People who suddenly begin dieting and exercising to lose weight, while in a stable relationship, may be preparing to break up with their other half, a study reveals.

In fact, happy couples don't worry about their weight because there is less pressure to look attractive, say the German researchers.

Professor Thomas Klein from the University of Heidelberg led the research and studied the link between happiness and body weight and said the warning signs could include a sudden obsession with the gym or getting involved in sports and diets.

"When you are on the lookout for a new partner people try to be as thin and attractive as possible," said Prof Klein.

Consequently, when one half of the pair in a relationship develops a sudden obsession with slimming down it could be interpreted as "preparing for the partner market".

"People in a relationship feel less pressure and often put on weight as they do not watch their weight so much," added Prof Klein.

"In a happy partnership, people tend to get fat. With less competition, the weight difference is low."

But a crisis in a relationship can end the gluttony, he says. And that could end up with both partners dropping weight.

More than 2,000 people aged between 16 and 55 were studied in the research. The study also found that people in couples generally weighed more than single people hoping to attract a partner in a competitive dating market.