In 1931, a Cleveland dentist named Weston A. Price embarked on a tour to observe local diets and health across all corners of the world. Price’s globetrotting—more than a dozen countries in a handful of years—would exhaust all but the hardiest backpacker today, but it’s truly impressive in light of his age (nearly 60 when he started) and the fact that this all took place before the advent of commercial aviation. Traveling on ships, trains, and by foot, he swung from isolated Swiss valleys to Scotland’s Outer Hebrides to far-flung native reservations in Alaska and Canada, to numerous Central and East African nations, to eight South Pacific archipelagos and the distant reaches of Australia and New Zealand, all the way to the thin air of the Peruvian Andes.

When he was finished, he wrote a book about his findings. Published in 1939, Nutrition and Physical Degeneration earned mostly dismissive reviews and soon fell into obscurity, where it stayed for most of the rest of the 20th century.

But in the last decade or so, Price’s teachings on nutrition have experienced a strange and dramatic renaissance in the diet and nutrition community. Though the average person has probably never heard his name, his work lies behind one of the most popular diet trends today—the low-carb, meat-loving Paleolithic (“Paleo”) diet.

Weston A. Price (Wikimedia)

The Paleo diet, as its name implies, espouses the theory that we should attempt to eat like our cavemen ancestors because this is what our bodies are evolved to do. Though what cavemen actually ate is far from clear (recent research suggests they enjoyed porridge, for example), the Paleo interpretation means lots of meat and seafood proteins, nuts, animal fats, and some vegetable oils, and little to no grains, dairy, or sugars. Considered fringe just a few years ago, the Paleo movement now has conferences, vacation retreats, magazines, and innumerable blogs and cookbooks. You can buy Paleo snack bars at Walmart and Paleo beef jerky at the supermarket. Jeb Bush eats Paleo. So does Miley Cyrus. Paleo also overlaps with an ever-growing number of buzzword-y diet philosophies—“whole foods,” “nutrient-dense eating,” “ancestral nutrition,” and so on—that eschew processed carbs and sugars in favor of meats, seafood, and a healthy dose of fat.

Many of today’s top-selling diet books reference Price by name, including The Whole30 (Amazon’s current number-one bestselling diet book), Perfect Health Diet (described by Vogue as “Paleo perfect”), and The Paleo Cure (a national bestseller). In the acknowledgements of his bestselling 2007 book, Good Calories, Bad Calories, Gary Taubes refers to Nutrition and Physical Degeneration as “[t]he book that may have been most influential in altering my perspective.” Price even makes an appearance in The Omnivore’s Dilemma, by Michael “don’t eat anything your great-grandmother wouldn’t recognize as food” Pollan.

In fact, he tends to pop up wherever the nutritional wisdom of our ancestors is being exalted—but in these instances, there’s a lot about him that’s usually left unsaid.

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In Nutrition and Physical Degeneration, Price presents remarkably consistent findings. Among the “primitives,” as he called groups who had not yet adopted a modern diet of processed foods, cavities were exceedingly rare. Tuberculosis was not a problem, nor were complications from childbirth. But according to Price, when a population started eating the “foods of modern commerce”—white flour, white sugar, canned fruits and vegetables—everything changed. Teeth rotted, TB ran rampant, and dental arches and nostrils became narrowed. (Price’s preoccupation with the width of dental arches and nostrils is a running theme throughout the book; modern medicine regards these as natural variations generally without impact on health.) Worse still, he wrote, were the “personality disturbances” and “mental backwardness” that modern food seemed to cause. These changes could only be due to diet, Price said: A group of “primitives” would be perfectly healthy, while a group in a nearby town or adjacent island, identical except for their access to modern foods, would be shockingly degenerated.

At the time of Price’s travels, the diets of the different so-called “primitive” groups varied widely. In the Swiss valley of Lötschental, for example, the villagers ate mostly locally produced milk and cheese on rye bread. Price had the dairy products chemically analyzed and declared them more vitamin-filled than commercial American dairy.

