Our bodies convert asparagusic acid into sulfur-containing chemicals that stink—but some of us are spared from the pungent odor. If you’ve ever noticed a strange, not-entirely-pleasant scent coming from your urine after you eat asparagus, you’re definitely not alone.
Asparagusic acid, as the name implies, is (to our knowledge) only found in asparagus. When our bodies digest the vegetable, they break down this chemical into a group of related sulfur-containing compounds with long, complicated names (including dimethyl sulfide, dimethyl disulfide, dimethyl sulfoxide and dimethyl sulfone). As with many other substances that include sulfur—such as garlic, skunk spray and odorized natural gas—these sulfur-containing molecules convey a powerful, typically unpleasant scent.
All of these molecules also share another key characteristic: They’re volatile, meaning that have a low enough boiling point that they can vaporize and enter a gaseous state at room temperature, which allows them to travel from urine into the air and up your nose. Asparagusic acid, on the other hand, isn’t volatile, so asparagus itself doesn’t convey the same rotten smell. But once your body converts asparagusic acid into these volatile, sulfur-bearing compounds, the distinctive aroma can be generated quite quickly—in some cases, it’s been detected in the urine of people who ate asparagus just 15-30 minutes earlier.
Of course, the whole asparagus-urine scent issue is complicated by an entire separate issue: Some people simply don’t smell anything different when urinate after they eat asparagus. Scientists have long been divided into two camps in explaining this issue. Some believe that, for physiological reasons, these people (which constitute anywhere from 20 to 40 percent of the population) don’t produce the aroma in their urine when they digest asparagus, while others think that they produce the exact same scent, but somehow lack the ability to smell it.