When we experience something painful, our brain produces natural painkillers that are chemically similar to potent drugs such as morphine. Now research suggests these endogenous opioids also play another role: helping regulate the body's energy balance.

Lauri Nummenmaa, a brain-imaging scientist at the University of Turku in Finland, and his colleagues measured endogenous opioid release in the brains of 10 healthy men. The subjects were injected with a radioactive substance that binds to opioid receptors, making it possible to visualize the receptors' activity using positron-emission tomography.

The study found evidence of natural painkillers in the men's brains after they ate a palatable meal of pizza. Surprisingly, their brains released even more of the endogenous opioids after they ate a far less enticing—but nutritionally similar—liquid meal of what Nummenmaa called “nutritional goo.” Although the subjects rated the pizza as tastier than the goo, opioid release did not appear to relate to their enjoyment of the meal, the researchers reported earlier this year in the Journal of Neuroscience.

“I would've expected the opposite result,” says Paul Burghardt, an investigator at Wayne State University, who was not involved in the work. After all, previous human and animal studies led researchers to believe that endogenous opioids helped to convey the pleasure of eating.

Nummenmaa, too, was surprised. His group's earlier research showed that obese people's brains had fewer opioid receptors—but that receptor levels recover with weight loss. “Maybe when people overeat, endogenous opioids released in the brain constantly bombard the receptors, so they [decrease in number],” he says.

Why more opioids flooded the brain after the goo versus the pizza remains a mystery, but the researchers speculate that faster digestion of the liquid meal may have produced more of the chemicals at the time of the scan, 15 minutes after eating.

The new results may indicate that opioids play a wider role in energy metabolism than scientists previously thought. One possibility is that the opioid system is triggered by the satisfaction of a full stomach and replenished energy, Nummenmaa says.

“If you take a step back and look at conditions that activate opioid release—pain, feeding, pleasure—they are all related to homeostasis,” or keeping the body's energy in balance, he explains. “The most interesting thing is that eating triggered the system even in the absence of sensory pleasure.”