A decade ago, we went crazy for Atkins. Now, a new grain-free, low-carb diet is sweeping the nation. The so-called “Paleolithic” diet – or “paleo” for short – instructs dieters to eat like their Stone Age ancestors ate. The premise of the diet is simple: your body evolved to eat a radically different diet than what most Americans eat today. Go back to that original diet, and you’ll lose weight and eliminate a host of diseases.

Have our bodies evolved to only consume foods found in a hunter-gatherer diet and not from agriculture? And, does the paleo diet, as outlined by the bestselling books by Loren Cordain and Robb Wolf, accurately capture what our cave-dwelling ancestors ate?

The Paleolithic era is defined as the “Old Stone Age.” Roughly speaking, it includes everything from the oldest use of stone tools by human ancestors in Africa up to the dawn of agriculture a mere 10,000 years ago. About 1.8 million years ago, our ancestors experienced a massive increase in brain size. The date humans acquired controlled use of fire is debated, but it likely occurred by about 300,000 years ago at the latest. And, at some point during this long period, our ancestors left Africa and spread throughout the world.

Needless to say, it’s impossible to accurately lump together the diet of every single human ancestor or even just the Homo sapiens who lived in this period. “The truth of the matter is there is no paleo diet,” summarizes Katharine Milton, a professor in the Department of Environmental Science, Policy, and Management at UC-Berkeley. “The only thing you can do is generalize very broadly and you can say beyond a shadow of a doubt that paleo peoples were eating wild plant and animal foods because there was no agriculture and there were no domesticated animals.” A piece in Nature backs her up, showing how difficult it is to reconstruct human diets of the distant past through a variety of means.

Proponents of the paleo diet attempt to distill it into an easy diet plan nonetheless. They tell dieters to eat grassfed meat, seafood, fruits and vegetables, eggs, nuts and seeds, and “healthful” oils (defined as coconut, olive, macadamia nut, avocado, flaxseed, and walnut oils). The list of prohibited foods includes what Cordain calls “Neolithic and industrial-era foods:” all grains, legumes (including peanuts), dairy, refined sugar, potatoes, processed foods, salt, alcohol, and refined vegetable oils.

According to Cordain, “The crucial aspect is to not precisely mimic the exact foods our hunter-gatherer ancestors ate, as this would be impractical or impossible, but rather to mimic the food groups they ate (fresh fruits, vegetables, meat, seafood, poultry, nuts) with commercially available foods from the supermarket.” According to him, “nearly 71 percent of the calories in the typical Western diet come from refined sugars, vegetable oils, cereal grains and dairy products – typically via processed foods. Our hunter-gatherer ancestors from any location on the planet or any time period rarely or never consumed these foods."

How does he know? Logic, he answers. It’s pretty easy to figure out what kinds of food you can’t get when you’ve got no agriculture and little more than stone tools to work with. But, he notes, there are other techniques one can use, including “ethnographic data from historically studied hunter-gatherers,” studying the chemicals in fossilized remains of human ancestors, and finding remains of butchered animal bones or even fossilized human feces.

No matter what, there are several aspects of this diet that deserve praise. Cutting down on sugar, salt, alcohol, and processed foods is a healthy move. So is switching to pasture-raised meat, if you eat animal products. And the oils recommended each provide healthy ratios of omega-6 to omega-3 fatty acids, thus addressing a common problem in the American diet. The diet also preaches variety, telling dieters to switch up what they eat every day, instead of relying on the same handful of foods. These are all concepts that are broadly recommended by many nutrition experts – and they can be adopted without turning back the clock 10,000 years to before the dawn of agriculture.

Let’s look at the actual food eaten by a hunter-gatherer societies in the recent past and then examine the elements of the popularized paleo diet one by one. In San Diego County and south of the border into Mexico, the Kumeyaay were hunter-gatherers until recent times and their diet is well recorded. Some foods and traditions are even maintained today. Their dietary staple was the acorn, which they gathered in the fall and stored. Once dried, around February, the acorns were ground into flour, leached to remove the bitter tannins, and eaten as a staple food called shawii. In addition to acorns, they ate wild game (deer, bighorn sheep, and rabbits), fish, seeds, seaweed, prickly pear cactus, greens, and wild fruit.

Much of their food was seasonal. During the winter rainy season, they could count on greens like miner’s lettuce. Around March and April, they harvested yucca and agave. Over the summer, they gathered seeds, including chia seeds and a wild grain, juniper and manzanita berries, mesquite beans (a legume), and pine nuts. Fruits included berries, prickly pear fruit, palm fruit, and some stone fruits. They dug edible roots, tubers, and corms from wild plants as well. And they had a source of salt, which they included in their diets.

The Kumeyaay were hardly vegetarians, but they did obtain protein from plant sources as well as animals, including from both grains and legumes. They did eat an enormous variety of foods throughout the year, but during some periods they might have been limited to a rather narrow range of foods simply because nothing else was available. Some of their foods are delicious, but you might not wish to eat some unless your only other choice was starvation. And, it’s likely that sometimes, that was the choice they were making.

One example does not make a rule, but the Kumeyaay diet blows through several claims made by paleo experts like Cordain. They ate salt, they ate grains, and they ate legumes. Logic tells us that our ancestors absolutely ate grains and legumes elsewhere in the world too. How do we know that? Because our ancestors ultimately domesticated grains and legumes and cultivated them as food on farms. What are the odds that an ancient people found an entirely inedible seed and began planting it and selecting it for desirable traits, trusting that eventually, perhaps in decades or centuries, it would evolve into an edible grain or bean?

