Vegetables From Stems

The edible portions of garden asparagus (Asparagus officinalis), a member of the lily family (Liliaceae), are the tender shoots before they have produced leafy branches. In the various types of ornamental asparagus, such as smilax asparagus (A. asparagoides), sprenger asparagus (A. densiflorus 'sprengeri'), sickle-thorn asparagus (A. falcatus) and fern asparagus (A. setaceus), the plants are grown for their interesting "leafy" foliage. In the genus Asparagus the true leaves are reduced to inconspicuous scales. What appear to be green, photosynthetic leaves are actually cladodes or cladophylls, flattened, modified branchlets that develop in the axils of the scalelike leaves.

Asparagus stems are rich in the amino acids asparagine, tyrosone and arginine, plus succinic acid and a methylsulfonium derivative of methionine. This combination of ingredients (particularly the latter compound) accounts for the distinctive odor of your urine shortly after consuming a mass of the stems. According to R.H. White (Science 189: 810, 1975), the malodorous agents in the urine are two sulfur-bearing mercaptans, S-methylthioacrylate and S-methyl 3-(methylthio) thiopropionate. Fowl-smelling, sulfur-bearing mercaptans are also responsible for the defense mechanism in skunks and the odor of garlic. In the case of asparagus, the odor isn't produced until the mercaptans are broken down by digestive enzymes. Apparently not everyone has the gene for the enzyme that breaks down mercaptan, so some people can consume asparagus without having their urine smell afterward. A study in the British Journal of Clinical Pharmacology Volume 27 (1989) found that less than half of British people tested produced the odor, while all French people tested produced the odor.