Soon-to-be mom Jessica Biel swears by it and Megan Fox is also a follower. With such celebs on the Paleo Diet, the eating plan that is intended to mimic the way our paleolithic ancestors ate, is more popular than ever. And now, its secrets are being revealed in a new cookbook by Paleo Diet founder Dr. Loren Cordain.

“Our genes haven’t changed a lot in the last 40 or 50 thousand years,” Cordain said today on “Good Morning America.” “So we do quite well on the foods that we’re genetically adapted to.”

The idea behind Paleo is to eat foods that our ancestors ate tens of thousands of years ago. Cordain claims you’ll stay in shape and protect yourself better from heart disease and diabetes. He even says following the diet can even help reduce or eliminate acne.

But it’s not an easy lifestyle. You can eat only grass-fed meat, fish, fruits and veggies, staying away from processed foods, sugar, wheat, grains and dairy.

A little-known secret, though? Cordain recommends following the diet at least 85 percent of the time, allowing for some cheats.

Some easy swaps are veggie chips in place of potato chips or nuts instead of granola in a trail mix.

But not everyone’s sold on the diet.

“The downfall is missing out on certain foods that many nutrition experts, including myself, feel are important to have in one’s diet, such as legumes that are rich in fiber and other good nutrients,” nutritionist Rachel Beller, author of “Eat To Lose, Eat To Win,” said on “GMA.”

Cordain responded to that criticism, though, by saying, “When we eliminate those, what happens is all of the other foods fill in and make the diet much healthier and nutrient dense.”