Vitamin D, calciferol, is a fat-soluble vitamin that is used in the absorption of calcium. It is found in food, but also can be made in your body after exposure to ultraviolet rays from the sun. Vitamin D exists in several forms, each with a different activity. Some forms are relatively inactive in the body, and have limited ability to function as a vitamin. The liver and kidney help convert vitamin D to its active hormone form.
A recent study estimates that tens of thousands of Americans die each year of cancers possibly caused by too little sun exposure and, in turn, too little vitamin D. (The average person gets about 90 percent of the vitamin from sunlight, the rest from dietary sources.) Moreover, shortfalls of sun and vitamin D may weaken the bones, possibly worsen arthritis, and perhaps increase the risk of heart disease, diabetes, and other disorders.
A deficiency of vitamin D can occur when dietary intake of vitamin D is inadequate, when there is limited exposure to sunlight, when the kidney cannot convert vitamin D to its active form, or when someone cannot adequately absorb vitamin D from the gastrointestinal tract.
Here are the most significant hazards of getting insufficient amounts of the vitamin:
- Weak bones, aching joints. Vitamin D helps the body absorb calcium from food, thereby making more of the mineral available to the bones. Outright deficiency of the vitamin prevents new bone tissue from hardening, a condition known as rickets in children, osteomalacia in adults. And vitamin-D deficiency clearly worsens osteoporosis, the brittle-bone disease.
Even moderately low levels of vitamin D, traditionally considered within the normal range, may also weaken the bones and increase the risk of fractures. Such moderate declines in the vitamin cause the body to churn out extra parathyroid hormone, which tends to pull calcium out of the skeleton. Studies show that consuming more vitamin D by itself can slow bone loss and possibly increase bone density in older women who have low-normal levels of the vitamin. Boosting the intake of both vitamin D and calcium can reduce the risk of fracture–and may help prevent tooth loss–in those women.
Finally, vitamin D may possibly slow the progression of osteoarthritis, the most common type of joint inflammation. In one test of that theory, Boston researchers studied 75 arthritic knees for up to 10 years. The disease was three times as likely to become worse in people who had average or lower vitamin-D levels as in those who had higher levels of the vitamin.
- Cancer. Researchers have long known that cancer-death rates in the U.S. are higher in the North than in the South, but they’ve failed to identify any dietary or lifestyle factor to explain why. Earlier this year, William Grant, Ph.D., an atmospheric scientist who investigates connections between the environment and health, compared cancer mortality in various parts of the country that receive different amounts of sunlight. Reduced sunlight correlated with increased risk for 13 types of cancer, notably of the breast, colon, ovary, and prostate, as well as non-Hodgkins lymphoma. Grant estimates that, if those correlations represent a true causal connection, nearly 24,000 Americans per year are dying from cancers caused by insufficient sunlight. Grant’s findings support the results of several previous studies that also found reduced cancer rates in sunny regions, both here and abroad.
To explain that connection, Grant and other scientists point to research linking low vitamin-D levels with increased risk of both colon cancer and prostate cancer. Moreover, vitamin D inhibits the development and growth of various cancers in animals. In fact, some cancer specialists are now testing vitamin D as a treatment for prostate and colon cancer.
- Heart disease. People with thin bones often have extensive calcium deposits in their arteries. Researchers theorize that the parathyroid-hormone buildup sparked by vitamin-D insufficiency may not only leach calcium out of the bones but also dump it in the arteries, contributing to the development of vessel-clogging plaque deposits. Moreover, parathyroid-hormone elevations may possibly raise blood pressure, further increasing coronary risk. A few studies have indeed linked low vitamin-D levels with increased likelihood of both calcified arteries and coronary disease. For example, an 11-year observational study of some 10,000 women in California found that those who took vitamin-D supplements were one-third less likely than others to die of coronary disease.
- Diabetes. The pancreas needs vitamin D to produce the hormone insulin, which controls blood-sugar levels. Lack of the vitamin appears to increase the risk of type 1 (insulin-dependent) diabetes, the less common but more serious type. People in the higher, darker latitudes face an increased risk of type 1, and they tend to develop it at a younger age. Three large studies have found lower rates of the disease in children who received vitamin-D supplements as infants or whose mothers took the supplements during pregnancy.
- Schizophrenia and multiple sclerosis. Vitamin D stimulates production of certain nerve or brain chemicals that may help prevent schizophrenia and multiple sclerosis (MS). Both conditions arise more often in populations that get little sunlight. And vitamin D has reversed MS in mice.
