Winter is characterized by less sunlight, which brings shorter days and the need to keep warm. That means that more of the body is hidden from the sun. Less sunlight may limit the body's ability to manufacture vitamin D, a nutrient vital for strong bones. Swedish researchers investigated the seasonal influence on the 'sunshine vitamin' and its effect on bone health. Their findings are presented in a recent issue of the Journal of the American Geriatrics Society.

Approximately 60 healthy, elderly men and women participated in the year-long study. Dietary intake of calcium and vitamin D was estimated based on their reported eating habits. Throughout the spring and fall, the participants spent at least 3 hours each week outdoors. While outside, they left their face, neck, and forearms uncovered and did not use sunscreen lotions. Four times throughout the year, blood samples were collected and analyzed for several markers of bone health, including calcium and vitamin D. Several times during the study, the researchers also measured bone density, an indicator of bone strength.

Sunlight helps build better bones

The volunteers who spent at least 3 hours each week outside were compared with those who spent less time outside. Volunteers who were exposed to more sunlight had consistently higher blood levels of vitamin D and calcium and had thicker bones than those who remained indoors.

Don't overlook diet

Although this study adds more evidence to the importance of sunlight for maintaining strong bones, it's important to remember that a healthful diet is needed, too. Unfortunately, the volunteers consumed only about half of the recommended amounts of vitamin D and calcium. This is a problem because, as one gets older, the need for these nutrients increases in order to ensure optimal bone strength and to prevent osteoporosis, a disease characterized by thin, weak bones that are susceptible to fractures. Adequate intake of these two nutrients would have maximized bone strength in all the volunteers, even those who did not spend much time outside.

Vitamin D requirements can be met with as little as 10 minutes of sunlight each day. However, this may be difficult for older people who live in an institutionalized setting or who are homebound. The inability to go outdoors, combined with a diet that does not provide enough vitamin D and calcium, may necessitate vitamin D supplements.

Combine sunlight and healthful foods for strong bones

For most people, though, diet and some sun exposure should meet vitamin D requirements. In the U.S., milk and many cereals are fortified with vitamin D. Calcium is abundant in dairy foods and calcium-fortified orange juice. Leafy greens and legumes contribute calcium as well. But the best overall source of calcium and vitamin D is milk, which provides about a quarter of the daily calcium and vitamin D needs in one glass.