"It's a population tool," said Dr. Michael Benedict, a physician in private practice in Richland, Wash., who has studied the effectiveness of health incentives programs. "It can be misleading on an individual basis because it doesn't take into account body composition."
For example, a person who had a high BMI but began an exercise regimen may still have the same BMI as they lost fat but gained muscle.
"If you did BMIs on NFL players, they would probably be pretty high," said Volpp. "[But] they're in superb physical condition."
In the Whole Foods program, a person who had a very high BMI but low cholesterol, low blood pressure and who did not smoke might not earn a higher discount, although most doctors would say a person meeting these criteria would be in good health.
"If I were advising them I would have advised them to reward both levels in changes, and changes at the better end of the spectrum more," said Newhouse.
Benedict agreed, explaining that while a 10 percent change in weight makes a difference in health, it might not be attempted by someone with a BMI of 35, since lowering that figure significantly is not realistic.
"I might do it a little bit differently," he said.
Newhouse explained that the incentives themselves are not particularly strong, since even the maximum change -- from a 20 to 30 percent discount, would only impact an employee's budget by 2.5 percent, assuming the employee spent 25 percent of his or her budget on food and all of the food was bought at Whole Foods.
For the same reason, some critics say, the program would not appear to be effective if it were a covert way for Whole Foods to avoid hiring employees who may not be in great health.
"I don't think [that] is likely, because this is a very gentle incentive," said Volpp.
"I would guess it would have a pretty minimal effect on who actually comes to work [at Whole Foods]," said Newhouse. "I don't have any evidence," he said, but "just looking at it, the magnitude of the dollars on the table seem likely to be offset just by other working conditions."
And also, he said, "Probably in this economy, lots of people would be happy with any job."
And that may be lucky for the company, said Kelly Brownell, director of the Rudd Center for Food Policy and Obesity at Yale University, given what would happen if the program led people to quit.
"To the extent the company gains anything by this policy they will lose by narrowing the number of qualified people who will work there," he said, noting that two-thirds of American are overweight or obese. "Losing a qualified employee costs the companies money."
Benedict noted that for a program hoping to change health behaviors to work, education, recruitment and follow-up would all be necessary.
Those are steps Whole Foods said it is taking.
"Along with this program, we have a tremendous amount of educational opportunities for our team members," said Wittenberg.
She explained that cooking classes and programs with doctors would be offered.
Benedict explained that programs to help employees develop better health habits have worked in other areas as well.
"There's very good data for return on company's investment in smoking cessation programs," he said. "Those things pay for themselves fairly quickly."
And despite the many critiques of the program, Volpp noted it should be praised because it "involves carrots instead of sticks."