A new appetite-controlling pathway that responds to molecules found in meat has been discovered in the brain. This brain signal system is triggered by specific amino acids and may lead to new ways of helping obese people lose weight, researchers say.

Certain amino acid molecules – the building blocks of proteins – exert powerful control over appetite, according to a new study in rats. Animals given injections of the amino acid leucine, which is found in high-protein meats and grains, gained only about one-third of the weight put on by their control counterparts.

Although levels of fats and sugars have been shown to influence the desire to eat, until now no team had demonstrated how protein molecules regulate appetite, the researchers claim.

Randy Seeley at the University of Cincinnati in Ohio, US, and his colleagues looked at an enzyme called mTOR, which responds to protein molecules and regulates their synthesis within cells. They found that mTOR was highly active in a region of the rat brain called the hypothalamus – a structure that is involved in regulating appetite in both humans and rats.

To see whether the mTOR pathway in the hypothalamus responds to amino acids, Seeley injected 1 microgram of leucine directly into the brains of rodents, near the hypothalamus. Over the next day, the rats that received the injection consumed 25 grams of food on average while the control rats consumed 30 g of food.

Fooled brains

A brief fasting period produced more dramatic results. When the growing rats were offered food after a 24-hour fasting period, the ones that had received the leucine brain injections gained just 4 g of weight in a day. The injection-free controls put on 12 g – three times as much.

Seeley believes the high levels of leucine in the hypothalamus of injected rats fooled their brains into believing that they had an ample supply of protein molecules circulating as available fuel in the body. This tricked their brains into suppressing hunger, he explains.

However, Seeley cautions that the findings do not necessarily explain why some people claim lose weight on the controversial high-protein Atkins diet. It remains unclear if ingested leucine has the same effect as leucine injected directly into the brain, he notes.

Journal reference: Science (DOI: 10.1126/science.1124147)