The Paleolithic diet (also called the paleo diet, caveman diet or stone-age diet) is based mainly on foods presumed to have been available to Paleolithic humans. Wide variability exists in the way the diet is interpreted. However, the diet typically includes vegetables, fruits, nuts, roots, meat, and organ meats while excluding foods such as dairy products, grains, sugar, legumes, processed oils, salt, and alcohol or coffee. The diet is based on avoiding not just modern processed foods, but rather the foods that humans began eating after the Neolithic Revolution when humans transitioned from hunter-gatherer lifestyles to settled agriculture. The ideas behind the diet can be traced to Walter Voegtlin,:41 and have been popularized more recently in the best-selling books of Loren Cordain.
Like other fad diets, the Paleo diet is promoted as a way of improving health. Limited data exist on the metabolic effects on humans eating the diet, though the available data suggest following this diet may lead to improvements in terms of body composition and metabolic effects as compared to the typical Western diet or as compared to diets recommended by national nutritional guidelines. Following the Paleo diet can lead to an inadequate calcium intake.
The digestive abilities of modern humans are different from those of Paleolithic humans, undermining the diet's core premise. Although little is known about the diet of Paleolithic humans, it is very likely that they consumed wild grains and legumes. During the 2.6 million year long Paleolithic era, the highly variable climate and worldwide spread of human population meant that humans were, by necessity, nutritionally adaptable; in contrast, supporters of the diet assume that human digestion has remained essentially unchanged over time. 
History and terminology
The idea of a Paleolithic diet can be traced to a 1975 book by gastroenterologist Walter Voegtlin,:41 which in 1985 was further developed by Stanley Boyd Eaton and Melvin Konner, and popularized by Loren Cordain in his 2002 book The Paleo Diet. The terms caveman diet and stone-age diet are also used, as is Paleo Diet, trademarked by Cordain.
In 2012 the Paleolithic diet was described as being one of the "latest trends" in diets, based on the popularity of diet books about it; in 2013 the diet was Google's most searched-for weight-loss method.
The diet is one of many fad diets that have been promoted in recent times, and draws on an appeal to nature and a narrative of conspiracy theories about how nutritional research, which does not support the supposed benefits of the paleo diet, is controlled by a malign food industry. A Paleo lifestyle and ideology have developed around the diet.
The diet advises eating only foods presumed to be available to Paleolithic humans; there is wide variability in the way this is interpreted. There is a debate surrounding the specific foods eaten by our ancestors.
In the original description of the paleo diet in Cordain's 2002 book, he advocated eating as much like Paleolithic people as possible, which meant:
- 55% of daily calories from seafood and lean meat, evenly divided
- 15% of daily calories from each of fruits, vegetables, and nuts and seeds
- no dairy, almost no grains (which Cordain described as "starvation food" for Paleolithic people), no added salt, no added sugar
The scientific literature generally uses the term "Paleo nutrition pattern", which has been variously described as:
- "Vegetables, fruits, nuts, roots, meat, and organ meats";
- "vegetables (including root vegetables), fruit (including fruit oils, e.g.,olive oil, coconut oil, and palmoil), nuts, fish, meat, and eggs, and it excluded dairy, grain-based foods, legumes, extra sugar, and nutritional products of industry (including refined fats and refined carbohydrates)"; and
- "avoids processed foods, and emphasizes eating vegetables, fruits, nuts and seeds, eggs, and lean meats".
The aspects of the Paleo diet that advise eating fewer processed foods and less sugar and salt are consistent with mainstream advice about diet. Like other low carb or high protein diets,[not in citation given] the Paleo diet's focus on protein from lean meat and seafood makes people feel full more quickly and so can help people eat less. Diets with a paleo nutrition pattern have some similarities to traditional ethnic diets like the Mediterranean diet that are healthier than the Western diet; however, following the Paleo diet can lead to nutritional deficiencies such as those of vitamin D and calcium, which in turn could lead to compromised bone health. There is also a risk of toxins from high fish consumption.
As of 2016 there are limited data on the metabolic effects on humans eating a Paleo diet, based on a few clinical trials that have been too small to have a statistical significance sufficient to allow the drawing of generalizations.[not in citation given] These preliminary trials have found that participants eating a paleo nutrition pattern had better measures of cardiovascular and metabolic health than people eating a standard diet, though the evidence is not strong enough to recommend the Paleo diet for treatment of metabolic syndrome. As of 2014 there was no evidence the paleo diet is effective in treating inflammatory bowel disease.
