Sweet, salty, sour and bitter — every schoolchild knows these are the building blocks of taste. Our delight in every scrumptious bonbon, every sizzling hot dog, derives in part from the tongue’s ability to recognize and signal just four types of taste.
But are there really just four? Over the last decade, research challenging the notion has been piling up. Today, savory, also called umami, is widely recognized as a basic taste, the fifth. And now other candidates, perhaps as many as 10 or 20, are jockeying for entry into this exclusive club.
“What started off as a challenge to the pantheon of basic tastes has now opened up, so that the whole question is whether taste is even limited to a very small number of primaries,” said Richard D. Mattes, a professor of nutrition science at Purdue University.
Taste plays an intrinsic role as a chemical-sensing system for helping us find what is nutritious (stimulatory) and as a defense against what is poison (aversive). When we put food in our mouths, chemicals slip over taste buds planted into the tongue and palate. As they respond, we are thrilled or repulsed by what we’re eating.
But the body’s reaction may not always be a conscious one. In the late 1980s, in a windowless laboratory at Brooklyn College, the psychologist Anthony Sclafani was investigating the attractive power of sweets. His lab rats loved Polycose, a maltodextrin powder, even preferring it to sugar.
That was puzzling for two reasons: Maltodextrin is rarely found in plants that rats might feed on naturally, and when human subjects tried it, the stuff had no obvious taste.
More than a decade later, a team of exercise scientists discovered that maltodextrin improved athletic performance — even when the tasteless additive was swished around in the mouth and spit back out. Our tongues report nothing; our brains, it seems, sense the incoming energy.
“Maybe people have a taste for Polycose,” Dr. Sclafani said. “They just don’t recognize it consciously, which is quite an intriguing possibility.”
Dr. Sclafani and others are finding evidence that taste receptors on the tongue are also present throughout the intestine, perhaps serving as a kind of unconscious guide to our behavior. These receptors influence the release of hormones that help regulate food intake, and may offer new targets for diabetes treatments, Dr. Sclafani said.
Many tastes are consciously recognized, however, and they are distinguished by having dedicated sets of receptor cells. Fifteen years ago, molecular biologists began figuring out which of these cells in the mouth elicit bitter and sweet tastes.
By “knocking out” the genes that encode for sweet receptors, they produced mice that appeared less likely to lap from sweet-tasting bottles. Eventually, the putative receptors for salty and sour also were identified.
In 2002, though, as taste receptors were identified, the evidence largely confirmed the existence of one that scientist had been arguing about for years: savory.
Umami is subtle, but it is generally described as the rich, meaty taste associated with chicken broth, cured meats, fish, cheeses, mushrooms, cooked tomatoes and seaweed. Some experts believe it may have evolved as an imperfect surrogate for detecting protein.
Since then, researchers have proposed new receptor cells on the tongue for detecting calcium, water and carbonation. The growing list of putative tastes now includes soapiness, lysine, electric, alkaline, hydroxide and metallic.
“The taste field has been absolutely revolutionized,” said Michael Tordoff, a biologist at the Monell Chemical Senses Center. “We’ve made more progress in the last 15 years than in the previous 100.”
One candidate for the next basic taste appears to have emerged as the front-runner: fattiness. The idea has been around for a while, and many scientists thought it was not a specific taste, more like a texture or an aroma.
But researchers recently identified two taste receptors for unsaturated fats on the tongue. And fat evokes a physiological response, Dr. Mattes has found that blood levels of fat rise when we put dietary fat in our mouths, even without swallowing or digesting it.
Hours after a meal, the taste of fatty acids alone can elevate triglyceride levels, even when the nose is plugged. But fat, like umami, does not have a clear, perceptible sensation, and it is hard to distinguish a texture from a taste.
Dr. Mattes says that fat may have a texture that we like (rich and gooey) and a taste that we don’t (rancid).
If so, the taste may serve as part of our sensory alert system. When food spoils, he notes, it often contains high levels of fatty acids, and the taste of them may be “a warning signal.”
Although there is still no consensus beyond sweet, salty, sour, bitter and savory, the research makes clear there is more to taste than a handful of discrete sensations on the tongue. Before long, scientists may have to give up altogether on the idea that there are just a few basic tastes.
“If you’re talking three, four, five, six, you can still call it a pretty exclusive club,” Dr. Mattes said. “If you start getting beyond that, is the concept really useful?”
www.londonmet.ac.uk › Faculties
In the fictional Star Trek universe, a tricorder is a multifunction hand-held device used forsensor scanning, data analysis, and recording data.
Three primary variants of the tricorder appear in Star Trek, issued by the fictional organization Starfleet. The standard tricorder is a general-purpose device used primarily to scout unfamiliar areas, make detailed examination of living things, and record and review technical data. The medical tricorder is used by doctors to help diagnose diseases and collect bodily information about a patient; the key difference between this and a standard tricorder is a detachable hand-held high-resolution scanner stored in a compartment of the tricorder when not in use. The engineering tricorder is fine-tuned forstarship engineering purposes. There are also many other lesser-used varieties of special-use tricorders. The word "tricorder" is a combination of "tri-" and "recorder", referring to its three device input keys, which by default cover GEO (geological), MET (meteorological), and BIO (biological) functions.
