The digestive abilities of anatomically modern humans, however, are different from those of Paleolithic humans, which undermines the diet's core premise. During the 2.6 million year-long Paleolithic era, the highly variable climate and worldwide spread of human populations meant that humans were, by necessity, nutritionally adaptable. Supporters of the diet mistakenly presuppose that human digestion has remained essentially unchanged over time.
The data for Cordain's book only came from six contemporary hunter-gatherer groups, mainly living in marginal habitats. One of the studies was on the !Kung, whose diet was recorded for a single month, and one was on the Inuit. Due to these limitations, the book has been criticized as painting an incomplete picture of the diets of Paleolithic humans. It has been noted that the rationale for the diet does not adequately account for the fact that, due to the pressures of artificial selection, most modern domesticated plants and animals differ drastically from their Paleolithic ancestors; likewise, their nutritional profiles are very different from their ancient counterparts. For example, wild almonds produce potentially fatal levels of cyanide, but this trait has been bred out of domesticated varieties using artificial selection. Many vegetables, such as broccoli, did not exist in the Paleolithic period; broccoli, cabbage, cauliflower, and kale are modern cultivars of the ancient species Brassica oleracea.
Grass-fed beef is often highlighted on the diet, which is promoted to contain more omega-3 fats than conventional beef (due to being fed grass instead of grain). It does contain small amounts of alpha-linolenic acid (ALA), a precursor to EPA and DHA. However, only a small proportion of ALA can be converted in the body to long-chain omega-3 fatty acids (EPA and DHA). The amount of omega-3 is also highly variable depending on the exact feeding regimen and differences in fat metabolism among cattle breeds.  In general, the amount of omega-3 in grass-fed beef is much lower than that in oily marine fish.  Cooked salmon contains 1000-2000 mg of EPA/DHA per 3-ounce portion, whereas 3 ounces of grass-fed beef contains about 20-200 mg of ALA.
This is a complete list of foods not allowed on the paleo diet. It’s a sad day when you first have to say goodbye to these foods but, once you start, it’s much easier and you find there are even better paleo substitutes for these foods. The first few weeks might be tough, but if you stick with it over time, it’ll be worth it. We promise. Here’s the ultimate list of foods not allowed on the paleo diet.
If you were to eat an unlimited amount of red meat (which the paleo diet technically allows), you may see your heart health suffer. While experts applaud the omission of packaged and processed foods like cake, cookies, chips, and candy — which are well known to be bad for your ticker — they’re not crazy about the fact that paleo doesn’t allow you to eat whole grains, legumes, and most dairy. Whole grains in particular have been linked with better cholesterol levels, as well as a reduced risk of stroke, obesity, and type 2 diabetes. (13) These are all comorbidities of heart disease. (14)
For instance, the fat allowance of the diet may be problematic. “My biggest hang-up with the paleo diet is all of the saturated fats it promotes with all of the meats,” explains Holley, noting that you could look for a locally sourced meat, whose origin and method of raising you're aware of, as a healthier option. Saturated fat from meat has been linked with an increased risk of early death. (9)
During this program there is no calorie counting, no measuring, and no weighing yourself for the entire 30 days. I know that can be hard — especially if you’re feeling totally bloated (which can happen in the beginning) or particularly svelte in the later weeks. Just don’t do it. Even though a lot of people’s weight begins to normalize during the 30 days, the program focuses a lot on the “non-scale victories,” as in how you’re feeling. Is your sleep better? How’s your skin? Do you feel like doing cartwheels with that extra energy? Cool.