Fruitfly’s food choiceGoing to choose between cheesecake and lean beef? Between beefcake and chicken Caesar salad? NBC’s storyboards may suggest that you eat lean, and you may want to become the biggest loser, but your food choices could be less the result of a conscious decision than a response to some tongue-twisting chemicals floating through your cerebral backwaters. In fact, there are signs that this decision could result from the same mechanism that operates in fruit flies — but we’re flying ahead of ourselves. A study of fruit flies published today in Current Biology explores a “dietary switch,” a chemical mechanism that forces the fly to eat what it needs. After a female mates, for example, the switch guides her toward the protein-rich fungus on a rotten peach. The same thing happens, through much the same mechanism, in flies that are deprived of protein. But when flies are deprived of sugar, they gravitate toward sugary food, says study author Pankaj Kapahi, a geneticist at the Buck Institute for Age Research. Fruit flies are an ideal organism for studying biology: their genetics have been heavily explored, and genes are widely shared among organisms. “The genes that regulate many processes are conserved,” Kapahi says. “Over 400 human disease genes have been found in fruit flies, so it is very pertinent to use a model like this, where we can get the answer at a fraction of the cost, and get it quickly.”
Rather fight than switch?“The dietary switch is pretty amazing,” says Kapahi. “During life history and development, all organisms, from simple insects up to humans, need to switch the type of nutrients they consume.” Growing animals need more protein than older ones, and the several dietary switches help ensure they get what they need. Kapahi admits that the existence of such innate mechanisms has been controversial, and they may seem more old wives’ tale than hard-core science, but he insists they are legit. “When you do not get much protein, or much carbohydrate, there are homeostatic mechanisms that tell the organism, ‘We need to find protein,’ or ‘We are running low on carbohydrate.'” It stands to reason that evolution would craft such switches, but reason and reality do not always agree. And yet in lab experiments with the fruit fly, Kapahi found evidence that such switches are working, and even tracked down some of their chemical mechanisms. The activity seems to center on the TOR (target of rapamycin — don’t ask us…) pathway, which helps detect the level of nutrients in the animal, and is also involved in diabetes and cancer. To explore how proteins work in the TOR pathway, Kapahi and colleagues found that certain proteins that triggered the TOR pathway caused un-mated females to eat more yeast. Although those proteins caused the virgin ladies to eat like mated females, they did not change the choices of mated females, suggesting that their dietary switch had already been activated.
The aging connectionTOR is known to be “a nutrition sensor that’s found from plants to humans,” Kapahi says. “It’s the link between nutrients in the environment and growth. When you eat protein, the TOR signal is part of an ancient natural sensor that says ‘There is now enough food around, let’s turn these nutrients into protein and grow bigger.'” But the current study “suggests that TOR is also an important pathway for balancing the nutrients, specifically protein and carbohydrate,” Kapahi says. Kapahi observes that TOR has also been linked to aging in four species. “If you inhibit TOR, you get a lifespan extension,” and the mechanism may help explain the anti-aging effects of caloric restriction. Eventually, monkeying with the TOR signal system could be the basis for drugs against diabetes, obesity, cancer or aging itself. But because TOR does multiple jobs, “that could potentially have effects on how the animal selects its diet,” Kapahi says. Already, finding that ancient genetic pathways can affect food choice in the fruit fly reinforces the notion that cellular processes are working to decide what we eat. “We humans unconsciously go toward certain foods,” says Kapahi. “Why do we combine rice and beans in so many cultures?” Neither rice nor beans has all the amino acids needed for a complete protein, but together “they make a very nice mix,” Kapahi says. “We have been practicing this for thousands of years, without realizing why, but it gives us an optimal amino acid balance.”
– David J. Tenenbaum