Around the world, child obesity rates are rising. In the Americas, 31 per cent of children are overweight or obese. In Europe, that number is closer to 40 per cent. Even in regions where obesity is less of an epidemic, it's becoming increasingly problematic.

And the trend, no doubt, has a lot to do with whatever it is that overweight kids and teenagers are eating. Is it sugary drinks that are doing the damage? Could processed foods be to blame? Is there a collection of popular foods that parents should stop feeding their children so often?

Curious to find an answer, researchers at Duke National University of Singapore took a closer look at the types of food associated with overweight and obese children. Using the Avon Longitudinal Study of Parents and Children, which recorded the diets and body mass index (BMI) of nearly 4,500 children in England in the 1990s, they tracked what the kids ate and what happened to their bodies over three years. What they found is convincing evidence that certain foods might be causing disproportionate harm.

The kids who regularly ate potato chips tended to gain the most weight. "We found potato chips to be one of the most obesity-promoting foods for youth to consume," the researchers wrote. "Potato chips are very high in energy density and have a low satiety index, yet they are commonly consumed as snacks."

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French fries, fried chicken and fish, processed meats and fatty spreads such as butter performed poorly, too. As did just about anything with added sugar - think desserts, sweets and sugary drinks. And refined grains such as bleached flour, found in most processed foods.

Foods cooked in oil, whether fried, sauteed or even roasted, were linked to weight gain, too.

But there's also a nuance to the things that appear to make kids fat: They pack calories, but don't fill anyone up.

"Just because a food has more calories doesn't mean it results in more weight gain," said Eric Finkelstein, who teaches at the Duke Global Health Institute at Duke University and is the study's lead author.

"There are foods, like potatoes, which aren't inherently bad for you because they fill you up," he said. "But when you turn them into french fries and potato chips, they tend to result in weight gain."

Why exactly that is is unclear, but it may have to do with the added fat.

Researchers have long studied why some foods are more satiating than others. A 1996 study found that fatty foods, surprisingly, tend to be less filling. Carbs and protein-dense foods, meanwhile, tend to be just the opposite. The problem, explained in Adam Drewnowski and Eva Almiron-Roig's 2010 book "Fat Detection," is probably that fats are more energy dense, but no more filling - so kids eat the same quantity, but consume more calories.

Finkelstein says that calories from liquids are particularly problematic, because they're less satiating than those from solid food. Sodas and other sugary drinks, in other words, are doubly harmful.