The scientific chef

Peter Barham on asparagus pee

Why does my wee smell when I've eaten asparagus?
Karen Marriott, Edinburgh

"Asparagus pee" has long been the subject of scientific debate. The problem is that if you ask people if their wee has an unpleasant, sulphurous smell after they've eaten asparagus, half say it does and half say it does not.

Scientists used to believe that people digested asparagus differently and divided them into "excretors" and "non-excretors". We now know that it is not whether or not individuals excrete chemicals that makes the urine smell, but whether an individual can smell these chemicals. Scientists still argue whether or not all of us produce the chemicals that make the asparagus pee odour. Most people certainly produce them, but many can't smell them as they lack the necessary sensory cells in their noses.

The most pungent compounds in asparagus pee are sulphur-containing molecules. How these are formed in the body is also a bit of a mystery. Only one sulphur-containing compound, asparagusic acid, is unique to asparagus. So it must be the way the body deals with this molecule that creates the odorous chemicals in urine. The concentration is greatest in young plants - which fits with the observation that asparagus pee is most pronounced when you eat young, white asparagus.

So 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.

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· Peter Barham is a physicist at Bristol University. His book, The Science of Cooking, is published by Springer.

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