WASHINGTON (AP) -- Many people who think they cannot digest dairy products might do all right if they eat a small amount at a time, but surprisingly little is known about just how many have true lactose intolerance, a government panel concluded Wednesday.

Lactose is a sugar in milk, and most babies are born producing enough of the enzyme lactase in the small intestine to digest it. But worldwide, most people's lactase levels progressively decrease throughout childhood. How low those levels drop determine whether a person can, for example, drink two cups of milk at a time without discomfort -- pain, diarrhea, gas or bloating -- by adulthood.

Mainly northern Europeans evolved to keep comfortably ingesting milk after childhood, while other populations where milk and cheese were less a part of the diet lost that ability.

Regardless of ancestry, how well people can digest lactose is highly variable, and there is no good count of how many experience symptoms that mean full-scale lactose intolerance, said a panel of specialists convened by the National Institutes of Health.

"There are huge gaps in knowledge," said panel chairman Dr. Frederick Suchy of Mount Sinai School of Medicine.

Some studies show that people who think they are lactose intolerant actually can digest the sugar, and the NIH panel worried that people who completely avoid dairy products miss an important source of calcium and vitamin D, nutrients important for bone health and possibly other conditions, too.

The panel's advice:

-Seek a doctor's diagnosis, as symptoms may be due to irritable bowel syndrome or another disorder rather than lactose intolerance.

-If diagnosed, studies suggest some people still can tolerate small amounts of dairy, up to a cup of milk, if taken together with other foods. Also, some products like low-fat cheeses contain less lactose.

-Get enough calcium or vitamin D from other sources, such as fortified orange juice, lactose-removed dairy brands, calcium-rich broccoli, soy products or supplements.

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