Oct. 11, 2010 (San Diego) -- Did you splurge at the birthday party with a big piece of chocolate cake, driving your daily calorie total way above average? Don’t feel guilty, because you're likely to compensate later, resulting in a negligible net gain, a study shows.

It turns out our body's feedback system doesn't tally up calories in, calories out quite as precisely as some experts believe, says Kevin Hall, PhD, a physiologist at the National Institutes of Health (NIH) who presented his findings at the Obesity Society meeting in San Diego.

''On a day to day basis, you can vary your food intake by plus or minus 600 calories a day and still have a stable body weight, as long as your long-term average [calorie intake] is not creeping up or down," Hall tells WebMD.

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Overeating and Weight Change

Hall and his colleague, Carson Chow, PhD, also of the NIH's National Institute of Diabetes and Digestive and Kidney Diseases, created a mathematical model to determine how precise the body's feedback mechanism is in handling the wildly fluctuating calorie intake and activity level most people experience.

"We used our mathematical model of human metabolism and 'fed it' with day-to-day fluctuations of calories that realistically simulate what happens in real people," Hall says.

The question they asked, Hall says, is "If we forced the mathematical model to eat what real people do, with fluctuations of about 30%, what would be be the expected change in body weight?"

"What we found was that the day-to-day fluctuations of about 600 calories a day led to only small variations of body weight of about 2%, or about three pounds over extended time period," Hall says. They simulated 10 years in the model.

But the results, he warns, aren't an excuse to splurge often. "Remember that the long-term average of our simulated food intake was constant over time."

And much previous research has shown that increasing average food intake over time surely results in weight gain.

The current research suggests people who eat sensibly and usually keep their daily intake to a reasonable number of calories to maintain their weight don't need to stress about an occasional splurge because they'll compensate later by undereating.
Calories Average Out

The mathematical model finding makes sense to Steven Heymsfield, MD, executive director of the Pennington Biomedical Research Center in Baton Rouge, La., and mimics what he has observed anecdotally.

He reviewed the study findings for WebMD. While the findings make sense, he says they run counter to what most people believe about eating and weight.

"Most people would say 3,500 [extra] calories is a pound [gained]," he says. "But [the researchers] are saying it doesn't translate that way quite so simply. With these large fluctuations in intake, up and down, your weight doesn't fluctuate on a relative basis as much."

Someone trying to limit calories to 2,000 a day, for instance, typically eats 1,500 one day and 2,500 the next, he says. But that person’s weight remains relatively stable.

Bottom line? "As long are you are trying to keep your intake around some general average [appropriate to maintain weight] you shouldn't worry about your weight going up or down too much because it is not going to," he says.

This study was presented at a medical conference. The findings should be considered preliminary as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.