Why does asparagus make your wee smell?

Chris Smith put this question to Lewis Dartnell...

Lewis - Yes, give the biologist the wee question. It is a really, really good question. And what's going on with the asparagus is that your body is metabolising the compounds and the food molecules and the asparagus that you eat and creating sulphur containing compounds. Sulphur containing compounds often tend to be a bit pongy. For example, the smell of rotting eggs is another sulphurous compound. So when you eat asparagus, the molecules pass through your body and you excrete them back out of your body through your pee. But the reason you don't smell it on your breath is that, unlike alcohol, where the alcohol is evaporating back out of your mouth and you can therefore smell it in your breath, the asparagus smell has been in your bloodstream, then excreted through your blood in urine. So there's two different systems going on there if you like.

Chris - But if it's making wee smelly, then it must be volatile out of the urine and coming up into the room. So if it's in your bloodstream to get into your wee, why doesn't it come out of your mouth?

Lewis - Well, Chris, you have backed me into this corner. If you were to drink your wee, then have that wet urine in your mouth and then breathe. You would now smell asparagus wee on your breath.

Chris - I'll leave you to do the experiment, like Ems did her maths equation you can do the experiment next time. Lewis, thank you very much for that.