Q. Why does our breath smell like garlic days after we've eaten it, no matter how many times we brush our teeth?

Concord

A. Eau de garlic emanates not from our mouths but from our lungs, long after we've eaten the pungent herb and brushed our teeth for the 15th time.

Dr. Michael Levitt, chief of research at the Minneapolis Veterans' Administration Hospital, says he discovered garlic's sneaky way around our toothbrushes and mouthwash when he developed the standard test for lactose intolerance. That's the inability of some people's digestive systems to break down the sugar, called lactose, that is common in dairy products.

Most people have an enzyme, lactase, that breaks down that particular sugar. In folks who don't, the lactose is broken down instead by bacteria that live in our digestive system. The byproduct of that is hydrogen gas. Some of the hydrogen is emitted as flatus. Some of it makes its way out on our breath.

Levitt's test measures the baseline hydrogen content of a patient's breath before ingesting dairy foods. If the hydrogen in the breath goes up after a glass of milk, it confirms lactose intolerance.

``If intestinal gas can get into your blood and breath, why doesn't your breath doesn't smell the way flatulence does?'' Dr. Levitt asks rhetorically. He explains that of the various gases in flatus, the tiny percentage that are offensive (less than 1 percent) are all sulfur-based.

``They are all poisons if they get into the blood,'' Levitt says. ``Hydrogen sulfide is one. It's as poisonous as cyanide.''

If those gases got into the bloodstream, you can imagine the results. So the lining of the intestines has evolved the ability to metabolize those dangerous sulfurs out of the gas that diffuses into the blood. Only the safe, non-smelly gasses like hydrogen, carbon dioxide, and nitrogen can get through to our blood, lungs, and ultimately our breath.

But then there's garlic. It contains a unique sulfur compound, allyl methyl sulfide. The body doesn't metabolize it as it diffuses through the intestinal lining.

``It's apparently non-toxic,'' Levitt says. ``This particular sulfur compound can get into the blood stream and up into the lungs. That's why we still have garlic breath hours and days after we've eaten it.''