ATLANTA -- How much money would it take to get you to lose some serious weight? $100? $500?Many employers are betting they can find your price. At least a third of U.S. companies offer or plan to offer financial incentives to get employees to lose weight or get healthier in other ways."There's been an explosion of interest in this," said Dr. Kevin Volpp, director of the University of Pennsylvania's Center for Health Incentives.Take OhioHealth, a hospital chain whose work force is mostly overweight. The company embarked last year on a program that paid employees to wear pedometers and get paid for walking. The more they walk, the more they win -- up to $500 a year.Anecdotal success stories are everywhere. Half the 9,000 employees at the chain's five main hospitals signed up, more than $377,000 in rewards have already been paid out and many workers tell of weight loss and a need for slimmer clothes.But will that kind of effort put a permanent dent in American's seemingly intractable obesity problem? Not likely. "It's probably a waste of time," said Kelly Brownell, director of Yale University's Rudd Center for Food Policy and Obesity.Brownell's assessment is harsher than most. But the science seems to back him up.Fifteen to 20 U.S. studies have tried to evaluate the effect of financial incentives on weight loss. None could reach conclusions about how much money it takes to make a lasting difference for most people.Perhaps the largest effort to date was an observational study by Cornell University. It looked at seven employer programs, and the results were depressing: The average weight loss in most was little more than a pound.Sure, there are grounds for optimism. Smaller experiments report some success. And other studies have shown promising results against tobacco. One problem: "Food is more difficult than tobacco," said Steven Kelder, an epidemiology professor at the University of Texas School of Public Health.People don't need to smoke to live. People must eat, however, and sugary drinks and fatty snacks are everywhere, Kelder and others said.Health officials lament that more than two-thirds of American adults are overweight and one-third obese and lecture on fat's role in deaths from diabetes, heart disease and other conditions. The problem has a huge economic impact, too, with obese workers costing U.S. private employers an estimated $45 billion or more annually in healthcare costs and lost labor. That's according to a report by the Conference Board, a research group focused on management and the marketplace.In a campaign led by first lady Michelle Obama, federal officials are emphasizing several approaches to slim the nation. Food companies, worried about potential anti-obesity rules and laws, have publicly endorsed Obama's message and recently pledged to offer lower calorie foods, change recipes and cut portion sizes.While watching to see whether foodmakers follow through, some experts remain fascinated by the idea of using economics to get people to eat better and exercise. Sales taxes have been used to drive up the cost of cigarettes and drive down smoking rates, and Brownell and others are pushing for similar taxes on soda.Companies tend to be more interested in incentives than disincentives like taxes. But the perks they attach to wellness programs come in a variety of forms and sizes.Some reward employees just for having a health evaluation or simply enrolling in a class -- whether they complete it or not. Others require measurable weight loss or exercise achievement. Some companies offer money, some vacation trips. Some refund the cost of Weight Watchers classes. Others reduce health insurance premiums.Companies "are making best their guesses about what might work and giving it a shot," said Robert Jeffery, a University of Minnesota professor. He has been experimenting with financial incentives and weight loss since the 1970s and is perhaps the most veteran researcher in the field.If companies asked the experts, they might be counseled to offer more cash or a bigger penalty in premium costs.Psychologists say people are more motivated by the risk of losing their own money than by a chance of winning somebody else's. Applying that idea to weight loss, some studies have set up refundable bond systems: Volunteers sign a contract agreeing to lose a certain amount of weight by a certain date or they forfeit their deposited money.One of the seven companies in the Cornell study offered a refundable bond option to employees. Its average weight loss was nearly 4 pounds. That doesn't sound like a lot but it's almost twice the average weight loss at companies that paid quarterly rewards.A 2008 University of Pennsylvania study found that after 16 weeks, people who put their own money on the line lost about a pound more, on average, than people who got cash from others.Many employers believe that the wisest approach is to use financial incentives as just one facet of a broader effort to create a culture that makes it harder to be lazy and gluttonous.Kevin Acocella illustrates their point. Acocella, a 35-year-old IBM marketing manager, was 5-feet-9 and a chunky 185 when he decided two years ago to enroll in the company's Web-based fitness program.IBM's work site wellness program is often called a model. But Acocella failed twice.The money got his attention. But the problem, he said, was the culture he was in. "In New York City it was, 'What restaurant can we go to or what bar can we go to?'" Early this year, Acocella moved to the IBM office in San Jose, Calif. "Here it's, 'What activity can you do, and what can you go see, and how can we figure out a way to not take a car there?'" he said.Acocella has lost 9 pounds in the three months he has been there.