Many people tout the benefits of a calorie restriction diet, but Dr. Oz says there are some risks.
Without proper research and medical advice, some people may eat the wrong kind of foods and become vitamin deficient. If you chose to get all your calories from burgers or sweets, Dr. Oz says you could develop health problems.
"You wouldn't get the right amount of calcium, so you might develop weakening of your bones," he says. "There are factors we have to take into account."
People who've adopted healthy eating plans that follow the program's principles, however, will see a difference over time. "If we took, and it has been done, people like John, and collect them together and look at how young their heart is or how young their arteries are, they end up turning out, over a couple of years, to get younger...not older," Dr. Oz says. "That's the best evidence to us."