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.
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For example, although white potatoes were recorded as being available during the Paleolithic era, they are usually avoided on the Paleo diet because of their high glycemic index. Processed foods are also technically off limits due to an emphasis on fresh foods, but some Paleo diets allow frozen fruits and vegetables because the freezing process preserves most nutrients.
To the outside world, though, as Glassman's company has exploded from a cult fitness website to a gym concept primed to have more locations than Curves by the end of 2013, CrossFit can appear risky, unhinged: Here is a fitness routine that has sent people to the hospital, overseen by people who may have had no more than two days of instruction. (Although every real trainer I met had considerable experience and was genuinely excellent, I attended the seminar and passed the test on my fourth day reporting this story. I am, believe me, no fitness savant.) And it's all led by a man who, in a 2006 CrossFit.com comment, wrote, "We have a therapy for injuries at CrossFit called STFU." As in, Shut the f-k up. That's enough to make even the most devoted laissez faire-ists get a little, well, Uncle Pukie.
CrossFit is promoted as both a physical exercise philosophy and a competitive fitness sport, incorporating elements from high-intensity interval training, Olympic weightlifting, plyometrics, powerlifting, gymnastics, girevoy sport, calisthenics, strongman, and other exercises. It is practiced by members of over 13,000 affiliated gyms, roughly half of which are located in the United States, and by individuals who complete daily workouts (otherwise known as "WODs" or "workouts of the day"). CrossFit has been criticized for allegedly causing people to suffer from unnecessary injuries and exertional rhabdomyolysis.