AKIPRESS.COM - Not being able to smell your food could have a surprising effect on your metabolism, potentially helping you remain thin even when you eat fatty foods, a new study of the University of California, Berkeleyin mice suggests.

To conduct the study, molecular biologist Andrew Dillin of the University of California, Berkeley, and colleagues turned to a variety of genetically altered mice. The scientists gave them regular doses of the diphtheria toxin—which causes a temporary loss of odor-sensing neurons—to suppress their sense of smell. They then fed the rodents either a normal diet or fatty foods—the mouse equivalent of cheesecake and pizza—that usually induce obesity.

After more than 3 months of noshing on regular chow, the odor-deprived rodents weighed slightly less than mice whose sense of smell was intact. In the group on the high-fat diet, however, the mice that couldn’t smell weighed 16% less than animals that could, which became obese. Losing the ability to smell also caused a different group of already-obese mice to lose weight, the researchers reveal today in Cell Metabolism.

The obvious explanation for this effect—that mice with impaired olfaction were eating less—turned out to be wrong. There was no difference in the animals’ food consumption. Nor were the slim rodents getting more exercise. They weren’t moving around their cages more than their porky counterparts.

Instead, the researchers determined, they stayed svelte because they burned more calories, especially in their brown fat. Unlike the white fat that accumulates on our bellies and thighs, brown fat is specialized to produce heat through metabolism. Adult humans have only a small amount of brown fat, but rodents carry plenty of it, and it’s crucial for maintaining their body temperature. Dillin and colleagues found that not only did the brown fat ramp up its activity in the smell-deficient animals, but some of their white fat also became more like brown fat. “They rewire their metabolic programming to burn more calories,” he says. When a mouse can’t smell its food, he speculates, it may “think” that it has eaten more than it really has, prompting it to use more energy.