Few people would eat three oranges in one sitting, but that’s the equivalent of one glass of juice. That, a new study finds, is a problem, and it’s why juice can lead to weight gain.

“One fundamental problem with juice is quantity; consuming fruit this way makes it so easy to overdose,” analysis co-author and leading nutrition researcher Walter Willett, a professor of epidemiology and nutrition at Harvard T.H. Chan School of Public Health and professor of medicine at Harvard Medical School in Boston, told CNN.

That glass of juice not only can be consumed in a couple of minutes, he added, but it also can easily be followed by another glass, which “will add many calories and lead to a spike in blood glucose.”

By analyzing previous studies, Willett’s team found a link between this juice consumption and a small weight gain in children and adults.

Although the extra pounds weren’t a lot, too much sugar can cause other health problems, including insulin resistance, diabetes, heart disease and obesity.

These risks led the American Academy of Pedriatrics to recommend no juice for babies younger than 1, no more than 4 ounces a day for kids 1-3, and only 6 ounces a day for children 4-6 years old.

Teens and adults should limit their daily intake to a max of 8 ounces of 100% juice, national guidelines suggest.

“General guidance is against ‘routine’ intake — in other words, relying on juice rather than water for thirst, or consuming juice specifically for health benefits,” Dr. David Katz, a specialist in preventive and lifestyle medicine who founded the nonprofit True Health Initiative, told CNN in an email.

He added juice should be seen as “an occasional sweet treat” to replace soda.

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