Several startups and research projects claim they can approximate the functions of the tricorder, the handheld device used to instantly diagnose a disease or analyze the atmosphere of an alien world.
Researchers have known for a while that many people alive today carry genes from human species other than Homo sapiens, the result of ancient interbreeding with Neanderthals and Denisovans. The specifics, though, have not been clear. But in one case they now are, for it is because of these occasional Denisovan ancestors that Tibetans thrive in Tibet http://econ.st/1j0IacI
Tibetans get high-altitude edge from extinct Denisovans' genes
Forget climbing Mt. Everest — for most humans, just eking out a living on the harsh Tibetan plateau is challenge enough. But Tibetan people have thrived there for thousands of years, and a new study says it's thanks to a genetic adaptation they inherited from an ancient human relative.
The study, published Wednesday in the journal Nature, identifies a long segment of DNA shared by the extinct people known as Denisovans and modern-day Tibetans. The segment contains the gene scientists think gives Tibetans a lung up over lowlanders at high altitudes.
No one knew the Denisovans ever roamed the Earth until four years ago, when scientists sequenced the DNA of a finger bone unearthed in a cave in the Altai Mountains of southern Siberia. The genome exhibited similarities to that of modern humans and our extinct Neanderthal relatives, but it was different enough to be considered a distinct species.
Like Neanderthals, Denisovans mated with their human contemporaries, scientists soon discovered. People of Melanesian descent who today inhabit Papua New Guinea share 5% of their genetic makeup with the Denisovans.
Now it appears that Tibetans can also trace part of their ancestry to this mysterious group.
In the new study, scientists collected blood samples from 40 Tibetans and sequenced more than 30,000 nucleotides on a segment of DNA containingEPAS1, the gene that makes Tibetans so well-suited for life at high altitude. Then the scientists compared that sequence with those of 1,000 individuals representing the 26 human populations in the Human Genome Diversity Panel. They found the high-altitude gene in only 2 of the 40 Han Chinese people in the panel and no one else.
“Natural selection by itself could not explain that pattern,” said Rasmus Nielsen, a computational biologist at UC Berkeley and an author of the study. “The DNA sequence was too different from anything else we saw in other populations.”
So they investigated whether the gene might have been imported from extinct Neanderthals or Denisovans, and, bingo, they found a match.
But how did the gene end up in the genome of modern Tibetans? The scientists used computer models to test two different hypotheses. Were Denisovans and Tibetans descended from a common ancestor that gave the gene to both? Or did humans acquire the gene by mating with Denisovans?
Early humans and Denisovans probably diverged around half a million years ago, and it’s very unlikely that the gene could be maintained in both populations for so long, Nielsen said.
“By the process of recombination, DNA segments become shorter and shorter and shorter,” he said. “But here we have a very long segment that is shared. That’s very unlikely, statistically.”
Alternatively, the gene could have entered the Tibetan gene pool more recently via sex. Once transferred, the gene would have spread rapidly in the Tibetan population because of the merciless selective pressures of high-altitude living.
“Genetically, Han Chinese and Tibetans are very similar throughout the genome,” Nielsen said. “But for this particular gene, they are extremely differentiated from each other, which is something you only see with very strong or very recent selection.”
The reason Tibetans need EPAS1 is that their mountainous home — a crease of buckled crust thrust upward by the tectonic collision of India and Asia — lies about 15,000 feet above sea level, on average. Up there, the air contains 40% less oxygen than it does at low elevations.
Although previous studies had identified the importance of EPAS1, scientists still don’t know exactly what the gene does. They know only that it leads to lower levels of hemoglobin — the oxygen-toting protein in blood — in Tibetans who live at high altitude compared with people from low elevations who have acclimatized.
“That may sound counterintuitive,” said Nielsen — after all, wouldn’t you want more oxygen, and thus, more hemoglobin to deliver it? But people without the gene tend “overreact” at altitude.
“As we acclimatize, we’ll start to produce a lot of red blood cells,” he said, speaking for himself and other non-Tibetans. Too many, in fact. “That will expose us to various diseases like hypertension, increased risk of stroke and preeclampsia. There are very negative fitness effects of having too many red blood cells, and the Tibetans avoid them.”
So how do Tibetans get more oxygen? Presumably they don’t, Nielsen said, and scientists are still trying to understand the physiological mechanisms that allow them to cope.
But one thing is now clear: They owe their extraordinary fitness to a rogue gene introduced into the human genome from their long-lost cousins.
Abigail Bigham, an anthropologist at the University of Michigan who was not involved in the study, said now the search for Denisovan DNA should extend to other groups not represented in the Human Genome Diversity Panel.
“When they looked in Han Chinese, they saw it in only two individuals,” Bigham said. “But other populations in Central Asia or East Asia — there are 49 other ethnic minorities in China that have different genetic backgrounds — would have been interesting to look at as well.”
In any case, she said, the new study adds to a growing body of work that has reshaped the way scientists think about human evolution and our relationship to our extinct relatives.
For a long time, most scientists believed Neanderthals and Denisovans had nothing to do with modern humans. Now they realize that these species are responsible for introducing some of the genetic diversity that allowed people to adapt to unique environments.
“We’ve come full circle,” Bigham said. “Not only has there been interbreeding, but in fact that interbreeding has led to important functional changes in the human genome.”
For all things science, follow me @ScienceJulia