Monday, December 21, 2009

Bittersweet: genes for taste

The bitter taster gene, I hadn't known that it is stronger in some groups. How does it fit with increased eating of mint/mustard? Cooking? Adding salt? Alcohol? Fermented milk vs fresh milk - lactase persistence? PTC/PROP? Malaria?

Some Neandertals and some humans lack it, some have multiple copies.

New research suggests that Africans have more sensitive palates than Europeans and Asians - at least for bitter tastes. A survey of numerous African populations in Kenya and Cameroon found a striking amount of diversity in a gene responsible for sensing bitter tastes. "If they have more genetic diversity, there's more variation in their ability to taste," says Sarah Tishkoff, a geneticist at the University of Pennsylvania in Philadelphia, who presented the findings at a recent conference. Europeans and Asians typically have only one of two forms of a gene called TAS2R38, which detects a bitter-tasting compound called PTC and similar chemicals in vegetables such as broccoli and Brussels sprouts. The gene makes the difference between people tasting a weak dilution of the compound or not, with little nuance in between.

However, the compounds that cause bitter tastes can be thyroid-damaging, notes Paul Breslin, a neuroscientist at Monell Chemical Senses Center in Philadelphia. If you have a healthy thyroid you want to eat these things because they're packed with vitamins, he says. [Cooking?]

Seafood benefits : A diet high in iodine - common in coastal-dwelling people - protects against such thyroid damage, but, iodine intake typically drops off the further people live from the ocean. So bitter-sensitive genes could help these people avoid toxic veggies, Breslin speculates. Tishkoff wonders why, then, Europeans lost some the ability to sense bitterness. Different diets and evolutionary forces offer one explanation, she says. Their lack of bitter taste diversity could also be due to a paucity of genetic variation in the small number of African migrants that became ancestors to the Europeans. In general, sub-Saharan Africans boast more genetic diversity than people native to Europe and other continents.

Avoiding potentially toxic plants might not be the only reason for diversity in bitter taste genes, says Theodore Schurr, an anthropologist at the University of Pennsylvania, who was not involved in the study. His team found lots of variation in bitter taste genes in a Siberian population that has historically eaten few vegetables.
People with a gene variation that dulls their taste buds to bitter flavours drink twice as much alcohol as those with more sensitive palates, suggests a US study. The bitter chemical 6-n-propylthiouracil (PROP) is often used in taste tests and in 2003 a gene influencing the sensitivity to PROP was discovered. The gene, TAS2R38, codes for a taste bud receptor and has several natural variations.

TAS2R38. About half of the world's population have at least one copy of the low-sensitivity variant AVI, he says.

A new study of ancient DNA offers preliminary support for that conclusion. Neanderthals possessed a gene mutation that would have meant they couldn't taste bitter chemicals found in many plants. There has been speculation that this mutation, which occurs in a taste receptor gene called TAS2R38, is beneficial to humans because it makes vitamin-packed vegetables more palatable. It probably arose in the common ancestor of modern humans and Neanderthals more than a million years ago. The gene encodes a receptor that detects a chemical called phenylthiocarbamide, which is closely related to compounds produced by broccoli, cabbage and Brussels sprouts.
He and colleague Erik Trinkaus at Washington University in St Louis, Missouri, compiled chemical measurements taken from bone collagen protein belonging to 13 Neanderthals and 13 modern humans, all recovered in Europe. They also added data collected from a 40,000-year-old human recovered in Romania's Oase cave.

Because our bones are constantly destroyed and rebuilt while we are alive, the atoms that make up collagen hold a record of what we've eaten. "When you take a sample of a bone you're getting all those breakfasts, lunches and dinners for 20 years," Richards says.
Telltale atoms

Measurements of the abundance of heavy isotopes of carbon and nitrogen hold the key. Marine environments contain a higher proportion of heavy carbon atoms (carbon-13) than land ecosystems, so lots of carbon-13 in the recovered collagen points to a seafood diet. Meanwhile, heavy nitrogen (nitrogen-15) tends to build up as the atom moves up the food chain, from plants to herbivores to carnivores.

High levels of heavy nitrogen can also come from a diet with lots of freshwater fish. Aquatic food webs tend to contain more steps than terrestrial ecosystems, so large fish often have higher levels of heavy nitrogen than land predators.
A very unusual mammal: the short tailed shrew, echolocates, clicks, venomous, cannibalistic...