If you happen to read this article from your home in Philadelphia or points north of there, your body doesn't make any vitamin D in January or February. At 40 degrees latitude--Philadelphia's location--the sun's rays just aren't strong enough during those months for the skin to produce the "sunshine vitamin."

But you still may not be getting enough vitamin D if you live in Baltimore, Washington or points south. Vitamin D deficiency is thought to be widespread throughout the country, particularly among people over 50. And without enough D, you can't absorb enough calcium, even if you consume a lot of it. The upshot: Proper bone building can't take place, leaving people who are deficient in vitamin D prone to more fractures.

Consider that in a study at Massachusetts General Hospital in Boston, one in three people who suffered hip fractures (average age 65) had low levels of vitamin D. It's even more serious than it sounds. Of the 300,000 Americans who suffer hip fractures each year, 60,000, or one in five, dies as an indirect result of their injury. Being bedridden during recovery makes a person less capable of clearing the lungs and thus more prone to pneumonia, which can be life-threatening. Bedridden patients can also develop blood clots in the legs that end up as fatal pulmonary blockages.

The problem isn't only one of sun exposure. Diet counts, too. And people aren't eating the right foods to get enough vitamin D. Consider that the largest single contributor of vitamin D to the diet--milk--is not drunk in large quantities by most people. On a typical day, many adults don't consume a whole cup's worth. Even if they did, a cup of milk contains only 100 International Units of the nutrient. That comes to just half the recommended allowance of 200 units for a young adult; 25 percent of the 400 units recommended for people 51 to 70; and a scant 16 percent of the 600 units advised for those 71 and older.

Other sources of D are not big players in the American diet either: fatty fish such as sardines, salmon, herring and mackerel; and organ meats and egg yolks (which many people limit because of their saturated fat and cholesterol content). And while some breakfast cereals are fortified with D, usually these products have only 40 to 80 units in a serving. All of this translates into a typical intake for someone 50 or older of only 100 to 125 units daily, according to surveys.

Even if you do take in the right amount of D from foods, the dietary intakes won't be enough unless you also get enough D from the sun. Michael Holick, director of the Vitamin D, Skin and Bone Research Laboratory at Boston Medical Center, explains that the recommended amounts for dietary intake were drawn up on the assumption that people will get enough of the sun's rays. "Sunlight provides 90 percent of our vitamin D," he says.

But simply living in or near the South isn't enough. You've got to get some sun exposure. And even many people who spend a lot of time outdoors now get limited access to sunshine. That's because they're (rightly) heeding the call to wear sunscreen to block the ultraviolet rays that can cause skin cancer. But blocking the cancer-causing rays means blocking the D-making rays. The more scrupulous you are about covering up, the less opportunity your skin has to produce the nutrient. Any sunscreen with an SPF of 8 or higher will block virtually all vitamin D production.

The dilemma worsens with age because the skin becomes much less efficient at vitamin D synthesis over time. The skin of an 80-year-old has only half the capacity to produce D as the skin of a 20-year-old. Age-related changes in the kidneys exacerbate the problem. As the decades pass, there's a loss of kidney enzymes that convert vitamin D to its active, or usable, state. (These differences between younger and older people are the reason that you're supposed to get more D from the diet as you grow older.)

Despite the seriousness of the matter, most people younger than 50 do not have too much reason for concern. Although they may be careful about using sunscreen, even the little bit of skin exposed to the sun on their hands and face for short periods of time will generally be enough to produce adequate amounts of the nutrient. Their dietary needs are also easier to meet. Furthermore, fatty tissues in the body can store D for up to six months. Thus, even if you're a young or middle-aged adult living in Philadelphia, you'll probably have enough to last through the winter, vitamin D in your diet or not.

If you're older than 50, make a conscious effort to expose your hands, arms and face to the sun for just 10 to 15 minutes twice a week. That will go a long way toward meeting your vitamin D needs, Holick says.

Think about incorporating more milk into your diet, too. If you don't like milk straight up, you can always mix it into soups, casseroles and puddings. (Note: Milk is the only dairy food that is fortified with vitamin D.)

If you really can't abide milk and you're homebound or indoors much of the time, consider taking a multivitamin supplement that contains 400 units of D.

But don't take more than 1,000 units daily. Large amounts can be toxic, harming the kidneys and causing bone loss rather than maintaining bone density. Ten percent of people evaluated for osteoporosis at the Cedars-Sinai Bone Center in Los Angeles were found to be taking upwards of 1,200 units daily--and losing three times as much calcium in their urine as those taking smaller amounts.

Finally, don't expect to produce vitamin D sitting by a closed window on a sunny day. The rays that are needed to produce the vitamin in the skin travel poorly, if at all, through glass.