The correct answer is 3: because asparagus contains asparagusic acid. If you’ve eaten asparagus before, you’ve probably noticed that not long afterwards, your urine has a strange, disagreeable odor. Or maybe you didn’t notice anything at all. Regardless, there’s a scientific explanation. It’s thought that the substances that cause this effect all derive from the same chemical compound: asparagusic acid. (It’s found in asparagus, thus the name.) It has been identified as the probable source of several organic compounds that modify the odor of urine. When you eat asparagus, the asparagusic acid molecules are digested and transformed into sulfur-based organic compounds. Research using gas chromatography and mass spectrometry has analyzed the “headspace” of urine produced after consuming asparagus. Headspace refers to the volume of gas situated just above the surface of a liquid, which is made up of the liquid’s light, volatile compounds, and its analysis is very useful for identifying compounds responsible for odors. This research has identified several compounds that are not present in normal (non-asparagus eaters’) urine. The substances, which were found at levels a thousand times higher than in normal urine, are methanethiol and dimethylsulfide. Some dimethylsulfoxyde and methylsulfonylmethane are also present and it is thought that they modify the odor by giving it a sweet characteristic. The human nose is very sensitive to “thiols” – we can perceive just a minute concentration of several parts per billion. Thiols are also found in the secretions of ferrets, which should give you an idea of how bad they smell. This is why the increased concentration of these compounds in urine after eating asparagus is a good explanation of the effect. The odor is detectable within 15-30 minutes of eating the vegetable. It’s interesting to note that our noses are not all alike. Research has shown that some people (2 out of 31, according to one study) cannot detect the change in odor. Previously we assumed that everyone’s urine changed after ingesting asparagus but that only a portion of us were able to detect it; but now, after new research, it is thought that perhaps asparagus doesn’t have the same effect on everyone’s urine, and only produces odor changes in 43% of us. From the book “Pourquoi l'asperge donne-t-elle une odeur au pipi?” PPUR.