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 which all the subjects could produce methyl mercaptan, but their ability to smell it in the urine differed. Those who were able to smell the odour in their own urine could smell it in the urine of anyone who had eaten asparagus irrespective of whether or not that person could smell it. The authors suggested that the ability to smell these substances in one's or, indeed, another's urine was also genetically determined.⁶

Apart from methyl mercaptan, other metabolites may be responsible for the smell of the urine after eating asparagus. Gas chromatography of the urine has shown that the odour could be caused by S-methyl-thioacrylate and S-methyl-3- (methylthio)thiopropionate,⁷ while a combination of methyl mercaptan, dimethyl sulphide, and small amounts of sulphur-oxidised compounds could also be responsible.⁸ The source of these metabolites is unknown, but the last paper postulates asparagusic acid, which is specific to asparagus and, when given to humans, causes the same characteristic smell in the urine.⁸

Don't panic

You should be reassured, however, that the smell in the urine is not a sign of disease, and so it is not a reason to stop eating asparagus, which contains more folic acid than any other vegetable. Asparagus is also low in fat, cholesterol, and sodium, but rich in fibre, potassium, thiamine, vitamins A, C, and B6, and glutathione, which is a powerful antioxidant.⁹ If you have never tried asparagus before, you should try it steamed or boiled and then smothered in butter or with Hollandaise sauce.

So next time you eat asparagus as a starter or a side vegetable, just wait until you visit the lavatory to see if you are one of the fortunate 40%. You also know what to tell a curious patient who likes the delicate flavour of asparagus. More information about asparagus can be found at http://www.asparagus.org/