The old potato chip commercial defying us to “eat just one” may actually have physiology on its side, researchers find, making it virtually impossible for anyone to win that bet. A group at the University of Texas Southwestern Medical Center has found that certain fatty acids in food actually send a message to the brain to keep on eating, overriding the body’s signals that we’re full.

“Normally, our body is primed to say when we've had enough, but that doesn't always happen when we're eating something good,” says researcher Deborah Clegg.

For the experiment, researchers fed various fats to mice in one of three ways – via a feeding tube, by injecting the fats directly into the brains of the mice, or by injecting fats into the carotid artery. The three types of fat used in the study were palmitic acid, monounsaturated fatty acids, and unsaturated oleic acid.

'What we've shown in this study is that someone's entire brain chemistry can change in a very short period of time.'

Normally the hormones leptin and insulin work control appetite and satiation. But what the researchers saw was that feeding fats to the mice changed their brain chemistry in such a way that they no longer listened to the body’s signals to stop eating when it was full.

“What we've shown in this study is that someone's entire brain chemistry can change in a very short period of time. Our findings suggest that when you eat something high in fat, your brain gets "hit" with the fatty acids, and you become resistant to insulin and leptin.”

The researchers found that of the three fats studied, palmitic acid, found in meat and dairy products, had the greatest effect on insulin and leptin, while oleic acid did not show the effect. What’s more, the phenomenon appears to be a lasting one, continuing for up to three days after the fatty food is consumed: Clegg says that this finding may help explain why people report being hungrier than normal on Monday if they have overeaten the Friday before.

The researchers say that their study provides an important key to unlocking diabetes, since the mechanisms behind insulin resistance – and the specific fats involved –are still somewhat unclear. Further, Clegg says that the finding that fats affect brain chemistry so quickly is of particular interest, as this phenomenon occurs long before the associated bodily cues (obesity) develop.