Asparagus, a green vegetable belonging to the lily family, has one notorious side effect for some diners who eat enough of it. Within a half-hour of asparagus consumption, some people notice their urine has acquired a very pungent odor, often compared to rotting cabbage, ammonia or rotten eggs. The effects of asparagus on urine are generally fleeting and harmless, but it's not necessarily the consumer's finest hour, bodily excretion-wise.
The good news is that asparagus does not affect everyone. Studies conducted on the "asparagus urine" phenomenon (aren't you glad you didn't volunteer!) indicate that roughly 40 to 50 percent of those tested developed the distinctive odor. Surprisingly enough, there is also a segment of the population who cannot smell the sulphurous fumes of asparagus-laced urine. It is believed that both the generation of the odoriferous urine and the ability to smell it are based on genetics. Only those with a certain gene can break down the chemicals inside the asparagus into their smelly components, and only those with the proper gene can smell the results of that chemical breakdown.
Scientists are still not entirely sure which set of chemicals or amino acids contained in asparagus actually cause the smelly pee. The stalks themselves do not acquire a similar odor as they are prepared, so whatever happens most likely happens after ingestion. Experts believe that those with a certain gene produce a digestive enzyme which breaks down the asparagus into various amino acids. One of those compounds is called methyl mercaptan, which is the same chemical which gives a skunk its defensive smell. One theory suggests that asparagus breaks down quickly in the body and an enzyme releases methyl mercaptan, which eventually goes through the kidneys and is excreted as a waste product in the urine.
Others suggest that the asparagus smell is created by other amino acid compounds called thioesters. There is also an amino acid called asparagusic acid, which is not surprisingly found primarily in asparagus. If these compounds are broken down and mixed with the genetically-created enzyme, the results could be a strong smelling urine. This smell is actually considered to be good news, since it proves that the asparagus eater's kidneys are functioning as they should.