The next time you do the sun salutation, think of it as a prayer of gratitude for the benevolent rays the sun showers down on us, and for the life-giving reactions they engender when they strike our skin. This biological miracle—a mammalian version of photosynthesis—produces vitamin D, an essential nutrient that researchers now link to the prevention of an ever-increasing array of diseases, from colds and flu to multiple types of cancer.
Vitamin D is an essential nutrient that researchers link to the prevention of an array of diseases, from colds and flu to multiple types of cancer.
Vitamin D isn’t really a vitamin, at least not like other vitamins, most of which are either antioxidants or co-factors or coenzymes. Instead, it is a prohormone, created when ultraviolet B rays activate a form of cholesterol in the epidermis. The liver and the kidneys turn this precursor into a potent steroid hormone that turns proteins on (or off) at some 2,700 binding sites on the genome, influencing more than 200 genes. As the Vitamin D Council points out, “those binding sites are near genes involved in virtually every known major disease of humans.”
There are two forms of Vitamin D: D2 (ergocalciferol) and D3 (cholecalciferol). D3, the form we get from the sun, is substantially more potent than D2, which we get from plants. Under normal circumstances—a sunny summer day and a healthy individual—the body is a remarkable vitamin D3 machine. Twenty to thirty minutes of full-body exposure will produce 20,000 IU of vitamin D3 in the bloodstream of a light-skinned person within 48 hours, and half that amount in someone with dark skin. When you consider that healthy people use about 3,000–5,000 IU of vitamin D a day, it’s easy to imagine that we get all the vitamin D we need from the sun. But that’s clearly not the case, even in summer.
The average person makes only about 1,000 IU a day. Most of us spend little time outdoors: we drive to work or take public transportation; we work inside under artificial light; we stay indoors during our leisure time to avoid the sun and heat; and if we do venture out, we smear ourselves with sunblock to ward off skin cancer. And those of us who live north of the 40th parallel (a line that passes through Philadelphia; Columbus, Ohio; Denver; and Redding, California) cannot produce Vitamin D from sun exposure from November to March. So it’s not surprising a 2009 study found that more than three out of four people nationwide have insufficient levels of vitamin D in their blood (less than 30 ng/ml).
This widespread deficiency stems in part from the difficulty we have in getting enough vitamin D from food. The version found in plants—the so-called vegetarian vitamin D (vitamin D2)—is some 70 percent less effective than vitamin D3. Eggs and meat provide small amounts of D3, but even fortified milk provides a mere 100 IU for each eight-ounce glass, barely enough to prevent rickets even if you drink multiple glasses. The exception is oily fish, such as salmon, mackerel, sardines, and tuna, but unless you’re prepared to eat like an Inuit, even fish won’t provide enough vitamin D to maintain a healthy level—never mind overcoming a deficiency. (See The Right Dose of D below.)
Research during the past decade provides compelling reasons to make sure you get enough vitamin D. For instance, a number of studies have linked low levels of vitamin D to type 2 diabetes. Two studies published last summer show that taking vitamin D can improve insulin sensitivity, a critical factor in the development of type 2. Researchers have also noted that the incidence of type 1 diabetes, an autoimmune disease, is much lower in countries closer to the equator (and thus sunnier) than in those closer to the poles.
Scientists have also found a strong link between vitamin D and upper respiratory diseases. In a recent Japanese study, taking 1,200 IU a day lowered the rate of flu by 58 percent; and a 2009 study published in Archives of Internal Medicine reports that people with the lowest levels of vitamin D were more than a third as likely to suffer from colds and flu than those with the highest levels.
More surprising is the link between vitamin D deficiency and cancer. At the cellular level, vitamin D favors differentiation over growth, which inhibits cell proliferation. It also regulates two tumor-suppressing proteins and limits angiogenesis, the formation of a network of blood vessels that can nourish a tumor. Studies show that cancer rates—like the incidence of type 1 diabetes—track with the sun: incidence rates of ovarian and colorectal cancer in the United States are greater in northern latitudes than they are in those near or in the subtropics. Worldwide, the same pattern holds with kidney cancer. And finally, two studies published in 2007 linked increased blood levels of vitamin D with up to a 50 percent decrease in the incidence of both breast and colon cancer.
In an age of health fads, vitamin D is fast being hailed as a wonder drug, so some level of skepticism is warranted. That said, strong evidence abounds for its ability to function as an immune system modulator: it can not only crank the body up to fight cold and flu viruses or the tuberculosis bacterium, but it can also calm it down and reduce the rate of autoimmune diseases, such as allergies, asthma, Crohn’s, irritable bowel syndrome, and Parkinson’s. Still awaiting further confirmation: vitamin D’s ability to ameliorate heart disease, high blood pressure, and age-related cognitive loss. All this for free with daily sun exposure (where it’s available) or less than 10¢ a day in supplements.
About a year ago, the Institute of Medicine (IOM) raised the RDA for vitamin D from 200 IU a day to 600, ignoring the informed outcry for much higher levels. In response, The Endocrine Society called for a daily intake between 1,000 and 2,000 IU, while the Linus Pauling Institute settled on 2,000 IU a day. The Vitamin D Council (VDC), a nonprofit educational group, recommends at least 5,000 IU a day for healthy adults and adolescents.
Your needs will vary depending on your age, skin pigmentation, sun exposure, location, and time of year—and your current blood level of 25-hydroxyvitamin–D, which, according to The Endocrine Society, should range from 30 to 60 ng/ml. (It takes roughly 100 IU extra vitamin D a day to raise blood levels by 1 ng/ml.)
How much is too much? The IOM has set the upper limit at 4,000 IU a day, but the VDC finds 10,000 IU a day perfectly safe. No one has ever reported an overdose from sun exposure. The only serious danger from too much D, hypercalcemia, seems to occur from doses exceeding 50,000 IU a day.
When choosing a supplement, be sure the label says vitamin D3, the form of vitamin produced by the skin. All D3 supplements come from animals. Plants produce D2, a far less bioavailable alternative. Some D3 is made from fish liver oil, but most high-quality D3 comes from the lanolin found in sheep’s wool. Another alternative: 10 to 15 minutes of midday sun exposure a day (or 25 minutes three times a week) in the lower latitudes, or in the warmer months in latitudes north of the 40th parallel.