Some fats are just better than others. Omega-3 fatty acids, including fats that make up fish oil, have been recognized for their health-promoting benefits.

Well, move over, omega-3s; now there's a fat that's even phatter. Researchers at Harvard University and Lipomics Technologies in West

An omega-7 fatty acid called C16:ln7-palmitoleate works as a health-promoting hormone, the researchers report September 19 in Cell. Palmitoleate is made by fat and liver cells, the team discovered. The fatty acid signals muscles to respond to insulin, prevents the buildup of fats in the liver and reduces levels of inflammatory chemicals made in fat cells.

The new study "really suggests that lipids--fatty acids--could have a signaling effect," says Clay Semenkovich of Washington University in St. Louis. "This is something people have postulated for a while, but has been difficult to get a handle on."

Palmitoleate is the first lipid demonstrated to work as a hormone, a job usually done by proteins, such as insulin, or by small molecules, such as adrenaline. If the lipid works the same way in people as in mice, it could someday be added to foods or given as a supplement to ward off heart disease and diabetes.

Researchers led by geneticist Gokhan Hotamisligil of Harvard had previously created extraordinarily healthy mice by preventing the mice from making two proteins that normally bind to fatty acids. Previous research had shown that blocking the proteins could improve health. But mice lacking both proteins had health "beyond the normal range," Hotamisligil says. "Almost indestructible. No heart disease, fatty liver disease, diabetes. No asthma, nothing." And their health held up even when they ate a high-fat diet.

But it wasn't obvious why the mice were so healthy. "The general dogma in the field is the more fatty acids you have in the blood, the sicker you are," Hotamisligil says. But the mutant mice had slightly higher levels of fatty acids in their blood than normal mice.

When examining which fats were present in the fiber-healthy mice, the scientists found that palmitoleate, normally rare, was the third most abundant fatty acid in the healthy mice's blood. The lipid improves muscle responses to insulin and prevents liver cells from accumulating other harmful fats, the researchers found. Because the mice were missing the two proteins, their fat cells were not able to store fat. The fat cells instead made lipids, primarily palmitoleate.

Liver cells also make the lipid hormone, the team found. In normal mice, the lipid is produced at low levels. When these mice eat a high-fat diet, their cells cut palmitoleate production in half. But the super-healthy mice continue making lots of the lipid, even when they eat diets rich in fat.

People probably also respond to high-fat diets by producing less palmitoleate, Hotamisligil says. "A caveman chasing a deer probably had active production of this material, but not us constantly stuffing ourselves with calories."

Rather than supplementing the diet, Hotamisligil thinks it would be healthier to persuade people's fat cells to produce more of the lipid. "What you make yourself is always the best," he says. "It's like homemade cooking."