Vitamin D promises some important health benefits

You're taking cover under hats and wearing sunscreen to protect yourself from skin cancer. But it turns out that those precautions might also be contributing to vitamin D deficiency. Up to 36 percent of Americans are low on the nutrient, recent research suggests, and that increases the risk of weak bones, certain cancers, and more. Vitamin D is the only nutrient your body makes itself, but you can't make it without the sun's help. So is the answer to get out and catch some more rays? Probably not. Instead, look at your diet and consider taking supplements.

"We are coming to recognize that even in sunny places like Texas there is evidence of widespread D deficiency," says Irwin Rosenberg, M.D., director of the Nutrition and Neurocognition Laboratory at Tufts University. Quest Diagnostics, a leading lab, reports that the demand for vitamin D tests increased by about 80 percent last year. But is it worth it to shell out $200 for a blood test? And how much vitamin D do you really need? Here's what some of the latest studies and experts had to say.

The level of vitamin D associated with the health benefits listed here is higher than the government's recommended level of 400 international units. For adults, our experts recommend 800 to 1,000 IU daily, which is hard to get without popping a pill. "As one who tends to be very conservative about supplements," Rosenberg says, "this is one that I recommend to half of my patients." Multivitamins and calcium supplements usually contain 400 IU of vitamin D, or you can buy vitamin D supplements. But there's no reason for most people to exceed the government's safe upper limit of 2,000 IU. So don't overdo it.

The best way to get any nutrient is through food, though there aren't a lot of sources of vitamin D, and sunlight can be a less-than-reliable source. Experts say spending time in the sun can help, but your body's ability to produce vitamin D can also hinge on whether you live in a sunny climate. Being older, heavier, or having dark skin can also limit the sun's effects.

Using tanning salons is downright dangerous. Ultraviolet radiation, including exposure from tanning beds, is linked to most of the 1.5 million skin cancers and the 8,000 melanoma deaths in the U.S. each year. "The difficulty and the challenge for research is to identify the dose of sun exposure that gives you adequate D without increasing your risk of skin cancer," says Patsy Brannon, Ph.D., R.D., professor of nutritional science at Cornell University. "Right now, we just don't know where to draw that line."

Reliable tests have only recently become available, and our experts say it's too soon to recommend routine screening for everyone. But testing is a good idea if you have conditions linked to abnormal calcium metabolism or vitamin D deficiency, such as osteoporosis, hyperthyroidism, and possibly multiple sclerosis. Testing might also make sense if your risk is higher due to your age, where you live, your skin color, or your weight. But unless you have a medical condition specifically linked to deficiency, insurance probably won't pick up the tab.

Vitamin D plays a key role in bone health, muscles, the central nervous system, immune system function, cell growth, reproduction, and reducing inflammation. Recent research has associated high intakes or blood levels of D with:

• A longer life. A June 2008 study of more than 3,000 heart patients published in the Archives of Internal Medicine found that those with the highest blood levels of D were the least likely to die of any cause during the nearly eight years of follow-up. Another study in August involving more than 13,000 men and women found that people with the lowest blood levels of D were 26 percent more likely to die during the six to 12 years of follow-up than those with the highest levels.
  
  
• Strong bones. A review of 167 studies concluded that people who got at least 700 IU of vitamin D daily and took calcium supplements had denser bones and had fewer falls and fractures than people who didn't.
  
  
• Protection against cancer. The list includes cancer of the breast, pancreas, prostate, and especially the colon.
  
  
• A reduced risk of many other ailments. D helps to protect you against heart attack, both types of diabetes, and multiple sclerosis. It's also associated with a reduced risk of depression in older people and greater immunity against respiratory and skin-wound infections.
  
  
• Higher birth weights.

Your body produces vitamin D from sunlight, but since excessive exposure can cause skin cancer, our medical experts recommend getting it mainly from your diet. Aim for 800 to 1,000 international units (IU) daily. If you don't get enough from food, consider taking a supplement.

A recent report concluded that many children have less-than-optimal blood levels of vitamin D. In the fall of 2008, the American Academy of Pediatrics announced that it had doubled its recommendation to 400 IU daily starting in the first few days of life. Milk, formula, and fortified soy milk are the biggest sources of D in most kids' diets, so make sure that your child drinks enough. If you're breast-feeding, talk to your doctor about supplements for your infant. Yet another reason to keep your kids from overeating: Excess body fat traps vitamin D, leading to lower levels of it in the blood.