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Mothers everywhere probably will not be surprised to hear that they were right. Drinking your milk is, of course, good for you. Not only is milk chock-full of calcium, most milk has been fortified with vitamin D -- and both nutrients are necessary for good bone health, among other things. And in recent years, a growing body of research has caused experts to conclude that many adults, particularly those over 50, are not getting enough calcium or vitamin D, says Dr. Michelle Germain, a part-time faculty member in the Department of Gynecology and Division of Urogynecology at Greater Baltimore Medical Center.

What is vitamin D?

Vitamin D is a fat-soluble vitamin. It is absorbed from food through the GI [gastrointestinal] tract and also is made by the body. The body needs it because it helps with absorption of calcium and phosphorus, both of which the body needs for bone health.

You said that, in addition to absorbing vitamin D from food, the body also manufactures it. How does the body make vitamin D?

The skin manufactures the vitamin D with the help of ultraviolet rays from the sun. So you need sun exposure to make it. Once we have vitamin D in our bodies, the liver and the kidneys convert the vitamin D into the active form that is used for bone health.

What foods contain vitamin D?

Cod-liver oil is the best way to get vitamin D, but we don't see many people taking that these days. So other really great sources of vitamin D are fish, such as salmon, mackerel and tuna, and vitamin D-fortified foods, such as dairy products and cereals.

What happens if we don't get enough vitamin D?

You can become vitamin D-deficient. Children can develop "rickets," in which the bone doesn't properly mineralize, and the children develop very soft bones and deformities of their skeleton. In adults, too little vitamin D can lead to osteoporosis. [Too much vitamin D, on the other hand, can make the intestines absorb too much calcium.]

How pervasive is rickets?

In developed countries like the United States, it is pretty uncommon because we fortify so many foods with vitamin D. But there has been a resurgence of rickets in African-American infants and children in poorer states, especially in the South such as Tennessee and Mississippi. The resurgence is being caused by poverty and poor nutrition. It also has to do with darker pigmentation of the skin, which converts sunlight less effectively into vitamin D.

Rickets also is more prevalent in immigrants from Africa, the Middle East and Asia because of their diets before they arrive in the United States and because they may not have enough money to afford good nutrition once here. Again, if they have darker skin, they are not converting UV rays into vitamin D as effectively as people with lighter skins.

Are there other populations for which vitamin D is particularly important?

Older women. If older women are vitamin D-deficient then they are at increased risk for osteoporosis. This is of special concern for menopausal women because once their estrogen levels drop, they begin to lose bone. The vitamin D is necessary to absorb calcium and utilize it in bone formation. And people who have any kind of gastrointestinal illness like Crohn's disease or celiac disease or any kind of liver disease also are at increased risk for vitamin D deficiency.

Are there any symptoms of vitamin D deficiency that might occur before bone loss?

Unfortunately, there are really no symptoms of it until you are diagnosed with osteoporosis.

What do you tell your patients about vitamin D?

Children and young adults need to consume a diet that is rich in vitamin D-fortified foods. At least 15 minutes of exposure to sunlight twice a week should be adequate to make sufficient amounts of vitamin D. But I also tell them that if you put on sunscreen with an SPF greater than 8, it can interfere with making vitamin D, so diet is very important. Particularly in winter, they should concentrate on eating a diet with plenty of vitamin D-rich foods.

What do you tell adults?

If a woman has osteoporosis, she should ask the doctor if she is potentially vitamin D-deficient. Very often, after a woman has a hip fracture, we check her vitamin D level, and it turns out to be ridiculously low. So, if you have osteoporosis or a fracture, ask about your vitamin D level.

How much vitamin D do we need?

Last year, the National Osteoporosis Foundation released new recommendations about vitamin D. The National Institutes of Health also are due to release this spring new guidelines. According to the National Osteoporosis Foundation, adults under age 50 need 400 to 800 IU [International Units] of vitamin D3 daily. [Also called cholecalciferol, vitamin D3 is the kind of vitamin D that best enhances healthy bones.] And menopausal women and adults over the age of 50 need 800 to 1,000 IU of vitamin D3 daily.

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