A major change for America's biggest restaurant chains went into effect this week, stipulating they must display calorie counts next to all menu items.

While some chains – notably McDonald's – have already introduced menus with calorie counts, all others must now follow after an Obama-era Food and Drug Administration (FDA) rule went into effect on Monday.

And it won't just be national chains that are impacted, as it applies to all companies with more than 20 locations, meaning smaller, regional chains could get caught up in the rules.

What's more, it doesn't just apply to fast-food and sit-down restaurants, but also pizza delivery services, and grocery and convenience stores, coffee shops, and movie theaters with 20+ locations that serve "restaurant-type food."

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In a statement, FDA commissioner Scott Gottlieb notes that America's obesity crisis continues to worsen, leading to chronic diseases like diabetes, heart disease and cancer.

At the same time, more Americans are eating food away from home, consuming 1/3 of their weekly calories eating out.

Gottlieb says studies have shown that calorie displays leads diners to consume 50 fewer calories on every restaurant trip, which adds up to a significant reduction over the course of a year.

The rule had been delayed to give restaurants more time to handle the changes, but it's now in force, with the FDA arguing it provides "a uniform standard to replace the patchwork of menu labeling laws that were appearing around the country."

While it will hasten the rollout, calorie counts on restaurant menus were increasingly becoming the norm anyhow.

The National Restaurant Association said that "thousands" are already complying partly because of the Monday deadline, but also because consumers "have been asking for more transparency on the nutritional content of the food they order."