If you can’t stand the taste of gin and tonic, or curl your tongue when you eat eggplant, that’s your evolutionary conditioning trying to save you from being poisoned. The bitterness in tonic water and certain vegetables alerts your body to a potential toxin, and high levels lead to a feeling of nausea.
Since most plant-based poisons taste bitter, it appears that somewhere in the higher branches of our evolutionary tree, our ancestors developed an in-built defence mechanism to repel toxins that we may unknowingly eat.
Pregnant women may already know about this, having experienced morning sickness that prevents them from keeping food down. Since this is a crucial time in the development of a foetus, the mother has an apparent safeguard in place against any dangerous chemicals in her food.
In a charming experiment, Paul Breslin and colleagues at the Monell Chemical Senses Center in Philadelphia set out to measure levels of nausea in 63 men and women fed intensely bitter substances. Test subjects swirled sucrose octa-acetate (SOA) around their mouths for three minutes, before spitting it out, which induced nausea in 65% of them. This is not surprising, since SOA is added to insecticides, giving them a bitter taste in order to prevent accidental poisoning of humans.
“We believe nausea is the negative affective state that comes with poisoning,” Breslin told me, “to punish you and make sure you don’t eat that again.”
The researchers then gave a different group of participants a drink of quinine – the substance that gives the zing to tonic water and bitter lemon – and similar numbers reported queasiness. It is interesting that while almost all people reported a horrible taste, not all responded with nausea, but this variation is expected in human tastes.
This aversion to intense flavours was proven to be specific to bitterness since no one reacted badly to strong sugar solutions, and less than a third felt sick after an MSG dose, which has both salt and umami flavours.
To ensure that the nausea-free participants didn’t have stomachs resistant to nausea, Breslin and his team positioned them inside a rotating drum painted with black and white stripes designed to induce nausea, and all except for one felt strongly nauseated.
And this distaste for bitterness isn’t unique to humans. Breslin also tests bitter tasting compounds on the fruit fly Drosophila, and “they clearly find bitter-tasting stimuli aversive and will avoid them,” he says.
Humans don’t avoid all bitter foods though; imagine a life without beer, dark chocolate and coffee. Breslin says, “Low intensity bitter is part of the natural experience of eating food, which you would not want to punish.”
Why one third of people did not feel nauseous after the bitter taste is unclear, but may show there is no accounting for taste.