A more D-manding diet

(This article was first printed in the June 2007 issue of the Harvard Health Letter. For more information or to order, please go to http://www.health.harvard.edu/health.)

Vitamin D has long been recognized as vital to bone health because the body needs the vitamin to absorb calcium. But research has suggested that it may be good for a lot more than just bones. Ample intake of vitamin D may help fend off a wide range of conditions, including colon cancer, diabetes, and physical weakness in old age.

Meanwhile, another batch of studies has found that many people, especially as they grow older, have low levels of vitamin D in their blood. Our skin has an amazing ability to produce vitamin D when it’s exposed to sunlight, but with age, the skin becomes less productive. The problem is made worse by older people spending more time indoors.

Other factors that contribute to low vitamin D levels include living in the upper latitudes, where winters are long, and having darker skin. Rickets is the classic children’s disease caused by vitamin D deficiency. It has re-emerged as a problem in some African American communities.

Because of the evidence for shortfalls and the possibility of added benefits, some experts think the recommendations for vitamin D are set way too low. The vitamin D enthusiasts say adults should be getting at least 1,000 IU (International Units) a day.

Up the D limit

The committees of experts convened by the Institute of Medicine to set daily nutrition requirements also calculate an upper limit (the technical term is Tolerable Upper Intake Level). It’s the too-much-of-a-good-thing level at which a normally healthful nutrient becomes possibly toxic.

The upper limit for vitamin D is 2,000 IU per day. Some of the same experts who think we should have a lot more vitamin D in our diets are saying that upper limit needs to be increased because, at its current level, it may be scaring people off so they don’t get the vitamin D they need.

An article published in the January 2007 American Journal of Clinical Nutrition reviewed dozens of vitamin D toxicity studies, including some that involved volunteers taking a whopping 100,000 IU a day. The authors concluded that the upper limit for daily intake of vitamin D could safely be set at 10,000 IU. This could be a biased view: Two of the four authors work for the Council for Responsible Nutrition, a trade group for the vitamin industry.

Still, a group of 15 nutrition experts cited the study in an editorial in the March 2007 issue of the same journal that called for an overhaul of vitamin D guidelines, although they stopped short of recommending definite amounts.

The experts — who include Dr. Walter C. Willett, chair of the Harvard School of Public Health’s nutrition department — noted that 400 IU doesn’t increase the amount of vitamin D circulating in the blood very much. Depending on how much a person started out with, they said a daily intake of about 2,000 IU — the upper limit — is necessary before blood levels get high enough for vitamin D to have its full disease-fighting effects.

More D

That’s a lot of vitamin D. Expanding and increasing fortification would be one way to increase intake. In the United States, almost all milk (but generally not other dairy foods) is fortified with vitamin D. The actual amount may vary, but the standard is 100 IU per cup.

Putting more D into multivitamins would be another way to go and helpful for older people, who tend to drink less milk and are among the biggest consumers of vitamin pills.

Many of the major brands of multivitamins, like Centrum Silver, contain 400 IU of vitamin D. Some of the Dr. Andrew Weil products contain 1,000 IU, but they are expensive.

Calcium supplements often include some vitamin D. And you can buy separate vitamin D pills that contain 1,000 IU or more. A few brands use vitamin D2 (ergocalciferol), which may be less potent than vitamin D3 (cholecalciferol), the form that is used most often to fortify milk and other foods, such as breakfast cereals.

Getting more sun exposure is another way to increase vitamin D levels, although doing so means changing the advice to avoid sun and wear sunscreen to protect against skin cancer and other forms of skin damage. Some analysis shows that any increase in skin cancer from adding a small amount of unprotected sun exposure would be offset by declines in other forms of cancer.

Some vitamin D researchers want to see “safe sun” recommendations that would actively encourage people to get 15 minutes or so of sun a few times a week — without sunscreen, which blocks the UV radiation that triggers vitamin D production in the skin.

The pressure to revise vitamin D recommendations is building. The American Cancer Society is preparing its first comprehensive set of guidelines for skin cancer prevention. So stay tuned.

There’s been a lot of disappointing news about vitamins, but vitamin D stands out as a bright spot.

(This article was first printed in the June 2007 issue of the Harvard Health Letter. For more information or to order, please go to http://www.health.harvard.edu/health.)

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