Asparagus urine and other strange food reactions


My esteemed colleague Tami Dennis recently wrote about “pine mouth,” the bitter flavor that can linger after some people eat pine nuts. That got us to wondering…What other strange taste things happen when people eat certain foods? And why do they happen, to the extent that scientists know?

Two very obvious ‘strange things” — ones that would be hard to miss — are changes in the urine when a person has eaten asparagus or red beets. A nasty odor, in the first case. And a very alarming hue, in the second.

Read all about it at an article by S. C. Mitchell titled “Food Idiosyncrasies: Beetroot and Asparagus” (has every subject on earth received a scholarly treatment?).

In the case of the red-beet urine, not everyone produces it — and scientists (to the extent people were going gangbusters studying the matter) early on suggested that the difference may be due to genetics.

These days, they’re more inclined to put it down to a host of environmental factors. Some are to do with how much beetroot pigment a food contains, based on the variety of beet, the condition under which it grew and whether a manufacturer added some extra beet juice to enhance a food’s color.

And then there are matters of human physiology. The pigment loses its bright pink hue when conditions are too acid or too alkaline — so how fast your stomach fills with acid and how quickly your stomach tends to empty can influence how likely you are to experience this. In one case report, a man who never had beet-colored urine suddenly got it — while taking ranitidine, a gastroesophageal reflux drug, which cut down on his stomach’s acidity.

For an exhaustive review of all the colors that your urine can turn — and, by God, there seems to be a rainbow of them — go to a Mayo Clinic page on the topic. They’re not all caused by foods. Certain drugs can have these effects as well.

And don’t forget your hands. Eat too many carrots and you’ll end up with orange palms as well as soles of the feet (I once saw this in a carrot-lover; it’s quite shocking), courtesy of an accumulation of the pigment carotene.

Unlike beets, the rotten-cabbage odor of urine after eating asparagus does seem to be based in genetics. And it’s complicated. Some people don’t produce smelly urine. Others just can’t smell it. Luckily, enough scientific studies have been done with independent sniffers (sometimes a whole panel) to distinguish between these two — in the best studies, using careful comparison samples of a person’s non-asparagus-y urine.

The first known references to the phenomenon were in the 18th century, even though the cultivation of asparagus goes way further back than that, to ancient Greece and Rome. This could be because farmers started using sulfur in asparagus fields, which improves the vegetable’s flavor — and, it turns out, enhances the pungency of the urine. Some effort was expended trying to find the responsible chemical, and the best candidate seems to be a sulfur-containing chemical, asparagusic acid.

Perhaps the funnest weird-thing-to-happen when you eat concerns the miracle fruit, which will make everything sour taste sweet for a while after you eat it because it somehow distorts the sweet-taste receptor. We ran an article about the miracle fruit a couple of years ago.

One could go on and on with all this, but we’ll end with a classic that’s clearly rooted in genetics: the ability (or not) to sense the chemical phenythiocarbamide (PTC), which also seems linked to how bitter some vegetables taste.

Anyone who’s done a human genetics lab in the last half-century has probably come across this one. You either taste it as bitter — or you don’t. And it’s hard to predict which you are until you actually do the test. If this much makes you curious, you should read a 2006 review of the topic by University of Utah geneticist Stephen Wooding, who has studied the trait.

The discovery was made in the 1930s “in a laboratory incident that would curl the toes of the most stoic OSHA officer,” Wooding writes. A scientist working in a lab spilled some white powder, which “flew around in the air.” That scientist noticed nothing, but a coworker complained it tasted bitter.

“The two then took turns tasting the powder and found that they really did differ dramatically in sensitivity,” Wooding writes. They then went on to test ”a large number of people.” Scientists later learned that a single gene confers the bulk of the trait: One version of the gene confers sensitivity, and a second version confers nonsensitivity.

Chimpanzees vary in their ability to taste this chemical too, as determined by geneticists at the Edinburgh Zoo in the late 1930s. Six of eight chimps there appeared able to taste the bitter chemical when offered up in a sugar drink — and the scientists’s early fears that they might not be able to tell whether the chimps tasted the bitterness were laid to rest when an outraged chimp spat at one of the geneticists, the famous R.A. Fisher — and, according to one on-the-ground report, tried to attack him.

Geneticists later learned that the gene responsible for the trait carries instructions for the bitter-taste receptors we have on our tongues. In both types of people, the receptors are functional — they just function slightly differently. So, depending on which one you have, you sense a different range of chemicals as bitter. (And if you have one copy of each — why, then you’ll detect a wider range of bitter chemicals. This versatility could be useful, given that bitter plant chemicals often can be poisonous.)

Phenythiocarbamide doesn’t occur naturally in plants, but people who can taste it also can taste other bitter chemicals found in vegetables such as cabbage and broccoli — which may explain why some folks don’t like these things.

Finally, here’s the link to the earlier “pine mouth” blog entry.

--Rosie Mestel

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