Dr. Oz weighs in on Katy Perry's 'Vogue' diet

For her July cover of "Vogue," Katy Perry said she didn't want to be Photoshopped -- so instead, she went on a rigorous diet, which included one curious ingredient which, Dr. Oz says, really does work.

Perry, who said she wanted to be "glowing" for the photoshoot by famed photographer Annie Leibovitz, credits her weight loss in part to green coffee bean extract, a supplement that Demi Moore has also famously used to shed pounds. And, according to Dr. Oz, there have been studies that show the supplement's effectiveness.

In studies, says Dr. Oz, control groups who kept food diaries and changed nothing about their diet except adding the extract, which has relatively little caffeine, doubled weight loss -- from one pound per week to two.

"The group that eats well with the green coffee bean extract lost an extra pound per week more," Oz noted on the Kent and Alan show, "which doesn't sound like a lot, but again, you're not doing anything else."

"Eating well" is the key, however. Green coffee extract won't do the work for you, says Oz, but it can speed weight loss "assuming you're eating intelligently."

Of course, Perry did more than just take green coffee beans -- she also told Jay Leno that she stopped drinking alcohol for three months, and that she "kind of went on a cleanse."