A hormone released by the stomach may activate the brain's reward system and motivate us to buy high-calorie food

The first law of home economics states that the amount of food in a supermarket trolley rises in direct proportion to the shopper's hunger level.

But while the danger is well known to shoppers who venture out on an empty stomach, how the problem arises in the brain has remained a mystery.

In a new study of hunger's effects on the mind, neuroscientists pieced together what happens in the brain that makes us buy more food when we are famished.

The findings point to ways of overcoming the temptations of the food aisles and throw fresh light on the rise of obesity over the past 30 years.

From a series of brain scans on volunteers, the researchers showed that a hunger hormone triggered activity in the brain that made people pay more for junk food than usual. Equipped with this knowledge, shoppers should be able to override the urge and stick to more healthy shopping, the scientists said.

"You should never make decisions about food when you are starving. When you go to the supermarket hungry, the food you are drawn to is high-calorie junk food," said Dr Alain Dagher, a neurologist at the Montreal Neurological Institute. "You assign way too much value to calories and so way too little to health and other things."

Dagher and his colleague Deb Tang recruited 29 volunteers for the study and, asked them to rate how much they liked various items of food, from chocolate cookies and potato crisps to apples and carrots.

The scientists next put the volunteers in a brain scanner and had them bid up to five dollars a time for food and other objects, such as baseball caps and T-shirts, that flashed up on a screen.

After each volunteer had bid for 300 items, one bid was taken and compared with a counterbid generated at random by a computer. If the volunteer bid the most, they got to eat the food or keep the item at stake.

Each person took the test twice over two days and beforehand received either an injection of ghrelin, a hunger hormone that is released by the stomach, or a shot of saline as a control. The ghrelin jab made volunteers ravenous for one to two hours.

Unsurprisingly, the hungry volunteers bid consistently more for food and less for inedible items than the volunteers who had the saline jab. The hungry people were willing to spend an average of $2.09 for food and $1.32 for other objects, but on saline bid only $1.82 for food and $1.58 for the inedible items.

The scientists traced the behaviour to increased activity in a brain region called the ventromedial prefrontal cortex, which is involved in motivation and reward.

"Ghrelin seems to increase the amount you are willing to pay for food through this area of the brain. We think this is why you assign greater value to the food when you are hungry," Dagher said. The research was presented at the Society for Neuroscience meeting in Washington DC.

But the study found that people did not simply pay more for foods they liked. Instead, the higher bids were for the high-calorie junk foods.

"When you are hungry you assign way too much value to calories and so way too little to health and other things," Dagher said.

Dagher believes that simply being aware of the way the brain works could help people overcome the urge to buy junk food when they are hungry. "We have this intrinsic value system but we can override it with our minds," he said.

The findings support evidence that shows skipping meals to lose weight can be counterproductive. "If you miss meals, you tend to gain weight because you get hungrier and eat calorically dense foods," Dagher said.

"The main reason for the increase in obesity over the past 30 years is the cost of food. When you look at an item of high-calorie food, its cost is low, so you're more likely to buy it. And once you have bought it, you are more likely to consume it.

"The problem is the decision-making before you even sit down to your meal. That's where you have lost the battle," Dagher said.