A single brain protein may be central to appetite control.

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A single brain protein may be central to appetite control.

There is a building block of protein that kills hunger in the brain,

researchers have shown in experiments with rats. The result backs

the idea that altering tiny quantities of particular nutrients in

our diets could help fight obesity and disease.

The study suggests that rats' brains monitor levels of amino acids,

the components of proteins, and use this to judge how much food to

eat. The researchers, at the University of Cincinnati, Ohio, found

that injecting an amino acid called leucine into the brains of

hungry rats curbed their appetite: they gained a third less weight

over 24 hours than rats that didn't have jabs. The team reports its

results in Science1.

The discovery implies that traditional thinking about diets — based

on monitoring the broader classes of carbohydrates, fats and

proteins — is rather crude.

Tinkering with our diets more subtly, to include particular

cocktails of 'micronutrients' such as amino acids, sugars or fat

components, might help to control weight, alter aspects of

metabolism and perhaps combat disease.

Nutrient cocktails

" We have to stop thinking of nutrients as just energy or protein

sources, " says Luciano Rossetti at Albert Einstein College of

Medicine in New York City, who is a proponent of this more subtle

approach to diet. Micronutrients are also biologically active

molecules, he points out.

Earlier studies have shown that lowering levels of oleic acid, the

major fatty component of olive oil, could dampen animals' appetites,

says Rossetti. And eating the amino acid L-phenylalanine, which is

found in some vegetables, juices, yogurt and artificial sweeteners,

has been shown to release a hormone called cholecystokinin, which in

turn seems to suppress human appetite2.

The notion that leucine gleaned from protein in foods, such as meat

and eggs, might do the same by having a direct impact on the brain

is so far speculative. The team doesn't know for sure whether

leucine needs to be injected to penetrate the brain, nor do they

know if it works in humans to suppress appetite.

" My worst fear is that someone is going to be selling leucine on the

Internet because of this, " says Randy Seeley, who led the research.

Brain food

Certain neurons in the brain's hypothalamus carefully regulate our

appetite and weight, by monitoring long-term fat stores. In the past

few years, researchers have also started to understand how the same

circuits keep an eye on the minute-by-minute changes in circulating

nutrients.

Seeley's team studied a protein called mTOR (mammalian target of

rapamycin), which is known to work in most cells of our bodies to

detect fuel levels and decide whether an individual cell has the

resources to grow. The researchers wanted to see whether it also

senses fuel in the brain, and so directs an animal's eating

behaviour as a whole.

The team found that mTOR is active in the neurons that control

appetite in the hypothalamus, and that leucine boosts the activity

of mTOR. This makes rats eat less than they would normally. The

researchers also showed that leptin, a hormone that tells the body

how much fat we are carrying, activates mTOR as well.

The team proposes that mTOR may be a molecule central to appetite

control, although probably one of many. The various weight-control

hormones such as leptin, plus the circulating fats, sugars and

proteins could all influence the activity of mTOR, which in turn

would regulate feeding.

" This idea could integrate all those things very nicely, " says

Myers, who studies mechanisms of appetite control at

University of Michigan Medical School in Ann Arbor.

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