"And those who do clear this hurdle are immediately faced with another: Keeping off lost weight is often more difficult than losing it in the first place," said Jan Prinzhausen, a dietary adviser and instructor at the BSA Academy in Saarbruecken, a German higher education institution responsible for health management and illness prevention.

A food and exercise journal helps weight watchers to keep themselves in line, he said. They can note times for exercise sessions and track the calories they consume.

There are two reasons for the notorious "yo-yo effect" after weight loss, Prinzhausen said. Firstly, the body's metabolism adapts to the changed situation. "Less important, energy-intensive processes are reduced.

“The body produces the same output with less energy," he pointed out. And secondly, people neglect the behaviour that helped them to lose weight.

"They quickly cut down on their exercise programme and everyday comforts gain the upper hand," Prinzhausen said. "The afternoon portion of vegetables gives way to cake and sweet pastries. Wine tastes better than mineral water with a meal."

Overall, the energy requirement decreases while calorie intake increases. "This energy-calorie surplus encourages replenishment of fat stores and weight gain," he said.

"So it's important that people integrate the added exercise that became habitual during dieting, along with the healthier diet, into their behaviour for the long term."