“It has been the achievement of the valley to build some of the finest physiques in all Europe,” he wrote, noting that many of the men from the area had been chosen to work as Swiss Guards.

But modern diets were already encroaching. Price was dismayed to see a new bakery in one of the villages selling white bread. In the Swiss resort city of St. Mortiz, he observed, children who dined on a modern diet of white bread, marmalade, and canned fruits and vegetables had teeth riddled with cavities. Price illustrated his findings from Lötschental, as he did throughout the book, with photographs of children having their mouths pried open for inspection by the large hands of an unseen adult behind them.

On the Scottish island of Lewis, where locals ate mostly fish and oats, Price described “teeth of unusual perfection,” in contrast to the “narrowed faces and nostrils” of “typical modernized Gaelics.”

Native Alaskans, who ate seal and whale fat, dried salmon, caribou, nuts, and native berries, demonstrated “an example of physical excellence and dental perfection such as has seldom been excelled by any race in the past or present.”

In East Africa, he met Maasai tribespeople who subsisted largely on raw animal blood, milk, and meat. Doctors in the region, Price reported, testified that the people suffered almost no cancer.

In the conclusion of Price’s book, he suggests a common theme in the diets he observed across the world: The healthy “primitives” ate plenty of meat, seafood, and fats. Americans would be wise to adapt their own diets accordingly, Price warned. Shortly before publishing the book, he’d gone to the Rutland State Fair in Vermont and had sat for an hour, observing the crowd. Three out of every four people he saw there, he said, showed signs of “prenatal injury” due to poor maternal nutrition.

Price died in 1948, and Nutrition and Physical Degeneration went out of print soon afterwards—though his ideas about dentistry, which included a tract on the evils of root canals, persisted throughout the 20th century despite being widely discredited in the 1930s. (Price pioneered “focal infection theory,” claiming bacteria in the tooth could escape during root canal treatment, causing anything from arthritis to heart disease. The American Association of Endodontists still distributes a fact sheet dispelling Price’s root-canal myths.) For decades, few Americans not acquainted with the history of dentistry knew his name.

Then, in the early 1970s, a Maryland mother of four named Sally Fallon walked into Yes Bookstore, a Washington, D.C. shop specializing in New Age titles. Fallon, who had a degree in English and an interest in nutrition, was in search of a new way to eat—she had several different health issues, and had become convinced that her supposedly “healthy” eating habits were the cause. An acquaintance had told her about an old book from the 1930s that talked the virtues of old-fashioned diets.

Fallon read Nutrition and Physical Degeneration and found that Price’s arguments made intuitive sense to her, especially his writing about the importance of animal fats. She started to feed her young children a high-fat, nutrient-dense diets—egg yolks, organ meats, cod-liver oil, the fatty pieces normally trimmed off cuts of meat—that she believes is largely responsible for her children’s lack of medical problems to this day.

“My kids didn’t need braces, they didn’t have any of the health problems that I had,” Fallon says. “No allergies. They were all just really healthy.”

In 1999, Fallon and the nutritionist Mary Enig created the Weston A. Price Foundation. As Fallon described it, the organization’s mission is “to encourage the consumption of animal fats and show how the science that demonizes animal fats is wrong.” It now has more than 400 local chapters across America, and some 40 international chapters from Australia to Malaysia to Romania.

A few years before launching the Foundation, Fallon published Nourishing Traditions, a cookbook based on Price’s teachings. Subtitled “The Cookbook that Challenges Politically Correct Nutrition and the Diet Dictocrats,” it was blurbed enthusiastically by Robert Atkins (he of the bacon and eggs diet), who said, “The first chapter of [Fallon’s] book is so right on target that I feel a little guilty for taking her ideas.” Full of recipes ranging from the prosaic (baked salmon) to the unorthodox (brain omelets), it’s since sold more than 600,000 copies.

