To meet the new recommendation of 400 units daily, millions of children will need to take daily vitamin D supplements, the American Academy of Pediatrics said. That includes breast-fed infants _ even those who get some formula, too, and many teens who drink little or no milk.

Baby formula contains vitamin D, so infants on formula only generally don't need supplements. However, the academy recommends breast-feeding for at least the first year of life and breast milk is sometimes deficient.

Most commercially available milk is fortified with vitamin D, but most children and teens don't drink enough of it _ four cups daily would be needed _ to meet the new requirement, said Dr. Frank Greer, the report's co-author.

The new advice is based on mounting research about potential benefits from vitamin D besides keeping bones strong, including suggestions that it might reduce risks for cancer, diabetes and heart disease. But the evidence isn't conclusive and there's no consensus on how much of the vitamin would be needed for disease prevention.

The new advice replaces a 2003 academy recommendation for 200 units daily.

That's the amount the government recommends for children and adults up to age 50; 400 units is recommended for adults aged 51 to 70 and 600 units for those aged 71 and up. Vitamin D is sold in drops for young children, capsules and tablets.

The Institute of Medicine, a government advisory group that sets dietary standards, is discussing with federal agencies whether those recommendations should be changed based on emerging research, said spokeswoman Christine Stencel.

The recommendations were prepared for release Monday at an academy conference in Boston. They are to be published in the November issue of the academy's journal, Pediatrics.

Besides milk and some other fortified foods like cereal, vitamin D is found in oily fish including tuna, mackerel and sardines.

But it's hard to get enough through diet; the best source is sunlight because the body makes vitamin D when sunshine hits the skin.

While it is believed that 10 to 15 minutes in the sun without sunscreen a few times weekly is sufficient for many, people with dark skin and those in northern, less sunny climates need more. Because of sunlight's link with skin cancer, "vitamin D supplements during infancy, childhood and adolescence are necessary," the academy's report says.

Recent studies have shown that many children don't get enough vitamin D, and cases of rickets, a bone disorder often associated with malnourishment in the 1800s, continue to occur.

Greer, a University of Wisconsin pediatrician, acknowledged that most studies suggesting vitamin D may play a much broader role in disease prevention have been observational, not the most rigorous kind of medical evidence.

Nonetheless, many doctors consider the research compelling and many have begun to offer patients routine vitamin D testing.

Adrian Gombart, a vitamin D researcher at Oregon State University, said the new recommendations are safe and conservative but that 400 units "is probably not enough."

Gombart's lab work in human tissue has shown that vitamin D helps increase levels of a protein that kills bacteria. He said many experts believe that between 800 and 1,000 units daily would be more effective at helping fight disease.

Several members of an academy committee that helped write the guidelines have current or former ties to makers of infant formula or vitamin supplements.

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