| | | | You should know, you're a medic: Why does urine smell odd after
eating asparagus?
by Roger Stevens
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/.
SOURCE: Student BMJ (studentbmj.com)
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| | | References | | | Vaughan JG, Geissler C. The new Oxford book of food plants. Oxford: Oxford
University Press, 1997;172.
McGee H. On food and cooking: the science and lore of the kitchen. New
York: Simon & Schuster, 1997;194.
Nencki M. šber das Vorkommen von Methylmercaptan in menschlichen Harn nach
spargelgenuss. Arch Exp Path und Pharmak 1891;28:206-9.
Allison AC, McWhirter KG. Two unifactorial characters for which man is
polymorphic. Nature 1956;178:748-9.
Mitchell SC, Waring RH, Land D, Thorpe WV. Odorous urine following
asparagus ingestion in man. Experientia 1987;43:382-3.
Lison M, Blondheim SH, Melmed RN. A polymorphism of the ability to smell
urinary metabolites of asparagus. BMJ 1980;281:1676-8.
White RH. Occurrence of S-methyl thioesters in urine of humans after they
have eaten asparagus. Science 1975;189:810-11.
Waring RH, Mitchell SC, Fenwick GR. The chemical nature of the urinary
odour produced by man after asparagus ingestion. Xenobiotica
1987;17:1363-71.
Michigan Asparagus Advisory Board. Nutrition infor- mation.
http://www.asparagus.org/ (accessed 12 May 2000).
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