FDA researchers, whose results appeared in Journal of the Academy of Nutrition and Dietetics, found that people were best at assessing things like chips and frozen meals - and comparing the healthfulness of multiple products - when the nutrition facts were presented for the entire container's worth of food, or for both one serving and the entire container.

This does away with the need to multiply the nutrition facts listed by the number of servings per package if people want to eat it all, researchers said.

"I think people really have a hard time interpreting what food labels mean," said Eric Matheson, a nutrition researcher from the Medical University of South Carolina in Charleston.

"It's almost like there's information overload," added Matheson, who did not work on the study.

Amy Lando and Serena Lo from the FDA surveyed close to 9,500 U.S. adults, showing them one of the 10 different types of food labels that presented calories and nutrients per serving, or per container, in a variety of ways.

Participants were asked how healthy they thought different products were, including how much fat, for example, was in one serving. They then compared types of chips or frozen meals to determine which was healthier.

Currently, manufacturers are given a lot of leeway when it comes to deciding how much a serving size is, according to Gina Mohr, a marketing researcher from Colorado State University in Fort Collins, who didn't contribute to the study.

To make products appear healthier, some companies have started increasing the number of servings listed per container, thus lowering the number of calories per serving. All of that adds to consumers' confusion, she said.

Researchers warned that it's still not known whether clearer nutrition facts would change what people choose to buy or eat, and it's also unclear if and when the FDA might issue changes to labelling requirements.

But having a system that lists the nutrients for one serving and an entire package - as some products do already - would help simplify things, Mohr said.

"It's so important to make the information as transparent as you can make it for consumers," she said.

Marion Nestle, a nutrition researcher from New York University, agreed, echoing Mohr's recommendation of a system like Britain's - in which the front of foods are labeled green, yellow or red on basis of their healthfulness.

"If you give somebody a big package of potato chips, they're not going to think there are five servings in it, they're going to think it has 100 calories," she said.