Some people, notice a strong odour after eating asparagus. This appears to be produced by several sulphur-containing compounds. As few as five or six spears may be enough to produce an odour. The odour may be noticed even within 15 minutes after eating the asparagus and disappears in a few hours when the asparagus is completely digested and absorbed.

Originally it was thought that the compound which causes the odour in urine after consumption of asparagus is methylmercaptan, which is a sulfur-containing derivative of the amino acid, methionine. Later studies suggested the compounds were methylthioacrylate and methylthiopropionate. Another compound, asparagine-amino-succinic-acid monoamide, which is derived from the amino acid asparagines, has also been mentioned.

In any case, the product is formed as a derivative during the digestion and subsequent breakdown of beneficial amino acids, which occur naturally in asparagus.