Our bodies need vitamin D to absorb calcium, the major building block of bone. Vitamin D boosts the body's ability to absorb calcium by up to 80%. It becomes especially important as we get older, when calcium is less efficiently absorbed. Together, calcium and vitamin D can prevent osteoporosis, a condition where bones become thin and brittle and break easily.
Vitamin D is often called "the sunshine vitamin," since our bodies can actually produce it themselves when exposed to sunlight. In the summer, having our arms, face, and hands out in the sun for just 10-15 minutes a day, 3 times per week, can make enough vitamin D to meet the body's requirements.
Unfortunately, the sun may not be the safest or most reliable way to get enough of this vitamin. Using sunscreens to prevent skin cancer blocks the rays needed to produce vitamin D. Dark-skinned people absorb less sunlight than those with light skins, putting them at risk of vitamin D deficiency. Also, many people, especially the elderly, are at risk, as they spend more time indoors. Finally, the long, cold, and dark winters in northern climes mean that the skin's vitamin D production shuts down from early October until late March every year.
So how can we get enough vitamin D - and just how much do we need? Babies, children, and adults under 50 (who do not have osteoporosis) need between 400 IU (international units) and 1000 IU per day. Adults over 50 should get at least 800 IU daily. Some doctors recommend up to 2000 IU per day for certain individuals.
In the US, a glass of milk (8 oz) is enriched with 100 IU of vitamin D, making it a good source of this nutrient. Small amounts of vitamin D are in margarine, eggs, chicken liver, salmon, sardines, herring, mackerel, swordfish, and fish liver oil. It may be hard to get enough vitamin D from food alone, though, so you may opt to take a supplement. Remember that most multivitamins contain 400 IU of vitamin D, which is enough for most people. Don't be tempted to take higher doses - too much vitamin D can lead to loss of calcium from bone, too much calcium in the blood, and kidney problems.
It's especially important that babies and children get enough vitamin D. Kids who are short on this vitamin can get rickets, a disease affecting bone development. Infant formulas are already fortified with vitamin D, so bottle-fed babies don't need supplements. Breast-fed babies, on the other hand, may need a vitamin D supplement since breast milk is usually low in vitamin D. Breast milk is still the perfect food for babies but, if you're breast-feeding, talk to your doctor about whether your baby needs extra vitamin D.