Asparagus contains a sulphurous compound called mercaptan (which is also found in rotten eggs, onions and garlic). When your digestive system breaks down mercaptan, by-products are released that cause the strange smell. The process is so quick that your urine can develop the distinctive smell within 15 to 30 minutes of eating asparagus. Not everyone suffers this effect; your genetic makeup may determine whether your urine has the odour -- or whether you can actually smell it. Only some people appear to have the gene for the enzyme that breaks down mercaptan into its more pungent parts.