CHICAGO (Reuters) - Losing weight is easier when there is money on the line, U.S. researchers said on Tuesday.

Subway riders walk through the turnstiles while leaving the U.S. Open in New York September 4, 2007. REUTERS/Lucas Jackson

They said weight-loss programs that reward people with money -- and remind them of the cash they stand to lose if they fail -- provided a powerful incentive to lose weight compared with more conventional approaches.

Dr. Kevin Volpp of the University of Pennsylvania School of Medicine was looking for an effective way to treat obesity, a growing problem that carries serious health risks.

He said many weight-loss programs fail because people are being asked to make sacrifices now for rewards in the future.

“We wanted to create a reward system which gave them rewards in the present,” said Volpp, whose study appears in the Journal of the American Medical Association.

Volpp and colleagues studied two kinds of incentive programs for weight loss. One was a lottery-based design in which participants played a lottery and were allowed to collect their winnings if they met their weight-loss target.

The lotteries were running daily, and people were told what their winnings would have been if they had met their weight-loss target.

“There is a very strong sense of loss aversion,” the theory that people are highly motivated to avoid losses, Volpp said in a telephone interview.

“The idea was to create a mechanism where loss aversion would help drive people’s motivation,” he said.

The other was a deposit contract, in which participants invested a small amount of their own money -- between 1 cent and $3 per day -- which they would lose at the end of the month if they failed to reach their goals. People in this group also got a bonus if they met their goal.

“You only received your reward at the end of the month if your weight was below the stated goal for the month,” Volpp said.

The researchers assigned 57 obese but otherwise healthy people to one of these two groups or a control group, in which people were simply weighed at the end of each month. All were aiming to lose 16 pounds (7.26 kg) by the end of four months.

People in the incentive groups lost far more weight than those who got no pay for their efforts, with about half of the participants in each group meeting their weight loss goals.

People in the lottery program earned a total of $378.49 and lost about 13 pounds (5.9 kg), while people in the deposit group got $272.80 and lost 14 pounds (6.35 kg).

Those in the control group, who were merely rewarded by better-fitting jeans, lost about 4 pounds after four months.

Volpp said the studies were highly effective at producing short-term weight loss, but when the money stopped flowing, the weight began to creep back on.

“We need to establish whether they can be effective in sustaining weight loss as well,” he said.