To me, it seemed to take a long time for the Institute of Medicine (IOM), which sets the recommended dietary allowance (RDA) for various nutrients, to revise its guidelines on vitamin D. When the new IOM report finally did come out at the end of 2010, I was shocked at how small the recommended increase was. Whereas the former daily “adequate intake” was set at 200 international units (IU) to 600 IU depending on age, the new RDA is higher, but just barely — only 600 IU daily for most Americans or 800 IU for those over age 70.

Like many people who track developments in the health and nutrition arena, I had heard that a number of experts expected the new RDA to be 1,000 IU to 2,000 IU. Concerned, I contacted professor Michael F. Holick, PhD, MD, of Boston University Medical Center, a top vitamin D expert. Diplomatically, he called the IOM recommendations “a step in the right direction” — but cautioned that they fall far short of what’s needed for optimal health. What’s more, the impact is widespread, given that two-thirds of Americans have blood levels of vitamin D that are too low. That’s why Dr. Holick’s new book, The Vitamin D Solution, calls this our “most common health problem.”

Why vitamin D deficiency is such a big deal. You know that vitamin D is needed for calcium absorption and therefore for strong bones. But in fact, this nutrient is important for the health of virtually every cell — heart cells, brain cells, muscle cells and all the rest. Dr. Holick said, “Essentially every cell in the body has a vitamin D receptor, which means that essentially every cell requires this nutrient for maximum functioning. That is why having sufficient vitamin D in the body can help prevent or treat a remarkable number of ailments.”

“Remarkable” might be an understatement! Health problems linked to low vitamin D include heart disease, stroke, breast cancer and other cancers, obesity, diabetes, depression, colds, flu, asthma, Parkinson’s disease, rheumatoid arthritis, multiple sclerosis, fibromyalgia, back pain, infertility, memory loss and mental decline.

Problems with the IOM guidelines. The IOM set too low a standard in saying that almost all individuals have sufficient vitamin D when blood levels are at or above 20 nanograms per milliliter (ng/ml), said Dr. Holick. He should know. The IOM set that level based in part on Dr. Holick’s research from the 1990s, which showed that blood levels above 20 ng/ml did not affect levels of parathyroid hormone (PTH), a metabolic signal that the body has enough vitamin D to promote bone health.

But bone health is no longer the only concern when it comes to vitamin D. Yes, levels below 20 ng/ml signal a severe deficiency — yet this does not mean that levels just above that are sufficient for optimal health. Rather, levels from 20 ng/ml to 30 ng/ml indicate a vitamin D insufficiency that is linked with weaker bones, a weaker immune system and the increased stiffness of artery walls that can lead to heart attacks and strokes. Dr. Holick said, “We need blood levels above 30 ng/ml — and, ideally, I would like my patients to have blood levels of 40 to 60 ng/ml.”

Another problem is the IOM’s position that most Americans receive enough vitamin D. Because there are few good food sources except vitamin D-fortified milk and wild (not farmed) salmon, the usual American diet does not maintain blood levels above 30 ng/ml, according to Dr. Holick, even if the person takes a typical daily multivitamin.

Symptoms of vitamin D deficiency include muscle weakness and pain, bone pain and fractures, falls and difficulty walking. The 25-hydroxyvitamin D blood test can determine vitamin D blood levels — but Dr. Holick said that routine testing is not necessary if people follow the supplementation recommendations below. Exceptions: You should get tested if you have symptoms of vitamin D deficiency… have a digestive condition that interferes with fat absorption, such as inflammatory bowel disease… are obese… or take medication that affects vitamin D metabolism, such as glucocorticoids, antiseizure drugs or AIDS medications.

How to keep blood levels above 30 ng/ml. In Dr. Holick’s opinion, taking a daily supplement is an easy way to ensure getting enough vitamin D. His daily supplementation recommendations: 600 IU to 1,000 IU for children age one to 12 years… and 1,500 IU to 2,000 IU for teenagers and adults.

Because vitamin D is synthesized inside the body when sunlight hits the skin, another alternative is to expose your bare skin to a sensible amount of sunlight during the spring, summer and fall. “Bare means bare of sunscreen, too. Yes, use sunscreen on your face to reduce wrinkling and skin cancer risk. But two or three days per week from May to October, get outside between 10 am and 3 pm and expose your arms and legs for about five to 10 minutes,” Dr. Holick said. Because vitamin D is a fat-soluble vitamin, it remains stored in the body’s tissues and is eliminated much more slowly than water-soluble vitamins are — which is why spring-to-fall exposure generally is sufficient to get you through the winter.

How much vitamin D is too much? The fat-soluble nature of vitamin D is behind the concerns about getting too much. And it is true that excessive vitamin D can lead to high blood levels of calcium, which can cause kidney damage and blood vessel calcification. Symptoms of vitamin D overload — loss of appetite, nausea, vomiting, excessive thirst, weakness, nervousness, high blood pressure — generally arise when blood levels exceed 150 ng/ml.

But the IOM’s “upper tolerable limit” of 4,000 IU of vitamin D daily is too cautious, Dr. Holick said, because “scientific data show that an intake of up to 10,000 IU daily for adults for up to five months is safe.” Still, unless testing reveals a vitamin D deficiency and your doctor prescribes a higher dosage, Dr. Holick advised sticking with 1,500 IU to 2,000 IU per day. That’s what I’m doing.

Source: Michael F. Holick, PhD, MD, is a professor of medicine, physiology and biophysics and director of the General Clinical Research Unit, the Bone Health Clinic and the Heliotherapy, Light, and Skin Research Center, all at Boston University Medical Center. The author of more than 300 scientific articles, Dr. Holick is widely regarded as one of the world’s foremost experts on vitamin D and health. His most recent book is The Vitamin D Solution: A 3-Step Strategy to Cure Our Most Common Health Problem (Hudson Street).