On the surface, describing the Paleo diet seems simple enough: It's the diet that early humans in the Paleolithic period were said to have eaten as hunter-gatherers -- that is, before the advent of agriculture. Although variations on this diet have been around in the media since at least the 1970s, author Loren Cordain has popularized the idea and trademarked the term "Paleo diet." Although the concept of the diet seems simple, determining what our early ancestors ate is not as straightforward as it appears, and even among followers of such diets, there's a lot of disagreement about what constitutes a "true" Paleo diet.
The basic premise of the Paleo diet is that humans are adapted to eat certain foods and that the advent of modern agriculture radically changed human diets, resulting in poorer health. Supporters of the Paleo diet believe some of our modern health problems, such as obesity, cardiovascular disease and diabetes, could be greatly reduced if people returned to eating a diet more similar to what hunter-gatherers ate.
What's In, What's Out
Proponents of the Paleo diet believe that a healthful diet should include only those foods that were available to early humans before they began cultivating crops and raising animals for food. The diet therefore often includes lean meat and fish; fruits, vegetables and certain roots; and eggs, nuts and seeds. The diet typically excludes all grains, legumes, processed oils, dairy products, and sugar and salt.
Critiques of the Diet
Many scientists' biggest critique of the Paleo diet is that early humans didn't all eat the same; humans evolved to be very flexible eaters and to thrive with a wide variety of diets. For example, traditional Inuit people ate almost entirely high-fat, meat-based diets. At the other extreme, traditional !Kung people ate very little meat, and most of their diet consisted of seeds and nuts. A second critique is that today's foods bear little resemblance to their wild ancestors: A big juicy tomato on store shelves bears little resemblance to the small berry-like tomatoes that our ancestors might have foraged for. The same is true for virtually all fruits and vegetables around today. A third common critique relates to the Paleo claim that humans have not evolved in the past 10,000 years: On the contrary, humans have indeed evolved in some ways to deal with modern diets. For example, a mutation in many modern humans allows them to digest lactose, which allows them to consume and digest dairy products as adults; their Paleolithic ancestors could not do this.
Disagreements From Within
Even within the Paleo community, there is often impassioned disagreement over which foods are or are not truly "Paleo." A casual Internet search of "Are potatoes Paleo?" or "Is alcohol Paleo?" demonstrates these debates. In general, people seem to take sides according to whether or not they believe the food in question to be "healthy" rather than whether evidence supports the idea that their ancient ancestors really ate it. If someone follows a Paleo diet and also happens to believe a potato is "healthy," she'll be likely to argue that it's "Paleo"; if she believes it to be unhealthy, she'll argue the opposite. At that point, the most relevant question seems to be less about "What did our early ancestors really eat?" -- which we can't know for sure, anyway -- and more about "What does science tell us is healthiest for us to eat today?"
Possible Benefits of the Paleo Diet
There's some experimental support for the notion that eating a so-called "Paleo diet" may have some beneficial effects. A review article published in 2009 in the "Journal of Diabetes Science and Technology" describes a small study that compared people with diabetes who ate a Paleo diet versus diabetics who ate the diabetes diet. According to the study, the Paleo diet resulted in "statistically significant lower mean values of hemoglobin A1c, triglycerides, diastolic blood pressure, weight, body mass index, and waist circumference." There were also significantly higher levels of HDL -- the good cholesterol -- in the followers of the Paleo diet. Both diets effectively controlled blood sugar. The authors of the same review article note similar findings -- lower blood pressure, better weight control and better blood lipid profiles -- in a handful of studies looking at the potential benefits of a Paleo diet on certain risk factors for cardiovascular disease.