BioEd Online

Eating less can lengthen an animal's life. But now it seems that — for flies at least — they don't have to actually cut down on the calories to benefit. Fruitflies can boost their lifespan just by not smelling their food.

The result suggests that flies might use their sense of smell — as well as the actual consumption of food — to help determine how rich their environment is, and how they should go about distributing their energy resources.

From flies and worms to rats and mice, animals fed on restricted diets generally live longer than those given abundant food. No one is sure exactly why this is. One theory is that when times are tough and there is little food about, animals channel more of their resources into maintaining their everyday body function, at the expense of putting energy into reproducing. That can extend lifespan.

Scott Pletcher of the Baylor College of Medicine in Houston, Texas, wanted to find out what governs this decision. Smell, he thought, might be one determinant. "We wanted to see whether we could use odor to trick the flies into thinking the environment was more nutrient-rich than it actually was," says Pletcher.

Normally, cutting a lab fly's usual food intake in half lengthens its lifespan by about 20%, from 41 to 50 days. But exposing hungry flies to the scrumptious smell of yeast, a favourite food, took away some of this benefit, the team found. "About one-third of the beneficial effects on lifespan are lost," says Pletcher.

The yeasty odor had no effect on the lifespan of fully fed flies.

Hold your nose

From these results the team speculated that a fly with no sense of smell might also live longer, as it isn't receiving odour cues about nutrients in the environment.

So they looked at mutant fruitflies that did not have properly functioning smell receptors. As expected, fully fed flies without a sense of smell lived longer than normal flies on the same diet — by 40 to 50%. The results are reported in Science¹.

It's difficult to tell what mechanism is at work, says evolutionary biologist Daniel Promislow of the University of Georgia, Athens, who was not involved with the study. It is possible that, instead of the smell being an environmental signal, the yeast gives off some toxic substance that has a direct physiological effect on a fly that shortens its life, he says.

Promislow also notes that lab animals are quite different from wild ones, in that they may be selected for their ability to reproduce early, even if that shortens their lifespan. So the effect of calorie restriction on longevity may be exaggerated in the lab, he says.

As for humans, the evidence that less food means a longer life is only anecdotal. But that hasn't stopped some people restricting their diet to try and live longer.

If this study had any relevance to people, it would suggest that walking through a bakery while on a diet intended to lengthen your life would be a bad idea — and holding your nose while tucking in might help you live a bit longer. But as a fly is a very different creature from humans, these results are far, far away from helping us to extend our lives.