Mark Bothwell, of Palmerston North, asks :-

What is in asparagus that changes the smell of my urine?

Peter Falloon, a botanist, plant breeder and asparagus grower at Aspara Pacific Ltd, responded.

The main product in asparagus that causes the distinct odour in some people's urine is methanethiol. There are other minor compounds but this seems to be the main culprit.

The production of methanthiol is controlled by a recessive gene in humans. So if both parents produce the smell then all their children will. If one parent is a "Smeller" and the other not then half the children will produce the smell. I'm sure you can probably work out the genetic ratios.

Methanethiol is excreted from the bladder within a very short period after eating asparagus, 10 minutes is sufficient if asparagus is eaten when the bladder is full. Because of this asparagus is considered to be an effective diuretic, some compounds in asparagus certainly work their way through the urinary tract very quickly.

Because of the speed with which the methanethiol is delivered to the bladder, one assumes that there is insufficient time for there to be much digestion of the asparagus or conversion of a methanethiol precursor to methanethiol prior to excretion. So perhaps methanthiol is in the asparagus and humans vary in their ability to excrete it. This would be an interesting area for some research.

To my knowledge noone has tried to breed this compound out of asparagus - I'm sure it would be possible to do so but there just doesn't seem to be the demand for such a variety. ie other traits like disease resistance, flavour, yield, etc seem to have a higher priority.