This question bugs me every single time I eat asparagus because it makes my pee stink worse than when I’m on antibiotics. And hey, what do you know, it only affects 40% of us. And to think I always thought it was The Hubs who had a problem with his nose.
Why does eating asparagus make my pee stink?
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 chemical compounds 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 chemical compounds. 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. (From WiseGeek)