Fruit flies hold the answer, maybe.
In the quest to learn more about how sugar intake may be correlated to the country's ongoing obesity epidemic, University of Michigan Professor Monica Dus is at the research forefront, working to find out exactly how our brains respond to sugar and, in turn, control our eating patterns.
To do so, she studies fruit flies—"We can't really use humans—and that's because biology is infinitely complex and we can't really do much just by studying ourselves," she said at a talk earlier this year.
By examining behavior in a simpler animal, such as the fruit fly, scientists can learn about the underlying laws of biology and nature—in fact, they've found that the same genes that make humans more susceptible to addiction to substances such as drugs and alcohol are also found in flies. Plus, fruit flies like to eat what we eat (which is why they're always in your kitchen), so they're an especially good subject for food testing.
What Dus has found so far in her fruit fly research is revelatory. Dus is operating based on two assumptions: "If you're not hungry, when you go to eat, you eat just because of the pleasure of food," and "When you're drinking diet soda or eating fake sugar, the rewarding property is only the taste."
Now, these might sound like master-of-the-obvious statements, especially to those of us who love mealtime, but the implications of these concepts combined with her research results show that the body could have nutrient-detecting facilities beyond just taste. The hungry fruit flies without use of their taste buds in her study preferred "real" sugar water just as much as the hungry fruit flies who did retain access to taste—the latter group also went for the fake sugar water over plain water (like humans to Diet Coke).
In a recent NPR interview, Dus explained how her fruit fly research has evolved over the course of the year. Her working hypothesis is that "a diet high in sugar actually changes the brain, so it no longer does a good job of knowing how many calories the body is taking in." This shift leads to overeating, and subsequent obesity. In fact, flies in her one of her more recent studies who were placed on high-sugar diets wound up ingesting more overall calories than flies who ate "normal fruit fly food," according to NPR.
"In other words, a steady diet of sugar makes you eat more than you need."