Rationale and counter-arguments
The rationale for the Paleolithic diet derives from proponents' claims relating to evolutionary medicine. Advocates of the diet state that humans were genetically adapted to eating specifically those foods that were readily available to them in their local environments. These foods therefore shaped the nutritional needs of Paleolithic humans. They argue that the physiology and metabolism of modern humans have changed little since the Paleolithic era. Natural selection is a long process, and the cultural and lifestyle changes introduced by western culture have occurred quickly. The argument is that modern humans have therefore not been able to adapt to the new circumstances. The agricultural revolution brought the addition of grains and dairy to the diet.
According to the model from the evolutionary discordance hypothesis, "[M]any chronic diseases and degenerative conditions evident in modern Western populations have arisen because of a mismatch between Stone Age genes and recently adopted lifestyles." Advocates of the modern Paleo diet form their dietary recommendations on its basis. They argue that modern humans should follow a diet that is nutritionally closer to that of their Paleolithic ancestors.
The validity of the evolutionary discordance hypothesis has been brought into doubt by recent research. Studies of a variety of populations around the world show that humans can live healthily with a wide variety of diets; humans have evolved to be flexible eaters. Lactose tolerance is an example of how some humans have adapted to the introduction of dairy into their diet. While the introduction of grains, dairy, and legumes during the Neolithic revolution may have had some adverse effects on modern humans, if humans had not been nutritionally adaptable, these technological developments would have been dropped.
Evolutionary biologist Marlene Zuk writes that the idea that our genetic makeup today matches that of our ancestors is misconceived, and that in debate Cordain was "taken aback" when told that 10,000 years was "plenty of time" for an evolutionary change in human digestive abilities to have taken place.:114 On this basis Zuk dismisses Cordain's claim that the paleo diet is "the one and only diet that fits our genetic makeup".
Diseases of affluence
Advocates of the diet argue that the increase in diseases of affluence after the dawn of agriculture was caused by changes in diet, but others have countered that it may be that pre-agricultural hunter-gatherers did not suffer from the diseases of affluence because they did not live long enough to develop them. Based on the data from recent hunter-gatherer populations, it is estimated that at age 15, life expectancy was an additional 39 years, for a total age of 54. At age 45, it is estimated that average life expectancy was an additional 19 years, for a total age of 64 years. That is to say, in such societies, most deaths occurred in childhood or young adulthood; thus, the population of elderly – and the prevalence of diseases of affluence – was much reduced. Excessive food energy intake relative to energy expended, rather than the consumption of specific foods, may underlie the diseases of affluence. "The health concerns of the industrial world, where calorie-packed foods are readily available, stem not from deviations from a specific diet but from an imbalance between the energy humans consume and the energy humans spend."
Adoption of the Paleolithic diet assumes that modern humans can reproduce the hunter-gatherer diet. Molecular biologist Marion Nestle argues that "knowledge of the relative proportions of animal and plant foods in the diets of early humans is circumstantial, incomplete, and debatable and there are insufficient data to identify the composition of a genetically determined optimal diet. The evidence related to Paleolithic diets is best interpreted as supporting the idea that diets based largely on plant foods promote health and longevity, at least under conditions of food abundance and physical activity." Ideas about Paleolithic diet and nutrition are at best hypothetical.
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 Paeleolithic ancestors; likewise, their nutritional profiles were very different from their modern 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; it and cabbage, cauliflower, and kale are modern cultivars of the ancient species Brassica oleracea.
Trying to devise an ideal diet by studying contemporary hunter-gatherers is difficult because of the great disparities that exist; for example, the animal-derived calorie percentage ranges from 25% for the Gwi people of southern Africa to 99% for the Alaskan Nunamiut. Recommendations to restrict starchy vegetables may not accurately reflect the diet of relevant Paleolithic ancestors.
Not all processed foods were introduced after the agricultural revolution; there is evidence early humans processed plant food and possibly prepared flour 30,000 years ago. Researchers have proposed that cooked starches met the energy demands of an increasing brain size, based on variations in the copy number of genes encoding for amylase.
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