The Sunshine Vitamin
by Sheila Buff

Your body needs 13 different vitamins for good health. Of those 13, almost all come from the foods we eat, with one important exception: vitamin D. Although vitamin D is found in some foods, your body manufactures most of what you need in a complex process that begins with ultraviolet B light from sunshine on your skin.

Researchers have long known that vitamin D plays a crucial role in building and maintaining healthy bones. More recently, researchers have come to realize that vitamin D plays a central role in preventing disease, especially cancer. And as the research increasingly shows, most of us have vitamin D levels that are far below optimum for disease prevention.

How Much Vitamin D Do You Need?
According to the federal nutrition standards, the amount of vitamin D you need every day for good health is small. (Vitamin D is measured in standard International Units, or IU. There are 40 IU in one microgram, and there are 1,000 micrograms in a milligram. Strictly speaking, the measurements aren't really comparable, because IU measures potency, not weight or mass.) The adequate intake for infants and children to age 18 is 200 IU; for adults age 19 to 50, it's 200 IU; for adults aged 51 to 70 it's 400 IU; and for adults over age 71 it's 600 IU.

There's a big problem with those numbers: Today most experts agree that they're way too low. To take just one example: An important study published in The New England Journal of Medicine in 1997 showed that men and women over age 65 can cut their risk of a bone fracture in half if they take 700 IU of Vitamin D and 500 mg of calcium every day. Additional studies have shown that even when older adults get the adequate intake amount (or AI) for vitamin D, nearly 40 percent are still deficient. Other studies show that on average nearly half of all Americans are deficient in vitamin D—even when they get the AI every day. In fact, even among people who get abundant sun exposure, about half are deficient in vitamin D. To many researchers, the true AI needs to be set at somewhere between 1,000 IU and 2,000 IU daily.

The best way to get more Vitamin D? Get outside into the sunlight more—without sunscreen. That's fine in theory, but in practice it's a problem. Most of us work indoors during the day and can't get outdoors for 20 minutes or so of direct sunlight at midday. On overcast, rainy, or cold days, the odds of getting direct sunshine on the skin are low. Also, the darker your skin, the less sunlight you'll absorb. And even if you do get outside more, it may not be enough. Older adults produce only about half the vitamin D they did when they were younger. There aren't that many foods that naturally have vitamin D. Beef liver, egg yolks, oily fish such as canned sardines, butter, and margarine all have some, though not a lot. Plant foods have almost none, although vitamin D is added to a lot of breakfast cereals. Today almost all the vitamin D people get from their diets comes from fortified milk. There isn't naturally much vitamin D in milk, but milk producers have been adding 400 IU of it to every quart of milk—whole, skim, low-fat, and nonfat—since the 1930s. It's the reason rickets (bowed and malformed bones caused by a lack of vitamin D) has practically disappeared. There's no vitamin D in most dairy products, though. Cheese, yogurt, cottage cheese, and other dairy foods aren't made with fortified milk, so they don't have much or any vitamin D. Also, raw milk, most organic milk, and goat's milk don't have added vitamin D; soy milk usually contains added vitamin D, but check the label to be sure.

Even if you spend time outdoors every day and eat a diet high in fish and dairy foods, you're still likely to be on the low side for vitamin D. The answer is supplements; most experts suggest 1,000 IU to 2,000 IU daily. Doses much higher than this are safe for most people, so there's little down side from taking supplements. (Check with your doctor anyway, especially if you take any prescriptions drugs or have a chronic health problem.) The upside is stronger bones and quite possibly a reduced risk of cancer.

When choosing a supplement, look for a product from a reputable manufacturer. Choose a brand that contains vitamin D3, also called cholecalciferal. This form of vitamin D is most effective in supplements.

Vitamin D and Strong Bones
The importance of vitamin D for building strong bones in children and young adults and helping to prevent osteoporosis (thin, brittle bones that break easily) in older adults has long been known. Recent studies have shown just how important high levels of vitamin D are for maintaining lifelong bone health. A study that appeared in the American Journal of Clinical Nutrition in 2007 showed that the currently recommended intake of 400 IU a day didn't help reduce fracture risk in older women, but taking 700 to 800 IU daily did—even without supplemental calcium.

Vitamin D and Cancer Prevention
Recent work has shown that vitamin D is essential for helping to prevent cancer, particularly of the breast, ovaries, prostate, pancreas, and colon. The studies suggest that vitamin D supplements could prevent at least 150,000 cases of breast cancer and 100,000 cases of colon cancer in the U.S. every year. In a study published in June 2007 in the American Journal of Clinical Nutrition, researchers revealed that women who took a supplement containing vitamin D and calcium over a four-year period had an astonishing 60 percent drop in their risk of any cancer. The evidence for the number of breast and colon cancers that wouldn't happen if everyone took supplemental vitamin D appeared in the respected journal Nutrition Reviews in August 2007, right after a major review article in the famed New England Journal of Medicine presented similar conclusions.

Vitamin D and Other Diseases
The crippling autoimmune disease multiple sclerosis is very rare among people living near the equator and becomes progressively more common the further you go to the poles. Researchers have long suspected a link between MS and lack of vitamin D from low sun exposure. Recent studies have shown that women who get at least 400 IU of vitamin D daily have only a 60 percent risk of getting MS compared to women who get lower amounts each day. Similarly, vitamin D may help prevent rheumatoid arthritis (RA), another crippling autoimmune disease. Women with the highest levels of Vitamin D are about a third less likely to develop the disease.

In addition, vitamin D plays an important role in overall immune system strength and may help prevent other chronic diseases, including age-related macular degeneration, inflammatory bowel disease, Parkinsons disease, and diabetes.

The evidence in favor of more vitamin D is strong and getting stronger. In 2006 and 2007 alone, more than 250 major research studies on the role of vitamin D in human health appeared in core medical journals. So powerful is the evidence that the Canadian Cancer Society now recommends all adults take a daily vitamin D supplement containing 1,000 IU. Authorities in the United States haven't made any recommendations for adults yet, but in 2008 the American Academy of Pediatrics doubled its recommendations for the minimum amounts of vitamin D that infants, children, and teens. The new recommendations says that they should get 400 IU daily, from supplements if necessary.