Obesity Beware!

Altered gravity plays an unexpected role in obesity and weight loss.

Astronauts have long known that space travel is a good way to diet. The excitement of launch. Thrilling vistas seen from Earth orbit. Floating weightless. Maybe a touch of motion sickness. Who can eat at a time like that?

Rats, apparently, feel the same way. Rats in space (they've been there onboard the space shuttle) also under-eat. They grow lean compared to rats on Earth. Curiously, rats experiencing high gravity (inside gently-spinning centrifuges) under-eat, too. And this suggests there's more to the story than thrilling vistas:

"Altered gravity somehow disrupts the natural ability of animals to maintain their own weight," says Barbara Horwitz, a professor of physiology at the University of California. No one understands exactly why that should be, but it's probably an important clue to the inner workings of weight control - something that interests people on Earth just as much as astronauts in space. Horwitz is studying the phenomenon in rats at her laboratory in Davis, California.

Although some of us who struggle with weight issues may find it hard to believe, animals, including humans, have evolved a complicated system for maintaining appropriate weight. You'd expect that: the bodies of animals that are too heavy, or not heavy enough, don't function properly.

Astronaut Loren Shriver eats M&Ms onboard the space shuttle Atlantis.

Feeding behaviour is essential, not only to the health of individuals, but also to the survival of whole species. The body stores energy in fat, and there's a minimum amount an organism must have before it can get pregnant. "Animals that lose a lot of fat don't reproduce," says Horwitz.

But the complex network that signals when to eat and when to stop eating can go awry. This could be a contributing cause of, e.g., the "obesity epidemic" in the United States, under-eating among astronauts, and maladies such as the "wasting syndrome" linked to AIDS.

Horwitz is particularly interested in leptin regulatory pathways. Leptin is a hormone that's key to regulating appetite: when it was first discovered in the mid-1990's it was regarded as a possible way to treat obesity in humans. Leptin is produced by fat cells. The more fat cells an organism has, the more leptin circulates through its body.

Leptin manages appetite by activating receptors in the hypothalamus, a part of the brain. These receptors control the production of small signaling proteins called neuropeptides. Leptin increases the amount of neuropeptides that make you feel full, and decreases the amount of neuropeptides that make you feel hungry.