Harvard researchers say more than half of US children will be obese by age 35

More American kids than ever are expected to carry a heavy load into adulthood.

In a study published Thursday in the New England Journal of Medicine, researchers at Harvard's school of public health extrapolated future obesity trends based on more than 25,000 children's current heights and weights. They estimate that nearly 6 in 10 kids in the US today will be obese by age 35.

"When we look at trends in weight gain over the past 40 years, its not too surprising that we're heading in this direction," lead study author Zachary Ward told Business Insider.

Still, the projection is quite a jump from the current obesity rate, which hovers between 35-40% for 35-year-old adults. Overall, The Centers for Disease Control and Prevention estimates that roughly 38% of American adults can be classified as obese.

Ward said that the younger kids are when they develop obesity, the harder it it is for them to become healthy adults.

Increased screen time and bad diets are two of the reasons that rates are on the rise, he said. Ward's team at Harvard is researching new ways to prevent childhood obesity, like serving healthier snacks, limiting kids' screen time, and cutting sugary drinks like juices at nursery schools.

"Children have already gained enough excess weight in young childhood that it puts them on a trajectory that's really hard to change as they grow older," Ward said.

The obesity epidemic has been growing around the world for decades, but rates in the US have been climbing faster than any other country. Between 2003 and 2014, obesity rates in American kids aged 2-5 actually decreased from 13.9% to 9.4%. But rates in the other age groups have remained stable or continued to rise.

"Unhealthy foods are cheaper and they're everywhere; if you go to any store, you can buy a candy bar at the checkout but not a piece of fruit," Andy Bellatti, a registered dietitian and the cofounder of Dietitians for Professional Integrity, told Business Insider earlier this year.

On Wednesday, the United States Department of Agriculture relaxed nutrition rules for school meals, so cafeterias are now allowed to serve more processed breads, fewer whole grains, and more flavored milks. They can also add more sodium to kids’ breakfasts and lunches.

Obesity is measured by comparing height and weight, a metric known as 'Body Mass Index' or BMI. A BMI score of '30' or higher is generally considered obese. (A person who is 5 feet 9 inches tall and weighs in around 203 pounds would likely be considered obese, which is a step up from 'overweight.')

Recent research has also shown that obesity is carcinogenic: A new report in The Lancet this week blames high BMI for 3.9% of all cancer cases worldwide, or more than half a million cases.