A lack of vitamin D — thought to be a problem of a bygone era — is showing up in growing numbers of women, children and the elderly, increasing the risk of bone disease and possibly other health problems.
Exposing only the face, hands and forearms to sunlight for 10 to 30 minutes, just two or three days a week, can usually produce all the vitamin D we need. Longer exposure doesn’t produce more of this vitamin. Yet today, many people’s lifestyles and locations do not allow them to produce enough, making dietary sources vital.
Many Americans have low blood levels of vitamin D, which might be caused by one of many reasons. For example, the dark skin of African-Americans tends to form active vitamin D in response to sunlight more slowly than does skin with less pigment. A lack of vitamin D is also common among the elderly, especially those who are institutionalized or homebound. Other reasons include low consumption of milk or breakfast cereal, no intake of vitamin supplements, seasonal changes and urban location.
Vitamin D helps our bodies absorb calcium from food and also plays a vital role in the constant remodeling of our bones. Calcium can’t be used properly for bone formation when children lack vitamin D, leading to skeletal deformities in the condition called rickets. Inadequate vitamin D isn’t as obvious in adults, but bone weakening can be significant. In one study of women with osteoporosis, those who consumed the most vitamin D from food and supplements developed 37 percent fewer hip fractures than did women who consumed the least. Other studies show that raising inadequate vitamin D intake can increase bone density, but results are not completely consistent.
Some studies link adequate calcium and vitamin D intake with a lower risk of colon cancer. At the American Institute for Cancer Research’s most recent conference on nutrition and cancer research, scientists discussed several mechanisms by which the active form of vitamin D seems to block the formation and growth of colon cancer cells.
Experts recommend different amounts of vitamin D for people of different ages, since the formation of vitamin D in the skin and activation in the kidney decreases after age 50. For adults 19 to 50, the official recommendation is for 5 mcg (micrograms; equal to 200 IU) of vitamin D daily. From age 51 to 69, that increases to 10 mcg (400 IU), and from age 70 onward, increases again to 15 mcg (600 IU).
Two eight-ounce glasses of milk supply the 200 IU of vitamin D recommended for young adults; older adults need four to six glasses if milk is their only source of vitamin D. Yogurt and cheese provide calcium, but they are not fortified with vitamin D. The primary food sources of vitamin D are milk (including soymilk, if it’s fortified), fortified breakfast cereals, naturally high-fat fish (such as salmon and mackerel) and egg yolks.
Excessive amounts of vitamin D can raise blood levels of calcium high enough to cause changes in heart rhythm and kidney damage. But such levels could generally only be reached by overdoing on supplements or cod liver oil. While we aim for 200 to 600 IU, some researchers suggest that elderly people with low blood levels, people who are not exposed to sunlight, people with intestinal diseases that keep them from absorbing fat properly, and people on chronic steroid medications may need even higher amounts. But even somewhat higher goals can be achieved without exceeding the 2000 IU (50 mcg) maximum set as the upper limit of safety.