If you think modern medicine has all the details of basic nutrition worked out, think again. Last week the American Academy of Pediatrics (AAP) issued a clinical report (PDF) recommending that all children, from newborn infants to adolescents, double the amount of vitamin D in their daily diet. That's 400 IU a day, twice the 200 IU amount that was established in 1997.

If you're Dr. Greg Plotnikoff, medical director of the Penny George Institute for Health and Healing at Abbott Northwestern Hospital in Minneapolis, twice as much isn't nearly enough. Plotnikoff has a strong background in interventional nutrition, and he's researched and written about vitamin D. He believes the AAP recommendations lag well behind the most current scientific literature. "If you're looking for a word: disappointing. It's a step forward but not nearly far enough. And it's not sufficiently evidence-based to be valid guidance for children in Minnesota."

Plotnikoff's enthusiasm for the health benefits of vitamin D is supported by an increasing amount of research suggesting that our understanding of vitamin D has been, up until recently, dull and ill-informed. He frames the issue of vitamin D this way: "Because vitamin D is so cheap and so clearly reduces all-cause mortality, I can say this with great certainty: Vitamin D represents the single most cost-effective medical intervention in the United States."

Controls calcium absorption
Most of what most people know about vitamin D is either from high-school health class or the American Dairy Association ads reminding us that milk builds strong bones. That's true in that milk contains high amounts of both calcium and vitamin D, and both are important in bone health. Calcium is a key ingredient in bone matrix, the cement that helps give bones their strength. Vitamin D controls calcium metabolism in our bones, and how much calcium we absorb from our diet. When blood levels of calcium are falling and dietary sources are lacking, calcium is pulled out of "storage" from the bones.

Though milk is a good source of vitamin D, the cows don't have anything to do with that. Vitamin D has been added to milk (more impressively, "fortified") because … because it gives you a reason to drink it. Elsie the Cow was, among other things, a master politician and marketing guru. At current fortification rates, the vitamin D added to an 8-ounce glass of the white stuff provides 100 IU — 25 percent of the minimum daily amount for a child.

If you're lactose intolerant or just not a milk drinker, you could get your vitamin D by chomping on fortified breakfast cereals (about 100 IU per serving, and not so good without milk), or other fortified dairy products like yogurt, butter and cheeses. If you prefer all-natural forms, you'll be disappointed to know that there are very few foods that naturally contain anything but trivial amounts of vitamin D. You'll need to focus on oily fish like salmon or sardines (a 3.5-ounce can delivers 300 IU of vitamin D), shiitake mushrooms, or be prepared to eat a whole lot of eggs (about 20 IU in one yolk).

No need to panic, though, because we humans can and do synthesize our own vitamin D. All we need is sunlight. In the deeper layers of our skin, ultraviolet radiation (technically, UVB) converts a vitamin D precursor into the real thing, which is further processed by the liver and the kidney into its active form.

Process complicated by clothes — and latitude
This is a fairly efficient process. A light-skinned adult lying naked in the summer sun for 10 to 15 minutes can generate 10,000 to 20,000 IU of vitamin D. Those are ideal conditions, of course. For many, body-image problems or simple modesty prevents us from lying around outdoors in the nude. And clothes drastically reduce the amount of UVB that can get to our skin — as does living at a latitude like Minnesota's, where the sun gathers strength in spring training and fizzles out by the World Series. Highly pigmented skin inhibits vitamin D synthesis; melanin is the substance that gives our skin its color, but when found in high quantities it can prevent up to 95 percent of UV light from getting to the deeper layers of the skin where vitamin D is produced. Sunscreen has the same effect as melanin.

None of the above is new information. So why has the preeminent pediatric association in the country, the AAP, decided to double the recommended daily intake for Vitamin D that was established in 1997? There are two answers to that question, and they are closely related.

It's becoming clear that the previously established normal vitamin D levels are probably too low. They were calculated with a very narrow view of the physiological role of Vitamin D: bone health. The question used to set the bar was simply, "How much vitamin D does one need to take to avoid rickets?" as if rickets were the singular problem associated with low vitamin D levels.

A complex biochemical role
But vitamin D is undergoing a rather dramatic renaissance, as an ever-increasing amount of research data suggests that vitamin D builds strong bodies, not just bones. Vitamin D receptors have been found in muscle, brain, prostate, breast, and colon tissues; and in white blood cells. We're beginning to understand the complex biochemical role vitamin D plays inside those tissues.

Living out on Keillor's frozen tundra has its emotional hazards, but living this far north also puts us at increased risk for certain health problems. It is well established that people living at higher latitudes are at increased risk for cancers from the colon, pancreas, breast, and ovaries, as well as Hodgkins lymphoma. And type 1 diabetes, multiple sclerosis, Crohn's disease (inflammation of the GI tract), hypertension and cardiovascular disease also occur more frequently at higher latitudes.

The hypothesis is that decreased sun exposure at higher latitudes decreases vitamin D levels and leaves us susceptible to these illnesses. And while this remains a hypothesis only, a review article on vitamin D in the New England Journal of Medicine in 2007 brandished a litany of research data showing the association to be consistent and powerful. 

Obese teens in Milwaukee
Want an example that's close to home? How about Milwaukee? Plotnikoff recounted a recent study that looked at obese 13-year-olds in that city. "There were 127 of them, and they were white, Hispanic and African American," Plotnikoff told me. "And depending on the racial group, they were between 74 and 88 percent deficient in vitamin D." The degree of deficiency correlated with the darkness of skin tone, but they were all deficient (in part because vitamin D gets deposited — and unavailable — in fat).

The researchers measured each participant's insulin resistance, a metabolic condition that precedes diabetes. "They all had insulin resistance," Plotnikoff noted, "but the question is, what predicted insulin resistance? Was body mass index [obesity] a good predictor of insulin resistance in this population?" Plotnikoff asked. Typically, the more obese a patient becomes, the more insulin resistant he or she gets. "No, in fact, BMI was a lousy predictor, it had no predictive value whatsoever," Plotnikoff recalled. "The highest predictive value for the degree of insulin resistance was the degree of vitamin D deficiency."

If not 400 IU, then how much?
Plotnikoff points out that the 400 IU daily dose should be considered a minimum. Trained in both internal medicine (adult medicine) and pediatrics, he recommends that children and adults get between 800 and 1,000 IU of vitamin D3 each day. That's in keeping with the recommendations made in the New England Journal of Medicine's 2007 review article. Keep in mind that vitamin D supplements come in two forms, vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). If you end up buying vitamin D2, you'll need to triple the dosage (2,400-3,000 IU daily) since vitamin D2 is about 30 percent as effective as vitamin D3.

Another thing Plotnikoff dislikes about the new recommendation is that it's a one-size-fits-all affair that doesn't take into account such things as skin color, obesity or latitude/sun exposure. "400 IU a day for children in Anchorage or Seattle or Minneapolis is supposed to be as effective as 400 IU a day for children in Tampa or Houston? There is no scientific logic behind the recommendations," Plotnikoff told me. It may be that measuring serum levels will be the only way to find out what the correct dose is for any individual.

One thing we can be sure of: It's October in Minnesota, and the sun is headed south for the winter. Go out and find yourself some "sunshine in a bottle" — vitamin D.