June 28, 3009, 2009 -- Sweet-toothed Brits have one less excuse for taking their morning tea with several spoons of sugar. They and other Europeans are among the most sugar-sensitive people in the world, a new genetic analysis concludes.
The vast majority of people in the UK, France, Italy and Russia boast a tandem of genetic variations in a sugar-sensing gene that allows them to detect trace levels of sweetness.
Around the world, populations that live at northern latitudes carry these genetic variations at far higher frequencies than tropical-living peoples, says Dennis Drayna, a geneticist at the National Institute on Deafness and Other Communication Disorders in Bethesda, Maryland.
His team presented 144 Europeans, Asians and Africans with nine solutions containing varying amounts of table sugar – sucrose – in amounts varying from 0 to 4 per cent. "Four-per-cent sucrose is very sweet to everyone, and to me it's intensely sweet," Drayna says. "Imagine some cloyingly sweet desert."
Volunteers arranged the solutions in order of their perceived sweetness numerous times, and from these, Drayna's team calculated a sucrose sensitivity score for each person.
When the researchers correlated the scores with variations in two sugar-sensing genes, TAS1R3 and TAS1R2, they found two variants just outside of the TAS1R3 gene that seemed to predict their volunteer's scores.
This puzzled Drayna because TAS1R2 is chock-full of single DNA letter differences between people, and research on bitter taste genes suggested that such mutations – which change the shape of the receptor – underlie these differences.
Instead, the two variations near TAS1R3 probably determine how much of a receptor protein is produced by the taste buds, Drayna says. Tests showed that the variations most common in Europeans crank up the expression of TAS1R3.
Although the gene variants were commonest in Europeans, they were also widespread in Japanese, Palestinian, Han Chinese and other Middle Eastern and Asian populations. Low-sensitivity variations were most prevalent among the several different African populations that the team examined.
The researchers could not estimate when the high-sensitivity mutations evolved. But since other great apes appear to have the low sensitivity version, the changes probably occurred sometime after the common ancestor of humans and chimpanzees split, roughly six million years ago.
An even bigger puzzle is why the low-sensitivity variations are more common among Africans. "The straight answer is we don't know, but there are some tantalising possibilities," Drayna says.
For instance, a dearth of sweet fruits and vegetables beyond the tropics might have favoured increased sugar sensitivity to help find energy rich carbohydrates in local food plants.
"All these things that have really high sugar stores are largely tropical in origin," Drayna says. "When you get into the higher latitudes, you don't find plants like that. We think people needed to turn up the [volume], so to speak."
Paul Breslin, a biologist at Monell Chemical Senses Center in Philadelphia, Pennsylvania, says that theory makes sense. "Maybe someone who couldn't detect sweetness very well would never realise that a carrot or a parsnip was something that was nutritious or yummy to eat because it wouldn't taste very good."
Alternatively, increased sensitivity to sugar could make starchy foods more palatable. When an enzyme called salivary amylase breaks down starch, it eventually produces a sugar called maltose. "It could be a way of finding starchy foods in the world," Breslin says.
A 2007 study led by Nate Dominyat the University of California, Santa Cruz, found that Japanese and European-American populations that load up on starchy foods tend to produce more salivary amylase than African populations that traditionally skimp on carbs.