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A PROTEIN that makes fat mice thin regardless of how much they eat has been
discovered. But after the failure of leptin, a much-hyped natural appetite
suppressant, researchers are wary of claiming that another 鈥渕iracle pill鈥� for
obesity is on the horizon.

鈥淚鈥檓 not going to speculate,鈥� says Harvey Lodish, a cell biologist at the
Whitehead Institute for Biomedical research in Cambridge, Massachusetts, who led
the research.

Fat cells were once thought to be just storage sacks for fat. But they have
come to be thought of as regulators of energy storage as well. They may even
form the core of the 鈥渁dipostat鈥�, a mechanism for keeping body fat constant. By
understanding how the regulatory proteins work, it may eventually be possible to
reset the adipostat.

In 1995, Lodish鈥檚 team purified a protein called Acrp30, similar to one
thought to control metabolism in hibernating chipmunks and ground squirrels.
Only a fragment of Acrp30, which is secreted by fat cells, turned out to affect
metabolism.

The researchers gave mice a high-fat meal, then injected them with a solution
containing the fragment. Four hours later, the animals had between 25 and 30 per
cent less fatty acid and glucose in their blood than mice injected with salt
water. Further tests showed that this was because muscle cells were soaking up
fatty acids much faster than normal. What鈥檚 more, obese mice given regular
injections of the Acrp30 fragment lost nearly 4 per cent of their body weight in
four days, despite continuing to overeat.

Lodish does not yet know how the muscles expend the extra energy. 鈥淧eople ask
me if the mice get hot,鈥� he says. But so far there is no evidence of extra heat
or muscular activity.

And whether the protein will work for people remains to be seen.
Leptin鈥攁nother protein secreted by fat cells鈥攚as hailed as a miracle
drug for obesity in 1995 after dramatic results in mice. Yet obese people
somehow resisted its appetite-suppressing effects. The same fate could await
Acrp30.

鈥淚t鈥檚 so early, all you can do is have fun speculating,鈥� says metabolism
researcher Michael Schwartz of the University of Washington. 鈥淏ut it certainly
looks good when you have an animal lose weight on a high-fat diet.鈥