If you eat too much fattening food one day, don't count on yourself to be good the next day and eat less.

People offered large meals will eat them day after day, according to a study released on Wednesday at a conference of North American obesity researchers in Vancouver.

Health experts have pointed to large food portions, such as "supersized" fast-food meals, as a culprit in the dramatic rise in obesity rates in the United States.

Sixty four percent of Americans are considered to be either overweight or obese.

"I think it's quite obvious we need innovative strategies to limit the impact of portion size on intake," said Barbara Rolls of Penn State University, who conducted the study.

The researchers tracked the eating habits and energy intake of nearly two dozen men and women over 11 days, making it one of the longest studies of its kind.

People would consistently eat more when offered large meals, except in the case of vegetables, according to the study.

"As someone who had been pushing fruits and vegetables for weight reduction I find this quite discouraging," Rolls said.

Another study presented to the annual meeting of North American Association for the Study of Obesity (NAASO) on Wednesday found that for snack food it may not be the size of the bag that determines how much people eat.

Researchers who compared consumption of normal and larger amounts of snacks such as potato chips found that people would eat the larger amount available if it was presented in one large serving or smaller individual increments.

"If we give people a greater amount of food, a greater amount is consumed," Hollie Raynor of Brown University in Rhode Island told the Vancouver convention.

A third study released on Wednesday found the link between overeating and availability of larger amounts of food in a meal may start in children as young as age 2.

But the researchers said they also found that children who tended to eat too much were less likely to do it when allowed to serve the food to themselves.

Experts say it is difficult to convince Americans to limit how much they eat, because of the convenience of large portions at fast food outlets and the financial attraction of buying low-priced food in bulk in stores.

"People like value. We've got to shift people away from this value way of thinking of simply getting the most calories for the least dollars, to value in terms of health," Rolls told reporters.

NAASO president Louis Aronne said the availability of larger portions is not the only thing pushing the increase in obesity.

"You start out massively over-eating because portions are larger, and things get activated in the brain and liver that fuel the upward movement in body weight," Aronne said,

A recent survey by food-service company Aramark Corp. found a majority of consumers would like restaurants to offer half-sized portions on menus and more information on the nutritional content of the meals.