In addition to calcium, vitamin D is crucial for healthy bones, partly because it allows the body to absorb calcium from the intestines and kidneys, which would otherwise pass right out of the body as waste. Unlike with calcium, there are few dietary sources of vitamin D, aside from fatty fish (tuna, mackerel, and salmon), fish liver oil, and food fortified with the nutrient.
The best source of vitamin D is the sun: When exposed to the ultraviolet B rays of the sun, the skin starts producing vitamin D. However, many factors affect UV radiation exposure and vitamin D synthesis, including cloud cover, sunscreen, race, and age. For example, research suggests that a 70-year-old person only produces about 25 percent of vitamin D as a 20-year-old person when exposed to the same amount of sunlight. There are also issues with not getting enough sun during the winter, and developing skin cancer from too much sun. Given these variables, it's difficult to recommend a set amount of sun exposure that works for everyone, but experts generally suggest briefly exposing the face and limbs to the sun every day.
"Many people will need to take vitamin D supplements," Dr. Lewiecki says. The National Osteoporosis Foundation now recommends 400 to 800 international units (IU) daily of vitamin D for adults under age 50, and 800 to 1,000 IU daily for adults 50 and older.