It’s true that grains and legumes can be inedible in their natural forms. Acorns are too. But the Kumeyaay solved this problem through technology. For acorns, they found a way to remove the bitter tannins before consuming them. For grains, they toasted them over a fire and then ground them into a flour which was eaten as a dish called “pinole.” South of the border in Mexico, indigenous peoples there figured out how to make niacin in corn bioavailable by treating corn with lime in a process known as nixtamal. This allowed them to constitute complete protein with grains and beans and to avoid the disease pellagra that is caused by niacin deficiency.

“All humans do is transform their foods,” says Milton, commenting on the human ability to turn inedible substances into healthy foods with technology. “That's what being a human is. People only evolve in response to selective pressures. Many different very important foods are not digestible and humans transform them through culture.”

Cordain dismisses grains, calling them “nutritionally inferior foods compared to fresh fruits, vegetables, meat, poultry, fish and seafood.” He adds that, “most grains in the U.S. are consumed as fiber-depleted refined grains, and as such represent one of the greatest dietary contributors to the ubiquitous high glycemic load in the U.S. diet, which underlies numerous health issues including obesity and the metabolic syndrome.”

True – but why not simply tell people to eat whole grains instead of refined ones?

Cordain adds a concern about gluten-containing grains (wheat, rye and barley) because they cannot be eaten by the small percent of the U.S. population that suffers from celiac disease or the slightly larger group of Americans with gluten allergies or sensitivities. Well, it makes sense that those with celiac disease or allergies should avoid gluten, but why does that mean we all should?

How about dairy? In his book The Paleo Diet Revised, Cordain explains the dairy prohibition, saying, “Paleolithic people ate no dairy foods. Imagine how difficult it would be to milk a wild animal, even if you could somehow manage to catch one.” Good point, but early humans did consume dairy in the same way all mammals do. Human infants drank breast milk. Humans did evolve to consume and digest dairy as infants.

After weaning, Paleolithic humans had no reason to continue producing lactase, the enzyme needed to digest lactose. But sometime after the emergence of agriculture, after humans domesticated livestock, some humans were born with a genetic mutation allowing them to continue producing lactase after weaning, into adulthood.

Milton calls this “a classic case of how human culture modifies their environment and then humans adapt to their own changes in the environment.” First, humans domesticated livestock, and then any individuals with the genetic mutation allowing them to digest dairy as adults had an advantage over those who did not.

She notes that whereas some societies with livestock evolved adult lactase secretion, others used technology instead of genes to consume dairy products. “They figured out a way to get the lactose to be eaten by bacteria or drained out — maybe they made a yogurt or something like that — and then they eat the material that remains and it isn't full of lactose anymore.”

Cordain acknowledges that some 35 percent of the world’s population can digest lactose into adulthood, but points to dairy as the cause of cancer risks, insulin resistance, and acne. A recent study did find a link between high-fat dairy and mortality from breast cancer, but it recommends replacing high-fat dairy products with lowfat or nonfat ones, not cutting dairy out entirely.

What about the paleo diet’s claim that one must eat meat? In his book The Paleo Solution, Robb Wolf writes, “Your protein source needs to have the following criteria:

1. It needs a face.

2. It needs a soul.

3. You need to kill it, and bring its essence into your being.

4. Really.”

Cordain gives vegetarians the bad news a bit more gently, but the data he cites does not back up his assertions. In fact, one study he names backs up the health benefits of a vegetarian diet.

“Ancestral hunter-gatherer diets were never vegetarian,” Cordain notes – perhaps correctly. But he goes on to claim, “If they were, these diets would have been rapidly culled by natural selection because they are eventually lethal. Humans require vitamin B12 which is not found in plants, but only in animal foods.”

It’s true that humans require vitamin B12, which is only found in animal foods – but vegetarians do eat animal foods in the form of dairy and eggs. It’s vegans who eschew all animal foods, not vegetarians.

Cordain cites two studies that found that vegetarian diets are not more healthful than omnivorous ones. The first was an Oxford University study published in 1999. Cordain quotes the study’s abstract, which reads, “There were no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, lung cancer, breast cancer, prostate cancer, or all other causes combined.”

First of all, this means that vegetarians are no healthier than meat-eaters, but it also means that they are no less healthy.

However, Cordain neglects to mention the sentences that precede what he quotes. These read, “Mortality from ischemic heart disease was 24% lower in vegetarians than in nonvegetarians… Further categorization of diets showed that, in comparison with regular meat eaters, mortality from ischemic heart disease was 20% lower in occasional meat eaters, 34% lower in people who ate fish but not meat, 34% lower in lactoovovegetarians, and 26% lower in vegans.”

In other words, Cordain is selectively quoting this study’s findings to give a false impression of the results. The study found that vegetarians are 24 percent less likely to die of heart disease than their meat-eating counterparts, and no more or less likely to die of anything else.

A second, more recent study he cites also found no differences in mortality between vegetarians and meat-eaters. Again, he quotes from it selectively, noting that “Within the study, mortality from circulatory diseases and all causes is not significantly different between vegetarians and meat-eaters,” leaving off the rest of the sentence: “but the study is not large enough to exclude small or moderate differences for specific causes of death, and more research on this topic is required.”

Yet Cordain says, “In fact, if the truth be known, your lifelong dietary deprivations will not prolong your lifespan but rather will produce multiple nutrient deficiencies that are associated with numerous health problems and illnesses. If you have forced plant-based diets upon your children, or unborn fetus they will also suffer.”

Long story short, while many aspects of the paleo diet are uncontroversial and beneficial, like increasing fresh fruit and vegetable consumption, switching to pasture-raised meat, and cutting out processed foods, the overall premise of the diet as well as some of its key components appear based on pseudoscience and unsubstantiated claims. But, you might notice that many of the most popular, well-known paleo diet Websites sell books, diet plans and memberships. It appears that this diet might be more successful in generating profit for its proponents than producing health for its followers.