammation and DNA repair.
Five studies on colorectal cancer and breast cancer, taken together, showed that people with levels higher than 34 ng/ml to 52 ng/ml had a 50 percent reduced cancer risk, says Cindy D. Davis, a researcher at the National Cancer Institute's Nutrition Science Research Group.
Such studies are not proof that the vitamin influences disease development, points out Dr. Karen E. Hansen, an assistant professor at the University of Wisconsin who studies bone health. "People with higher vitamin D may just be healthier for other reasons."
But evidence linking higher blood levels to diabetes and cardiovascular disease is also mounting. A study in December 2008 in the Journal of the American College of Cardiology found that deficiency may increase the risk of cardiovascular disease. Other studies have tied lower levels to an increased risk of hypertension, diabetes, stroke and congestive heart failure.
Even for bone health, some studies suggest that about 700 IU to 800 IU per day are needed to prevent fractures in people over age 50, Hansen says. She recommends 800 IU per day, with calcium, to her patients.
Meanwhile, studies show that the previous estimation of a toxic dose - 2,000 IU per day - is most likely too conservative. Toxic doses can lead to a dangerous level of calcium in the blood, high blood pressure and even kidney failure.
"It's likely they will increase their recommendation for all ages," Hanson said of the Institute of Medicine committee, which will release its report next year.
But not everyone is convinced that the advice should be changed.
In a report ordered by the federal government to assist the committee, researchers concluded that there is a lack of strong evidence to support altering recommendations. The committee is not expected to change calcium recommendations.
"We did not find data that indicate a specific level of vitamin D intake is associated with adverse outcomes or beneficial health outcomes," said Dr. Thomas A. Trikalinos, co-director of the Tufts Evidence-Based Practice Center, which prepared the report.
He said the report is meant to inform the committee but does not make recommendations.
"The report sees the totality of the evidence and tries to put everything into perspective," Trikalinos says.
Already, however, the American Society of Clinical Oncology has recommended a higher intake for breast cancer patients who are deficient. In October, the American Academy of Pediatrics said children should get 400 IU a day, double the current recommendation.
In November 2008, 18 University of California researchers issued a statement saying 2,000 IU is appropriate for most people.
"I think some of the more vocal advocates are pushing the medical community to move forward" before adequate research is completed, Fleet says.
Dozens of more scientifically rigorous studies are in progress that could help resolve the questions about how much people should consume.
"I think they held this IOM meeting two to four years too early," Fleet says. "They are working without the big wave of vitamin D research that was initiated after people started pushing for it."
Some health experts say people should consume at least 600 IU of vitamin D per day, with an upper limit of 10,000 IU.
Current Recommendations on Vitamin D Intake:
Age 49 and younger - 200 IU per day
Pregnant and lactating women - 200 IU per day
Ages 50 to 70 -- 400 IU per day
Age 71 and older - 600 IU per day
Upper limit - 2,000 IU per day
- Source: The National Academy of Sciences