Having leftovers may cut down on food waste - but it may derail your diet and exercise goals, a new study claims.

Many dieters try to stay trim by only eating some of their meal, saving the rest for later.

However, marketing researchers at the University of Michigan have found that this often leaves people with the false impression that they "under-ate" - meaning they then "reward" themselves with extra food or skipping the gym.

This issue is becoming more of a concern as portion sizes grow, and those who "just eat half" their meal are still consuming plenty of calories.

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"We know that growing portion sizes increase consumption, but grossly enlarged portions also cause consumers to face more and more food leftovers," said Aradhna Krishna, a professor of marketing at the University of Michigan Ross School of Business.

Krishna and co-author Linda Hagen of the University of Southern California carried out five studies to see how leftovers may skew our perception of our own eating habits.

Across the board, no matter how much food someone ate in their first sitting, if their leftover portion was large, they seemed to think they hadn't eaten much.

That misconception had a direct knock-on effect on a person's lifestyle in the hours and days after.

People with larger leftovers were more likely to allow themselves unhealthy snacks and bigger portions in the meals that followed, because they felt they'd "earned it".

In the same vein, a person with a hefty portion left over felt less driven to exercise than someone who didn't leave anything - even if both ate the same.

"The psychological drivers of this phenomenon are twofold," Hagen said.

"Larger leftovers reduce perceived consumption, which leads people to feel better about themselves. And feeling better about themselves, in turn, reduces people's motivation to compensate."

America's expanding waistband

America has always been known for giving its rotund population more bang for their buck - and increasingly so.

In the last 25 years, the average serving size in any given establishment has doubled or tripled.

Bagels are now six inches wide, not three; a medium bag of popcorn is 11 cups, not five; and a soda is 20 ounces not 6.5.

Americans aren't necessarily eating more in one setting. A study by other researchers found that when portion sizes grow by 100 per cent, people only eat 35 per cent more.

However, that means their leftover portions are growing - and Krishna and Hagen warn that, too, affects us.

"This study demonstrates that even leftovers stemming from these enlarged portions can impact consumption subsequently, expanding the scope of portion size research and highlighting the complex ways in which enlarged portions can influence consumption behaviours," Krishna said.

"Our research reveals that unconsumed food can exert meaningful influence on people's perceptions, affect, motivation and important health-related behaviour."