Slide 4 of 21: The Paleolithic diet, or paleo diet, allows only foods that cavemen would have eaten 500,000 years or so ago. Nuts, fish, lean meat, fruit, and vegetables without a lot of starch are OK. Dairy foods, grains, processed foods and starchy vegetables are forbidden. Everyday Health points out: ``Experts say the long-term results aren't proven and the diet is difficult to maintain.''