In the cookbook, Fallon rails against the so-called “diet dictocrats,” her umbrella term for doctors, nutrition researchers, and various governmental and medical institutions like the Food and Drug Administration, the American Medical Association, university medical schools, and the American Cancer Society, all of which recommend limiting saturated fats to lower your risk of heart disease. Far from being villains, saturated fats, Fallon writes, are utterly critical to health.

On a typical day, Fallon tells me, she might eat eggs and meat for breakfast, or oatmeal with butter or cream. For lunch, she has cheese or pâté or fish eggs (“I call our diet the ‘pâté and caviar’ diet,” she jokes). For dinner she has meat (sometimes organ meat) or fish, usually with a sauce made from bone broth, and buttered vegetables with potatoes or brown rice. She’s frustrated that it’s so difficult to get animal brains these days.

Fallon’s cookbook does not mince words. Rejecting the “sustaining, nurturing food folkways of our ancestors,” it says, “cheats mankind of his limitless potential, destroys his will, and condemns him to the role of undercitizen in a totalitarian world order.” It calls vegetarianism a holier-than-thou form of “spiritual pride that seeks to ‘take heaven by force.’” It encourages readers to eat clay, for the “trace minerals.”

It is not the sort of cookbook, in other words, that one might expect to sell more than half a million copies.

But the publication of Nourishing Traditions coincided with—and likely influenced—a slow shift in Americans’ mentality about eating fat. Eggs and butter, demonized throughout the 1980s and 1990s as sources of heart-destroying cholesterol and saturated fat, are now generally considered fine in moderation. Margarine’s star has fallen. The fat-free carbs beloved by a generation of dieters are now believed to promote weight gain, replacing more filling fats and causing blood sugar to spike. Increasingly, sugar wears the devil horns in nutritional literature.

But perhaps the key to Price’s—and Fallon’s—current success is the way they venerate the wisdom of the past. We’re currently going through a period of romanticization of the dietary days of yore: Suburban homesteaders raise chickens in their yards, 20-somethings can their own jam, and Paleo dieters imitate their imagined ancestors with breakfasts of half-cooked steak. When your Facebook feed is full of stories about animal antibiotics and spinach recalls, it’s easy to see the past as a simpler and more wholesome time.

“If you’re convinced that modern food is dangerous, then it’s easy to believe that in the Paleolithic [era] when humans were just evolving and separating from the rest of the animal kingdom, it was more natural and we were healthier,” says Rachel Laudan, a food historian at the University of Texas.

But older diets tended to reflect reality more than choice—as many critics have pointed out, a high-fat diet was necessary when calories were hard to come by, a situation that likely doesn’t apply to most Paelo devotees today. “We’re in a funny stage,” Laudan says. “We have a huge food abundance, and so the old idea that you eat up or go hungry is no longer the case.”

When it comes to food, uncritically equating “old” and “good” can lead to unhealthy results. The Paleo diet was recently ranked as the worst of 2015’s most popular diets by a panel of experts assembled by U.S. News and World Report, earning low ratings on everything from weight-loss potential to safety. And the Price Foundation has aligned itself with a number of dangerous pseudoscientific ideas. Its board of directors includes a naturopath specializing in “magnetic, intuitive, spiritual, or psychic healing.” Its 28-page “Principles of Healthy Diets” brochure has a section on “holistic dentistry,” and recommends having your root canals removed. Its online “Ask the Doctor” column recommends homeopathic medicines for the flu and topical bloodroot paste as a home remedy for skin cancer. It’s firmly anti-vaccination, with the stance that a Price-approved diet is protection enough.

“Childhood diseases are either mild or non-existent when parents practice the kind of good nutrition that we advocate,” Sally Fallon writes. “Diets rich in vitamins A and C can protect children against disease much better than vaccinations, and with side effects that are good, never harmful.” Nearly 70 years after Price’s death, it seems, his faith in the old way of eating has resurfaced in more ways than one—some more dangerous than others.