Overeating Makes Flies Obese, Diabetic, Dead
Drosophila are pretty distant relatives to us—as should be obvious from their wings and external skeletons—but our bodies make and manage insulin in similar ways. Insulin is the hormone that regulates blood sugar; we humans manufacture it in our pancreases. Obesity and bad diet can fry our systems, making us resistant to our own insulin (like we become tolerant of caffeine or alcohol or other drugs, feeling less effect with greater use). This immunity to insulin is the hallmark of type 2 diabetes.
Humans with type 2 diabetes have to constantly monitor their blood sugar and may eventually suffer blindness, amputations, kidney damage, and early death. In search of a new tool for studying diabetes in humans, researchers at Southern Methodist University set out to create diabetic fruit flies.
Flies were raised on food that was overly rich in either sugar or protein (the only two ingredients they need to survive). "The flies live in their food," senior researcher Johannes Bauer explained in an email, "so each 'bite' they take contains more nutrients than those of flies living in [normal] food."
Adult flies that ate too much sugar gained weight dramatically, just like humans who overdose on sweets and sodas. The flies had grown about 30% heavier after just 10 days. Flies that were moderately overfed on protein also gained weight. But when they ate the highest amounts of protein, their weight began to drop again.
Bauer says the protein-guzzling flies didn't look as much like diabetics as the sugar eaters. Instead, they looked like carb-avoiding humans who enter a metabolic state called ketosis. "Carbohydrates are the major fuel source of the body," he says, "and when this is in short supply, the body will break down fat for energy. This effect is thought to be the cause for the initial weight loss seen, for example, on the Atkins diet."
Even though the Atkins flies were keeping trim, they weren't healthy. Previous research showed that overfed flies die early, whether they're eating protein or sugar. And ketosis isn't a normal state for flies' bodies or for ours, despite its appeal to dieters. The scientists described the high-protein flies as sluggish and frail.
The most exciting result (for the researchers, if not the flies) was that both overfed Drosophila groups developed insulin resistance. Like diabetic humans, their bodies stopped responding to insulin. It happened whether they ate too many calories from sugar or from protein.
This means researchers might be able to study how diabetes develops—and how to treat it—in fruit flies, one of their favorite model organisms. Fruit flies are unfussy and take up hardly any space; they make baby flies at an amazing rate; and we know their genomes inside and out.
Naturally there are limitations to studying a complicated human illness in a one-milligram insect. "The underlying principles are similar, if not identical," Bauer says. "But the more complex an organism is, the more complex the individual steps become."
Fruit flies could help researchers deepen their understanding of diabetes or develop new drugs to treat it. Those findings, though, might not always translate directly into humans. Even when our diseases look similar to flies', the big, intricate bodies that house us are our own problem.
Morris, S., Coogan, C., Chamseddin, K., Fernandez-Kim, S., Kolli, S., Keller, J., & Bauer, J. (2012). Development of diet-induced insulin resistance in adult Drosophila melanogaster Biochimica et Biophysica Acta (BBA) - Molecular Basis of Disease, 1822 (8), 1230-1237 DOI: 10.1016/j.bbadis.2012.04.012
Image: Mo Kaiwen 莫楷文/Flickr