WHO’S AT RISK
While researchers have not yet determined exactly how much vitamin D the body needs, many experts lean toward a blood level of at least 20 nanograms per milliliter; that’s the amount required to keep the parathyroid-hormone blood level down.
Young and middle-aged white people in sunny regions, including the American South, almost always get that much vitamin D from sunshine alone, just by going about their daily affairs. But almost everyone else is at risk for vitamin-D insufficiency, especially in the winter. Numerous studies have found substantial wintertime drops in vitamin-D levels. Indeed, average bone density falls and fracture risk rises in winter, the latter caused only partly by slips on ice and snow. In addition to winter and living in dark, high-latitude regions, the following factors also increase the chance of vitamin-D insufficiency:
- Older age. Studies have repeatedly shown that most people over age 65 or so have insufficient vitamin-D levels, for several reasons. They tend to wear protective clothing, apply strong sunscreens, and stay indoors more often than younger people. Their skin’s ability to synthesize the vitamin is reduced. And they’re more likely to take certain drugs, notably laxatives and the cholesterol-lowering drug cholestyramine (Questran), that interfere with the absorption of the vitamin.
- Darker skin color. The darker your skin, the more sunlight you need to generate vitamin D. Researchers from the Centers for Disease Control and Prevention recently concluded that on average some 40 percent of black women were low in the vitamin. In theory, says Michael F. Holick, Ph.D., M.D., a vitamin-D expert at Boston University Medical Center, the rates in black men should be roughly the same.
- Excess weight. Vitamin D is fat soluble. So in overweight individuals, substantial amounts of the vitamin manufactured by the skin get trapped in the excess fat cells. As a result, obese individuals on average have about two-thirds less vitamin D in their blood than other people and are thus typically deficient in the vitamin.
THE VITAMIN-D PRESCRIPTION
Judicious sun exposure can provide most people with all the vitamin D they need. The best time of day is midmorning or midafternoon–earlier or later than that during summer in the South–when the sun is typically neither too strong to damage the skin easily nor too weak to stimulate vitamin-D production. To obtain enough vitamin D for the entire year, seek sun exposure during three seasons: spring, summer, and fall in the North (because there the winter sun is too weak to stimulate vitamin production), and any three seasons in the South.
Women who are overweight or older than age 60 or so should expose their hands and lower arms as well as their face or lower legs without sunscreen about three times a week for roughly one-quarter the time it takes their skin to start turning red. Younger women or thinner ones need to expose only their hands and lower arms. During the optimal hours in June, it typically takes about 40 minutes for the skin to start to redden in white people living anywhere in a rough line linking Boston, Chicago, and southern Oregon–so they’d need about 10 minutes of daily exposure. Darker skin increases that time by up to 50 percent; it also increases with higher latitudes and in months before or after June. Conversely, the time decreases with lighter skin and lower latitudes.
Women who can’t or won’t spend the necessary time outdoors need to consider dietary sources of the vitamin. Our medical consultants say that middle-aged and younger women who get some but not enough sunshine need about 400 International Units (IU) of vitamin D per day; older women need 600 to 800 IU. Women of any age who rarely or never get out in the sun probably need about 1,000 IU.
However, obtaining even the lower amount of vitamin D from food can be difficult. Few foods other than fatty fish are good natural sources of the vitamin, and the most common fortified food is milk, which many people avoid. If you don’t consume enough of those foods to reach the recommended dietary intake for your age and sun-exposure level, consider taking a supplement.
However, too much vitamin D (more than about 2,000 IU per day) can lead to elevated calcium levels and, in turn, kidney damage and calcium deposits throughout the body. But it’s impossible to get a toxic dose of vitamin D from sun exposure; that’s a self-regulating mechanism.
Insufficient vitamin-D levels appear to be far more hazardous and more common than previously believed. Young and middle-aged white southerners generally don’t need to worry. But other people have to ensure an adequate supply by taking these steps:
- Regularly expose some skin to the sun for about one-quarter the time it takes it to start reddening; the older or heavier you are, the more time and exposed skin you need. (But be sure to cover up or apply sunscreen well before your skin starts to redden.)
- Consume plenty of skim or low-fat milk and fatty fish.
- Consider a vitamin-D supplement if you don’t get enough of the vitamin from sunshine and your diet-particularly if you are an older, postmenopausal woman or have other risk factors for osteoporosis. A multivitamin containing 400 IU of D is sufficient for most people who get at least some but not enough sun exposure, particularly if they consume milk or fish regularly or are not older than middle age.