Hargrove also noted that

 Hargrove also noted that 

a joule can be calculated based on electrical currents, which physicists preferred, while determining the amount of heat needed to raise a liter of water by 1 degree Celsius can vary based on elevation and room air temperature.

But the U.S. Ended up remaining loyal to the calorie. “People had known about the Calorie as a unit of food energy (measured as heat) since 1880, and there was no valid reason to confuse the public with a technical name (joule) that is only a man’s name,” Hargrove added.  

Because consumers are so familiar with the calorie, Ford said she thinks transitioning to the metric equivalent, the kilojoule, would be “pretty disruptive.”

Lisa Harnack, a professor of epidemiology and community health at the University of Minnesota, said she thinks the lack of consistency with other countries may create confusion. However, she said it would be “a huge effort” to educate and help the public understand a new unit. 

Whether a meal tells you that it has 500 calories or 2,000 kilojoules, consumers are really using these units to understand how they fit in relative to their diet, and the actual unit being used arguably may not matter. 

“Most people do not know what a calorie, kilocalorie, or kilojoule actually measures in terms of raising the temperature of water, and how this translates to human metabolism,” said Ford of American University. “Because of that, they’re used more as a tool for monitoring intake.”