Crude, but I have to know - why does asparagus make your pee smell?

Question

And why is the effect almost instantaneous?

Answer 1

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.

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. 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. 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.

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, while a combination of methyl mercaptan, dimethyl sulphide, and small amounts of sulphur-oxidised compounds could also be responsible. 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

Answer 2

"If your wee smells after you have eaten asparagus it is because your body deals with asparagusic acid in such a way that it produces smelly chemicals, and you have the ability to smell these."

Some people actually can't smell these smelly chemicals, others can. Because of some chemical process, still unclear to scientists, sulphur-containing molecules are produced in your body when you eat asparagus.

It's actually a bit of a mystery.

Answer 3

The smell is caused by several S-methyl thioesters, specifically S-methyl thioacrylate and S-methyl 3-(methylthio)thiopropionate.

(Thioesters are compounds that result from the reaction of an acid with a sulfur-containing alcohol. They tend to be smelly.)

Answer 4

metabolite chemical in asparagus.

Answer 5

I have never noticed!