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  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.
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.
Technorati Tags: asparagus