Can you explain why eating asparagus causes one's urine to have a strong, unique odor?

H.B.

Brookline

A. About half the people reading this will think your question is crazy. The other half will be personally acquainted with the phenomenon. That's because only about half of the population finds that their urine smells funny a few hours after they've eaten asparagus.

Asparagus is filled with sulfur-containing amino acids that break down during digestion into six sulfur-containing compounds. These can impart a unique smell to urine as they are excreted. "It's the same sulfur group that makes skunks smell," said Barbara Hodges, a dietician with Boston University's nutrition clinic, the Evans Nutrition Group.

Scientists remain divided on why people have different urinary responses to eating asparagus. One camp thinks only about half of the population have a gene enabling us to break down the sulfurous amino acids in asparagus into their smellier components. Others think that everyone digests asparagus the same way, but only about half of us have a gene that enables us to smell the specific compounds formed in the digestion of asparagus.

"There's something of a dispute," said Dr. David Stollar, chairman of biochemistry at Tufts University Medical School.

The unusual smells are nothing to worry about, though. According to the Dictionary of Medical Syndromes, which includes an entry on the urinary excretion of odoriferous components of asparagus: "The syndrome does not have any pathological significance."