If you answer this question by "What a dumb question. How can asparagus be smelled in pee?", we will know that you are in the 34% of humans that do not have the genetic marker in your DNA that affects being able to detect an asparagus odor. The other 66% of readers know exactly what I'm talking about!
The ability to smell asparagus in urine is due to a compound called asparagusic acid, which is found in asparagus. When we eat asparagus, our digestive system breaks down asparagusic acid into several sulfur-containing compounds, including methanethiol, which has a strong, pungent odor. These compounds are absorbed into the bloodstream and eventually excreted in the urine.
Some people have a genetic variation that allows them to smell these sulfur-containing compounds in their urine, while others do not. This variation is related to a specific gene, called OR2M7, which codes for a receptor in the olfactory epithelium, the part of the nasal cavity responsible for detecting odors.
Interestingly, not everyone who can smell asparagus in their urine has the same sensitivity or timing of detection. Some people can detect the odor within 15 to 30 minutes after eating asparagus, while others may not notice it until several hours later. Additionally, some individuals may not produce the odor at all, even if they have the genetic variation.
If you have this genetic variation, keep in mind that your dog can smell over 10,000X better than you. They have from 125-300 million smell receptors and under the right conditions can smell their humans over 12 miles away. I don't know if they can pick up the odor of asparagus.
More info for the EverythingHealth nerds:
Methanethiol—an organosulfur compound with the chemical formula CH3SH. This colorless gas gives a distinctive rotten smell.
Dimethyl sulfide—an organosulfur compound with the formula (CH₃)₂S and has a characteristic disagreeable odor.
Dimethyl disulfide—an organic chemical compound with the molecular formula CH₃S2 and is associated with an unpleasant and garlic-like odor.
Bis (methylthio) methane—an organosulfur compound with a strong odor.
Dimethyl sulfoxide—an organosulfur compound with the formula (CH₃)₂SO.
Dimethyl sulfone—an organosulfur compound with the formula (CH₃)₂SO₂