Why Urine “Smells”
You may have noticed a strong but strange odor of your urine after you have eaten asparagus. However, not all people show the same reaction.
The high concentration of Potassium and the aromatic substance asparagine help maintain a steady level of water by stimulating the urination of excess fluids. This urination of excess water implies the renal clearance. Asparagus, which is made of 94% water, contains a number of dietary fibers that are not only instrumental for the regulation of the bowel movement, but also provides the body with the liquid necessary for the swelling. It is not surprising to rush to the restroom after a delicious asparagus dinner. The stimulation of the renal activity and, thus, the so-called “purification” must be seen as simply the outcome of the intake of a portion of asparagus, which may well exceed 500g. Because ingesting so much water and dissolved salts and other substances requires their eliminating. But don’t forget potassium as another driving force behind this process.
Asparagine Is Odorless
But how does this relate to the smelling of urine and, particularly, to the aromatic substance asparagine? It is true, that asparagus has a relatively high concentration of asparagine. But asparagine does not smell, because the odor stems from an amino acid, which tends to be relatively polar (even ionic), and does not evaporate. For those having doubts, just get a sample from the chemical storage unit and test it.
A first hint regarding the smell of asparagus might be the evidence of sulfur compounds in the shoot parts. Assumedly, it is these of all the compounds and decomposition products that develop the strange odor. Just think of the sulfur-containing decomposition products of cats or skunks.
Let us be advised by the “Römpp-Lexikon Lebensmittelchemie:” Among the many confirmed aromatic substances, sulfur compounds, such as 1,2-dithiol and 1,2-dithiolan-4-carboxylic acid methyl ester, particularly dominate the flavor of asparagus. Esther and its 1,2-dithiolan-4-carboxylic acid outweigh quantitatively other substances. The latter is also known as asparagus acid. This is probably the reason why many explanations in popular science hold to the view that the aromatic essence of asparagus was asparagine.
The typical odor of urine that comes with the intake of asparagus can be traced back to the plant’s decomposition products, such as s-methyl-thioacrylate and its methanthiol addition product s-methyl-3-(methylthio) thioproponiate.
This chemical reaction is run by our body. And it’s not even a strange or exotic reaction. Because the dithiolan-ring is also present in another biochemically effective substance, the lipoic acid, which plays a decisive role in oxidative decarboxylation reactions.
Lipoic acid (similar to the contents of asparagus) is made of cysteine. The metabolism of lipoic acid suggests that in most human beings urine does not smell after the eating of asparagus, simply because they do not have the genetic conditions, that is, the enzymes, to metabolize the aromatic substances of asparagus, which in effect prevents them from producing the odor substances. Asparagus is therefore not the silver bullet that “purifies” your body – whatever that means. Quite logically, only those substances will be urinated that have entered your body.
Source: Arbeitsgruppe Didaktik der Chemie II (”DC2″) led by Prof. Dr. R. Blume, Department of Chemistry, University of Bielefeld (Germany)