A new study identifies leptin, a hormone made by fat cells, as a key mediator in the transition to fat metabolism.

To keep the human brain supplied with energy when food was scarce, mammals evolved the ability to switch from burning carbohydrates to burning fat in order to preserve skeletal muscle that would otherwise be metabolized and converted to glucose.

Scientists have long believed that just a drop in insulin instigated the transition to fat metabolism.

The new study, led by Gerald I. Shulman, professor of medicine and cellular & molecular physiology at Yale University, examined the rate of fat and carbohydrate metabolism in rats during starvation as they transitioned from a fed to fasting state.

During starvation, Shulman says, plasma leptin levels fall, activating a pathway that promotes the breakdown of fat and mediates this critical shift from glucose to fat metabolism. While a drop in insulin also occurs, a decrease in leptin is also necessary for this process to happen, the researchers discovered.

“We found a new role for leptin in terms of energy maintenance, and the maintenance of nutrient supply to the brain during starvation,” Shulman says. “This leptin-mediated glucose-fatty acid cycle may be evolutionarily important for survival during times of famine.”

The paper appears in Cell.

Source: Yale University

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