Why does asparagus make your pee smell?

Asparagus contains asparagusic acid, and when your body digests it, the acid is broken down into a group of volatile sulfur-containing compounds. These are light enough to evaporate quickly, so they show up in urine and reach your nose fast, sometimes within 15 to 30 minutes of eating, producing that distinctive, slightly rotten sulfur smell. This happens to essentially everyone who eats asparagus; the chemistry is universal.

The fascinating part is that not everyone can smell it, which is why the phenomenon feels mysterious and personal. A significant portion of people carry genetic variations in their smell receptors that make them asparagus anosmic, unable to detect the specific sulfur compounds, so they conclude asparagus does not affect them when in fact it does, they just cannot smell the evidence. Studies mapping this trait have found the ability to smell asparagus pee varies widely across populations. So the honest answer to why does asparagus make my pee smell is that it makes everyone's pee smell, and whether you notice comes down to your nose's genetics, not your kidneys.