I happen to love asparagus and eat it often. But an acquaintance once told me she carefully avoids this wholesome vegetable simply because it makes her urine smell bad.

I was tempted to ask her who is likely to know or care, except perhaps a stranger in a public restroom. Surely there are worse offenses.

As one of the four routes by which substances normally exit the body (the others being feces, breath and sweat), urine has a uniquely valuable role in medicine: It holds clues not just to what people eat and drink, but also to how well their bodies are functioning. The search for these telltale signs is why doctors routinely request urine samples from patients, whether they seem healthy or are obviously sick.