Americans kids' junk food diet is making them shorter as adults compared to the rest of the world. That's one finding from a massive new study of nearly 19 million people across the world.

On the eve of World War I, American men were third tallest on the planet. By 2014, they dropped to 37th, while U.S. women have slipped from 4th tallest to 42nd. While the average American grew throughout much of the century, that growth leveled off between 1974 and 1994, according to researchers from Imperial College London who presented their work at the EuroScience Open Forum (ESOF2016) in Manchester and published in the journal eLife.

The main reason that we haven't kept up with adults of Netherlands, Belgium, Latvia and Estonia (ranked first through fourth) is that American children are eating too much crappy food, according to Majid Ezzati, professor of medicine at Imperial College.

RELATED: Diet May Change Gut Microbes for Generations

"There was a time that America was the land of plenty, but increasingly nutrition has become worse there," Ezzati said. "It has become a lot more unequal. Obesity in children has gone up."

In fact, childhood obesity among U.S. children has doubled in the past 30 years, and in 2012, more than one of three children and adolescents were either overweight or obese, according to the Center for Disease Control and Prevention.

Pregnant women aren't getting proper nutrition either, said Elio Riboli, director of the school of public health at Imperial College, while kids don't eat enough milk or meat during early childhood through adolescence.

Other high-income countries that have leveled off include the United Kingdom, Finland and Japan. In comparison, Spain, Italy and many Latin American and East Asian nations are continuing to grow taller over time. The researchers analyzed data from nearly 19 million people living in 200 countries.

WATCH:Should Short People Take Human Growth Hormone?