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You should know, you're a medic: Why does urine smell odd after eating asparagus?

Student BMJ; London Vol. 8,  (Aug 2000). DOI:10.1136/sbmj.0008277

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Why does urine smell odd after eating asparagus? Roger Stevens investigates Asparagus, or Asparagus officinalis, is a member of the lily family and grows throughout central and southern Europe, north Africa and west and central Asia. The British asparagus season is short, with asparagus being available during the early summer from late April to early July. 1

In the 18th century, a physician to the French royal family wrote in his Treatise on all sorts of foods that asparagus "eaten to Excess... causes filthy and disagreeable Smell in the Urine." 2 The smell cannot be noticed in raw or cooked asparagus, so it is believed that the body converts a compound within asparagus into a metabolite, which can then be smelled in the urine. The odour is often described as the smell of rotten or boiling cabbage, or even ammonia, and is believed to be due to the presence of methyl mercaptan, also known as methanethiol, which is a sulphur containing derivative of the amino acid methionine. 3

The mechanism

Allison and McWhirter first showed that the ability to produce methyl mercaptan after eating asparagus is not universal. 4 Some people would produce detectable amounts in the urine after eating only three or four spears of asparagus, while others would produce none even after eating as much as one pound (0.45 kg) of asparagus. In their random sample of 115 human subjects, they demonstrated that this ability occurred in about 40% of the population, with an autosomal dominant inheritance pattern. 4 In a larger study of 800 volunteers, Mitchell and co-workers verified these findings in both men and women, and from a pedigree analysis of two families, with one spanning three generations, the autosomal dominant mode of inheritance was also confirmed. 5 Interestingly, the BMJ later reported a study in...