A valid criticism is that there are inconsistencies in the tenets of the paleo diet, especially when it's practiced dogmatically. I focus on keeping junk fats, junk carbs, processed foods, legumes, gluten and casein out of my diet, I don't get dogmatic, I don't believe that there is one rigid right way to do Paleo and I'm good.
Here are some inconsistencies pointed out in one review of Paleo.
If the argument is that because "some" lectins are toxic to "some" people, then "all" people should avoid "all" lectins, we have a problem. We live in a world where food exists as part of a chain, with predators eating prey – and the prey develops defenses to protect itself from being eaten. Lectins are part of the circle of life and can’t be avoided; they permeate the food chain as predator eats prey. This means that if you wish to avoid all lectins, you would have to avoid all food, since all food contains lectins. To do otherwise implies selective belief in your theory. At least the Blood Type Diet acknowledges this issue and says that at least some groups of people have adapted to eating grains, beans, potatoes, and dairy.
Fruit and root vegetables such as carrots, turnips, and beets are okay, but not tubers such as potatoes, sweet potatoes, and yams. Incidentally, I find the exclusion of tubers requires a bit of theoretical bending. The argument is that potatoes are a "new world" crop and humans have only been eating them for maybe the last 35,000 years. But in truth, yams are an African crop that people have been eating since the dawn of time. So why are they excluded? And if that's your logic for excluding potatoes, then why is turkey okay? After all, turkey is a "new world" species, not even introduced into Europe until the 16th Century.
As for fruits, berries of all kinds are good -- strawberries, blueberries, and raspberries etc. are good. From there, differences in Paleo's abound. Tree fruits are controversial. For example, some say apples are great. Others call them "bags of sugar." And still others say they're okay if you eat the low sugar varieties. And yet, if the theory is based on eating what hunter-gathers ate, then tree fruits would have to be top of the charts. Not to go Biblical, but I think it's pretty safe to say that tree fruits such as apples and pomegranates have been part of the human diet since the very first man and woman walked the earth. And I don't believe hunter gatherers selected their fruit based on the glycemic index.
And, more on lectin:
Also, fruits contain lectins3 -- just like grains. Apricots, bananas, cherries, kiwis, melons, papayas, peaches, pineapples, plums, and even berries are all known to contain lectins and cause allergies. In fact, fruit allergies make up about 10 percent of all food related allergies. So why are fruits allowed? Incidentally, new research has shown that allergies to fruit are actually made possible by pectin, the soluble fiber found in fruit. The pectin surrounds the fruit allergens in the digestive tract so that they don't get broken down and enter the bloodstream intact. Using a digestive enzyme supplement that contains added pectinase can help moderate that problem by breaking down the fruit pectin, which then exposes the allergens to digestive juices and enzymes.
And, another inconsistency:
The assumed diet of the hunter-gatherers modeled by the Paleo's is reflective of cave people living in Northern Europe in cold climes where plants did not readily grow. But the simple truth is that hunter-gatherer societies in other locations ate decidedly different diets. As Katharine Milton points out in an editorial in the American Journal of Clinical Nutrition: "The !Kung might live in conditions close to the "ideal" hunting and gathering environment. What do the !Kung eat? Animal foods are estimated to contribute 33% and plant foods 67% of their daily energy intakes. Fifty percent (by wt) of their plant-based diet comes from the mongongo nut, which is available throughout the year in massive quantities. Similarly, the hunter-gatherer Hadza of Tanzania consume "the bulk of their diet" as wild plants, although they live in an area with an exceptional abundance of game animals and refer to themselves as hunters."
And, on grains:
And it's not just modern examples of hunter-gatherer tribes. There is solid evidence that suggests that Paleolithic peoples commonly ate grain, and even flour, as far back as 30,000 years ago.5 In fact, there is quite reasonable evidence that people were processing cereal grains for food as much as 200,000 years ago.6 The bottom line is that the fundamental premise that Paleolithic peoples did not eat grains and that they ate large amounts of meat is only "suggested" by historical records, not necessarily supported by them.
And, one more:
The idea that the so-called Paleo Diet is inherently healthier is simply not supported by the evidence, either ancient or modern. What is supported is that eating modern highly processed, high-glycemic foods is unhealthy. Diabetes was virtually unknown in China until people began eating the modern Western diet. But before people started eating modern diets in China, they weren't eating anything remotely close to the Paleo Diet. They were eating a largely vegetarian diet grounded in rice and noodles. For centuries, they ate grains without problems. It was the introduction of refined sugars and oils and processed fast foods "what done em in," to quote Eliza Doolittle. As a side note, although meat consumption has gone up dramatically in China, with disease rates climbing right alongside them, it's probably not the meat that's causing the problem. It's most likely all of the refined, processed, fast food that's killing them.