Dieters who skip meals often end up gaining weight over the long term, but why this happens is not well understood.
Researcher Tony Goldstone, of the MRC Clinical Science Centre at Imperial College London, scanned the brains of people who skipped meals and found mechanisms at work that could help explain the conundrum.
They found that prolonged fasting of any kind seemed to prime certain brain regions to gravitate towards higher-calorie foods when the person did eventually find a meal, the 'Guardian' reported.
"That makes evolutionary sense if you're in a negative energy-balance situation.
You're not going to waste your time going for lettuce," Goldstone said.
Goldstone scanned the brains of 21 men and women, all around the age of 25, on two separate days while they were shown pictures of food and asked to rate how appealing they found everything from chocolate and pizzas to vegetables and fish.
On one of the days, the volunteers skipped breakfast before their scans; on the other, they were given a 750-calorie breakfast of cereals, bread and jam an hour beforehand.
The volunteers were given lunch, where they could eat as much as they liked.
"Not surprisingly, when they are fasted they are hungry and they rate the high-calorie foods as more appealing than when they are fed.
"For low-calorie foods, the effect is not as marked. When they come out of the scanner, they are given lunch and they eat more when they haven't had breakfast," said Goldstone.
When the volunteers had skipped breakfast, they ate around 20 per cent more at lunch, compared with days when they had eaten a normal breakfast.
Their brain scans also showed that activity in the orbitofrontal cortex, which is just above the eyes, was especially responsive to high-calorie foods.