The idea is that these are the foods that our bodies were designed to eat while many of the heavily processed modern foods that fill our diets today contribute to chronic disease and health problems. Proponents of the diet suggest that a well-planned Paleo diet could lead to improvements in many aspects of health, such as enhanced weight loss and reduced inflammation.
The theory is our bodies were designed, and still optimized, to eat what our Paleolithic ancestors ate. Like your hunger-gatherer forefathers, on Paleo you get all the meat from wild animals and unlimited fruits and vegetables you can eat. But no starchy vegetables (like potatoes), no legumes (like lentils or beans), no wheat, and no grains (like quinoa or corn) because those plants were invented by human beings during the agricultural revolution after our Paleolithic ancestors left the planet. You get one cheat day where you can eat whatever you want (“Occasional cheating and digressions may be just what you need to help you stick to the diet.”) No oil because it puts omega 6 and omega 3 ratios out of whack which should never exceed 2:1, except olive oil if you must. Dairy is also prohibited. And meat must come from animals that weren’t fed grains (like corn) because grains lead to inflammation and increased fat.
The WHO trial (so named because the international team of principal investigators contained World Health Organization members) tested the potential of clofibrate, a “pre-statin” cholesterol-lowering agent, to reduce heart attack morbidity and mortality. The investigators ultimately concluded that clofibrate "cannot be recommended as a lipid-lowering drug for community-wide primary prevention of ischaemic heart disease.” Nevertheless, clofibrate remained in use until 2002, when it was pulled for increasing cancer rates. In their review of studies such as the WHO trial, Uffe Ravnskov and David Diamond observe, “Despite the largely disappointing findings from 50 years of cholesterol lower[ing] trials, the indictment and conviction of cholesterol as the causal agent in CVD [cardiovascular disease] has stood the test of time. … [Yet] the grand effort to reduce cholesterol as a strategy to improve health has failed.”