Why some people can smell asparagus in their urine while others cannot
Eating asparagus breaks down asparagusic acid into volatile sulfur metabolites like methanethiol. However, whether you notice the resulting pungent smell depends on your DNA. While nearly everyone produces these compounds during digestion, mutations in olfactory receptor genes—particularly OR2M7—prevent a large portion of the population from smelling them at all. This harmless genetic divide is known as asparagus anosmia.
The Chemistry of Asparagusic Acid
The distinct scent that can appear after eating asparagus begins with a unique chemical compound known as asparagusic acid. Chemically classified as 1,2-dithiolane-4-carboxylic acid, this molecule contains a five-membered ring featuring two adjacent sulfur atoms linked by a disulfide bridge, attached to a carboxylic acid group. While sulfur is common in biological systems, this specific organosulfur architecture is largely exclusive to asparagus (Asparagus officinalis). Plants use these sulfur compounds as part of their natural metabolic processes and defense mechanisms, storing them within young, growing spears.
When asparagus is eaten, human digestive enzymes and metabolic pathways break down asparagusic acid into a cocktail of smaller, highly volatile organosulfur compounds. The primary byproduct is methanethiol, also known as methyl mercaptan, accompanied by related molecules such as dimethyl sulfide, dimethyl disulfide, and bis(methylthio)methane. Further metabolic steps can also generate dimethyl sulfoxide and dimethyl sulfone. These sulfur-bearing molecules are closely related to the compounds responsible for the sharp aromas of rotten eggs, garlic, and skunk spray.
The key characteristic of these metabolites is their extreme volatility. Because compounds like methanethiol and dimethyl sulfide have very low boiling points, they do not remain dissolved in liquid for long. As soon as urine leaves the body and is exposed to the surrounding air at room temperature, these lightweight sulfur compounds rapidly evaporate. The resulting airborne vapor travels into the nasal passages, where it can encounter sensory receptors.