UT Southwestern conducted a study on the effects of different types of fats on the hypothalamus and other parts of the brain. “Normally, our body is primed to say when we’ve had enough, but that doesn’t always happen when we’re eating something good,” said study author Dr. Deborah Clegg, assistant professor of internal medicine at UT Southwestern. What they found was that certain fatty acids blocked hormone signals from insulin and leptin, hormones that regulate weight and appetite.

The study examined the effects in rats. The animals received equal amounts of different types of fats. The first group of rats was fed a diet high in palmitic fatty acids, the second monounsaturated fatty acids, and the third with oleic fatty acids. These groups were then divided further into two groups. In the first group, fats were directly injected into the bran via the carotid artery, while the second group of rats received fats via a tube.

“We found that the palmitic acid specifically reduced the ability of leptin and insulin to activate their intracellular signaling cascades,” Dr. Clegg said. “The oleic fat did not do this. The action was very specific to palmitic acid, which is very high in foods that are rich in saturated fat.”

Oleic acids are naturally-occuring “good” fats found in foods such as olives and some types of soybeans. Palmitic acid, in contrast, is a type of saturated fatty acid typically found in foods high in animal fats, such as beef, butter, and cheese. These types of fats are intended for long-term energy storage, but when a person consumes a diet high in palmitic acid it can accelerate obesity.

“What we’ve shown in this study is that someone’s entire brain chemistry can change in a very short period of time,” Dr. Clegg said. The study showed that the effects of these types of fats can last as long as three days.

“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,” she said. “Since you’re not being told by the brain to stop eating, you overeat.”

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