Losing weight without dieting, going hungry or using an expensive high-protein liquid diet can be as simple as eating a smaller lunch, reports a new Cornell study that is online and will be published in the journal Appetite in October.

Most importantly, the researchers found that when volunteers ate a lighter lunch, they were no hungrier than usual and didn't compensate by eating more later in the day or week to make up for the fewer calories eaten.

David Levitsky, professor of nutritional sciences and of psychology at Cornell, and graduate student Carly Pacanowski set out to test their theory that one reason why high-protein liquid meal replacements are effective for weight loss is they consist of smaller portions and compensation does not occur at subsequent meals.

They devised a five-week eating study, where the food intake of 17 paid volunteers was measured Mondays through Fridays. For the first week, all 17 ate whatever they wanted from a buffet. For the next two weeks, half the group selected their lunch by choosing one of six commercially available portion-controlled foods, such as Chef Boyardee Pasta or Campbell's Soup at Hand, as a substitute for the buffet lunch, but they could eat as much as they wished at other meals or snacks. For the final two weeks, the other half of the volunteers chose a portion-controlled lunch.

Over the 10 days of consuming a portion-controlled lunch, the participants consumed 250 fewer calories per day than usual and lost, on average, 1.1 pounds (0.5 kilograms).

"The results confirm that humans do not regulate energy intake with any precision," said Levitsky, adding, "Over a year, such a regimen would result in losing at least 25 pounds."

"Roughly two-thirds of the American adult population are overweight or obese. On average, American adults gain weight at a rate of one pound per year, which can cause people with normal body weights to become overweight and overweight people to become obese," said Pacanowski, a registered dietitian and a doctoral student in the field of nutritional sciences.

"Making small reductions in energy intake to compensate for the increasing number of calories available in our food environment may help prevent further weight gain, and one way of doing this could be to consume portion-controlled lunches a few times a week," said Pacanowski.

The study suggests that both high-protein and high-fiber meal replacements result in weight loss not by suppressing appetite, but by providing fewer calories, and because humans do not possess accurate mechanisms to compensate for the smaller intake at a previous meal, they end up taking in fewer calories.

"To stop the increase in obesity, we are going to have to learn to consume fewer calories and here is one simple, low-cost way to do it," Levitsky concluded.

The study was supported by the Division of Nutritional Sciences.

Stephanie Salato is a writer in the Division of Nutritional Sciences.