Days are short, nights are long, and if you’re like a lot of Americans, you’re not getting enough vitamin D. A recent national survey found that many adults, particularly those over age 50, do not reach their recommended daily intake of vitamin D.
Many of us are vaguely aware that vitamin D helps preserve bone, but we are now learning that this essential nutrient is also important in protecting us from diseases, including cancer of the prostate, breast, and colon as well as non-Hodgkin’s lymphoma, rheumatoid arthritis, type I diabetes, macular degeneration, multiple sclerosis, fibromyalgia, gingivitis, and muscle aches and pains in elderly people.
Vitamin D is vital in various bodily processes. It maintains normal blood levels of calcium and phosphorus and, by promoting calcium absorption, it helps to form and maintain strong bones. It also works with other nutrients and hormones to promote bone mineralization, which in turn prevents osteoporosis. Vitamin D also seems to play an important role in protecting our immune systems, regulating cell growth and differentiation, and exerting an anti-inflammatory effect.
The Sunshine Vitamin
Vitamin D has a unique feature among essential nutrients: While it’s available from food sources, it’s also manufactured by the skin and requires ultraviolet light for this process. Women ages 19 to 50, as well as men over age 51, eat the least vitamin D-rich food. And, of course, in the winter everyone’s exposure to sunlight is limited. Now is a good time of year to check your vitamin D consumption, as your levels of this important nutrient may be lowest.
How Much Vitamin D Do You Need?
The National Academy of Science set the latest daily vitamin D intake on an age-related scale:
19 to 50 years – 200IU
51 to 70 years – 400IU
71 and above – 600IU
To put this in perspective, an 8-ounce glass of milk has about 100 IU of vitamin D. Many of us get most of our vitamin D from fortified foods. In the 1930s, when rickets were a major public health problem in the U.S., they began to fortify milk with vitamin D. Today, about 98 to 99 percent of the milk supply in the U.S. is fortified. A cup of vitamin D-fortified milk supplies one half of the recommended daily intake for adults between ages nineteen and fifty, one quarter of the RDI for adults between ages fifty-one and seventy, and about 15 percent of the RDI for those over seventy-one.
Here are some excellent fish sources of vitamin D:
Vitamin D per 100 grams (3.53-oz serving)*
Alaskan sockeye salmon 687 IU
Alaskan albacore tuna 544 IU
Alaskan silver salmon 439 IU
Alaskan king salmon 236 IU
Alaskan sardines (canned in olive oil) 222 IU
Alaskan sablefish 169 IU
Alaskan halibut 162 IU
Ready-to-eat vitamin D-fortified cereals are also an excellent source. Depending on the brand, they supply approximately 40 IU of vitamin D per serving. As you can see, other than fortified milk, fortified cereals, and fish, few foods provide a rich supply of this vital nutrient.
Who’s at Special Risk?
Some folks are at special risk for vitamin D deficiency:
• Older adults. As people age, their skin is less efficient at synthesizing vitamin D and their kidneys are less able to utilize the vitamin. It’s been estimated that as many as 30 to 40 percent of older people with hip fractures are deficient in vitamin D.
• People with limited sun exposure. In the winter, this includes many of us. For example, sunlight exposure from November through February in Boston won’t produce significant vitamin D synthesis in the skin. Complete cloud cover halves the energy of UV rays, and shade reduces it by 60 percent. Industrial pollution, which increases shade, also decreases sun exposure. As more of us use sunscreens that prevent skin exposure to UV rays and/or limit our outdoor time to prevent skin cancers, we can become vulnerable to vitamin D deficiencies.
• People with greater melanin in their skin. Melanin is the pigment that gives skin its color. Darker skin is the result of more pigment. Darker skin is less able to produce vitamin D from sunlight, so African Americans and other dark-skinned people should consume foods containing adequate amounts of vitamin D.
• People with mal-absorption disorders. People who suffer from Crohn’s disease, pancreatic enzyme deficiency, cystic fibrosis, sprue, or liver disease, or who have undergone the surgical removal of part or all of their stomach or intestines can also suffer from vitamin D deficiency.
What’s the Solution?
Try to get adequate vitamin D from your diet. Eat fortified low- or nonfat dairy products like yogurt as well as vitamin D-fortified cereals. Eat plenty of fish. Spend some unprotected time in the sun. While it’s important to use sunscreen most of the time, a sun exposure of 10 to 15 minutes without sunscreen allows sufficient time for vitamin D synthesis and should be followed by the application of a sunscreen with an SPF of at least 15. When you see the sun shining in the winter, take a brisk 15-minute walk.