"This idea of regulating body weight by being a very active individual that exercises a lot is not being supported by our study," said Dale Schoeller, a professor at the University of Wisconsin and the senior author of the study.

"That doesn't mean you shouldn't exercise," he added, "because there are tremendous health benefits of being physically active and having a high energy expenditure."

Schoeller's team collected information on body size from 443 middle-aged, mostly overweight men and women in September or October of 1999, and again after the holidays in January or February of 2000.

At the beginning of the study, the group also measured the total amount of energy people used through a technique called "doubly labeled water," which involves drinking water that is tagged with oxygen and hydrogen atoms that are slightly different from the kinds usually found in drinking water.

The researchers measured over two weeks how much of the labeled hydrogen and oxygen was passed through urine, and then calculated how much of the remaining labeled oxygen had been used to burn calories.

Total energy includes everything a person burns up -- even while sleeping, watching TV, exercising and walking around.

The measurements taken in the autumn also determined how often people were physically active.

At the end of the study, men had gained close to two pounds and women a little over a pound, which equaled about a one-percent gain in body weight.

People who burned the most calories in a day and those who were the most active were just as likely to put on weight as those who used fewer calories and those who were more sedentary.

"You'd think people with a higher physical activity level would be protected from holiday weight gain," said Susan Racette, a professor at Washington University School of Medicine who was not involved in the study.

Presumably, the holiday parties, cookies, and religious feasts caught up with them.

It could also be that people were less active during the holidays, Racette said.

The researchers did not track how much people ate or how much they exercised during the season.

Schoeller said he would have expected that the high energy users would have been less affected because they should have an easier time compensating for the extra calories through more exercise or eating less later on.

For instance, if Thanksgiving dinner adds 500 calories to a person's daily energy needs, that's only a 17 percent increase for someone who burns up 3,000 calories a day, compared to a 25 percent increase for someone who uses only 2,000 calories.

"This extra 500-calorie meal would be a smaller part of their expenditure and therefore easier to compensate for than someone with a low energy expenditure," Schoeller told Reuters Health.

As his results showed, "it's not the case," he added.

"It really does come down to the fact that those extra calories are problematic for everybody," Racette told Reuters Health.

A study published a decade ago by Dr. Jack Yanovski at the National Institute of Child Health and Human Development also found that Americans gain about one pound during the holidays (see Reuters Health report of March 22, 2000).

In an email, Yanovski said the latest results "confirm the importance of the holiday interval from Thanksgiving to Christmastime for weight gain. Further studies are needed to understand how to prevent weight gain during this vulnerable time of the year."

Schoeller said from an obesity-prevention standpoint, the study backs up the idea that food - and not exercise alone - is important.

"An obesity prevention campaign built around physical activity only without addressing food intake is not likely to succeed," he said. "You need both."

SOURCE: bit.ly/AcNyDZ American Journal of Clinical Nutrition, online February 1, 2012.

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