AsparagusThank God for Greg’s post “Odeur d’Asperge” over at Carbon Tet! The mystery is solved, and I thought my kidneys were failing and I would have to spend the rest of my life on dialysis.

This spring I had become an avid admirer of the young asparagus shoots. And practically everyday for breakfast I had steamed or, sometimes, boiled organic asparagus served with cold pressed flaxseed oil and a little bit of soy sauce.

The taste was incredible, and I could never get enough of it. I even “promoted” asparagus to my second most favorite vegetable (after the fresh green peas, of course) but then came the smell…

It took me a few days to confirm the pattern:

asparagus for breakfast –> 15 – 20 min later –> a pungent odor of rotten cabbage in urine.

I immediately assumed that there is something wrong with my metabolism because when I questioned my friends they had no clue what I was talking about it. None one them smelled anything unusual in their urinary discharge after consuming asparagus.

I never got around to research my “problem” until yesterday when I was reading “Carbon Tet” blog and I saw Greg’s post about asparagus. Well, it turns out that I am ok. In fact, I am the lucky one who has the enzyme to break down asparagusic acid found in young asparagus plants.

The asparagus odor “problem” was first described by a Scottish mathematician and physician John Arbuthnot (1667-1735). In his book published in 1731 he wrote:

“asparagus… affects the urine with a foetid smell (especially if cut when they are white) and therefore have been suspected by some physicians as not friendly to the kidneys; when they are older, and begin to ramify, they lose this quality; but then they are not so agreeable”.

And since then more than a dozen of a research work was published on the subject.

Two of the studies conducted in 1956 and 1987 revealed that about 40-43% of the United Kingdom population produced the odor. Other similar studies were undertaken in Israel and China but concluded that all individuals excrete odorous urine following asparagus ingestion; however, these investigations have been subjective, the urine was smelled by individuals.

But in 1987 Waring and colleagues [1] examined the volatiles above urine samples. GC/MS (Gas Chromatography / Mass Spectrometry) identified six compounds above the “smelly” urine samples that were absent in the samples without the odor: methanethiol, dimethyl sulfide, dimethyl disulfide, bis(methylthio)methane, dimethyl sulfoxide, and dimethyl sulfone.

Out of these six, the most pungent compounds methanethiol (CH3-S-H) and dimethyl sulfide (CH3-S-CH3) probably give most of the odor, but the presents of bis(methylthio)methane (CH3-S-CH2-S-CH3) and methylsulfonylmethane (CH3)2SO2 could add sweet aroma.

Of the many sulfur-containing compounds found in asparagus only asparagusic acid (1,2-dithiolane-4-carboxylic acid) and its derivatives may be reduced in the body to its free thiol form, which could be methylated and then be a substrate for thionase-β-lyase activity liberating methanethiol.

Dimerization of methanethiol would yield dimethyl disulfide, while methylation and subsequent sulfur oxidation would give dimethyl sulfide, sulfoxide, and sulfone.

References:
1. “The chemical nature of the urinary odour produced by man after asparagus ingestion.”, Waring RH, Mitchell SC, Fenwick GR., Xenobiotica. 1987 Nov;17(11):1363-71.

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