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In America, where food is abundant, it's fairly easy to overeat. But as this ScienCentral News video explains, scientists say animals, possibly including humans, have a brain mechanism that makes them reject foods they don't need.
Temptations not so tempting
Long ago it may have been a lot easier for people to pass up extra food they didn't need - sweet and salty foods like chips and cookies that so many of us crave. Scientists at the University of California, Davis have described for the first time how a protective brain mechanism, evolved long ago, makes sure animals eat a balanced diet of amino acids, the building blocks of protein, while turning from away foods they don't need. It's an ability that people may have once had, but is now lost to modern temptations.
Getting enough amino acids is a priority for the body because they're essential to protein synthesis - the process that powers all the work our bodies do, like maintaining muscle tone, and repairing cuts - basically keeping us healthy.
"If we don't have them in the diet or in our food the way we should," says lead researcher and professor of veterinary medicine Dorothy Giezten, "then we have to start breaking down our own body proteins."
Scientists have known for decades that if an animal is deficient in any of the eight amino acids it needs to survive, it will reject foods that also lack those amino acids and search for other food. But until now, they never understood what controlled this choosy behavior. The researchers reported in the journal Science how a series of biochemical reactions runs this behavior in rats and mice. They say the reactions are identical to those seen in yeast, one of the world's simplest organisms, as well as in pigs, birds, and cats. Because so many different organisms share this mechanism, Giezten says it is second only to that which drives animals to eat when they are hungry. "It's so fundamental that it's conserved across all, what we call eukaryotic species, from all the way across evolution - from the simplest single cell organisms [like yeast], all the way up to people," says Gietzen.
Gietzen says rats will not go directly to foods rich in the amino acids they need. They have to use more of a trial and error approach, tasting a bit of food here and a bit there until they satisfy all of their nutritional needs. She says that if they stumble on a food deficient in the amino acid they need, they reject it within 20 minutes. But if the food has the nutrition they crave, "They plow into [it]," says Gietzen. "You can almost imagine that they're saying 'oh boy this is really good, this is what I was looking for'."
How it works
Transfer RNA latching on to an amino acid.
The brain mechanism that controls this behavior requires a lot of teamwork between several different players. They include amino acids and molecules called transfer RNAs, as well as an enzyme known as GCN2.
Normally when there's a healthy balance of amino acids in the body, each transfer RNA latches on to a specific amino acid and carries it away to build proteins. But if the body lacks an essential amino acid, or is "man down," the transfer RNA hooks up with the enzyme GCN2 instead. GCN2 is particularly sensitive to amino acid levels in the brain and has only one job: if the amino acid levels are out of balance, it activates a second enzyme called "p-eIF2a."
Gietzen says this chemical cascade, which all happens just under the animal's brow in an area of the brain called the anterior piriform cortex, results in one of three things - either the animal has enough amino acids and it can make proteins normally, it's missing amino acids and it will go look for other food sources, or it will start breaking down its stored body proteins, sacrificing them for the proteins it needs immediately.
In order to determine how this brain-trick works, Gietzen's team blocked the attachment of amino acids to transfer RNAs in the brains of rats. When they did that, the rats thought they were eating a diet deficient in amino acids so they went off in search of other food, when in fact their diet was well balanced.
In a second experiment, the researchers inactivated the enzyme GCN2 in mice. When GCN2 was out of the game, the mice ate amino acid deficient diets with no problem, indicating they had lost the ability to sense the food was missing the nutrition they needed. "The good thing about this system is that it activates the search for more food before the [the brain tells the body] to start breaking down its own proteins," says Gietzen.
One of Gietzen's mentors, University of Wisconsin-Madison professor emeritus and nutritional scientist Alfred Harper says the team's research is on target and probably true for people, but he says, "Its not the final answer."
image: NBC News
Harper says animal brains, and peoples' in particular, are armed with several ways of ensuring a balanced diet. He points out that the study depended on starving animals of a specific amino acid. "Under those conditions, I think this mechanism would explain [an animal's eating behavior] very nicely." But adds Harper, "There are other things that will influence selection or intake that don't depend on this severe depletion."
Adam Drewnowski, director of the Nutritional Sciences Program at the University of Washington, says that if there's one eating habit that is "under some kind of innate regulatory control" it's the intake of amino acids for protein synthesis. But for people, just the smell of a food sends us on mission to satisfy our cravings, as can things we see, or the emotions we feel.
While the obesity epidemic in the United States shows people are now clearly eating food they don't nutritionally need, Gietzen points to the combination of rice and beans in traditional South American diets as historical evidence that people did once, though perhaps not consciously, select foods to keep their amino acid levels balanced. Another example, found in every Asian culture is "The processing of soy beans to make a tofu to be used with rice," says Gietzen.
This research was published in the March 18, 2005 issue of Science and was funded by the National Institutes of Health and the U.S. Department of Agriculture.
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