Ask a group of marathoners why they run, and then count how many of them say, half-seriously, “So I can eat as much as I want.” The wish list usually includes carb-heavy foods like pasta, fruit, mac and cheese, pancakes, and Cap’n Crunch. For decades now, the value of carbs for improving performance and preventing bonks has been drummed into the heads of runners, cyclists, and triathletes by Ph.D.’s like Ed Burke, Tim Noakes, and Ed Coyle.

The way these experts framed it, high-mileage runners weren’t eating enough sugars and starches, no matter how hard they tried. Whenever a new diet came along that opposed this basic carb-loading idea, as the paleo diet famously began doing a few years ago, runners tended to yawn and make a third trip to the pizza bar. Paleo was especially easy to ignore given its restrictions: no pasta, no legumes, no bread, no dairy. No beer.

Through diet crazes like the Zone, Atkins, and paleo, I too stuck with carbs, chugging sports drinks on a daily basis. At the aid stations of Ironmans, I chased PowerBars with Gatorade and Coke. During a period when I dug into a bag of Chips Ahoy several times a day, I weighed 158 pounds and could run a 2:38 marathon. My belief was in sync with other running geeks: just burn it off.

Perhaps the first to break ranks with this way of thinking was Mark Sisson, who was once a 2:18 marathoner and an elite Ironman triathlete. Sisson says the high-carb doctrine is disastrously wrong, for runners and everyone else. Over the past decade, he’s been using his popular blog, Mark’s Daily Apple, to warn people that such a diet will sooner or later rob you of health. Sisson believes that regular payloads of carbs disrupt the body’s hormonal balance, leading to chronic inflammation, insulin resistance, and even leaky gut—porous, nutrient-wasting intestines. These are complications you won’t be able to cure by running farther.

Sisson was 28 when his health collapsed, despite 30 hours a week of exercise. He’s 62 now. Instead of distance runs and bike rides, his weekly routine consists of a couple of trips to the gym for workouts that may last as little as 15 minutes, an Ultimate Frisbee game with twentysomethings, and maybe a long hike on the trails of Zuma Canyon, near his hilltop home in Malibu, California.

“Sometimes I think about going for a run the way I used to,” he says, “but I get about a half-block before I think, What’s the point?” And yet, Sisson’s body-fat level, around 10 percent, is roughly the same as it was when he was a young athlete. His secret, he says, is that he eats a diet composed mostly of animals and vegetables, free of cereal and grains, relatively high in fat, and low in carbs. Somehow this has reengineered his cells to make them super-efficient at burning fat.

These days, Sisson earns a very good living by selling a modified caveman lifestyle to fitness buffs, defined by ten “primal” laws, including “Move around a lot at a slow pace,” “Get lots of sleep,” and “Play.” Mark’s Daily Apple gets two million hits per month, and Sisson has sold more than 400,000 copies of his book The Primal Blueprint. Though his wavy hair has gone from surfer blond to gray, he looks just as fit now—maybe fitter—as he did when he appeared on the cover of Runner’s World in 1986.

I started poking around Sisson’s blog early last year. I’d been frustrated by a chronic Achilles-tendon injury in my left leg, and I had moved from the Bay Area to Boston, where blizzards limited my exercise to occasional walks around the block with my wife, our infant son, and our dog. That, coupled with a few workouts at an overpriced CrossFit gym, put me in a funk. TV and Dogfish Head IPA became appealing countermeasures, and the consequences soon rolled in. At my annual checkup, I stepped on a scale and watched the pointer go past 200. My body-mass index was around 27 percent.

Sisson caught my eye because of an old blog post he wrote that pinpointed my excuse for not exercising—the expense. He touted a “prison workout” that required “no specialized equipment, no gimmicks, no late-night infomercial tchotchkes, no gym membership.” All you needed was a cell-size amount of space and the will to work hard. “It is you against everyone else in prison, so you better get in shape,” he barked.

The basic routine involved lots of burpees. Back in the 1940s, an exercise physiologist named Royal Burpee invented this full-body exercise, which was used to assess the fitness of U.S. Army recruits. From a standing position, you drop down into a squat, then thrust your legs backward so that you land in the push-up position. You do the push-up, then jump back to upright and stand for a count of one. Then you do it all again, and again, fatigue building rapidly as you go.

Aside from pared-down exercise regimens, Sisson’s main sell was his take on the paleo diet, which stems from a belief that modern humans would be much healthier if we ate the way our ancestors did thousands of years ago, before agriculture gave us wheat, grain, and, more recently, packaged foods made with vegetable oils and high-fructose corn syrup. Sisson’s advocacy of paleo initially made me reluctant to buy in. I had talked to a few paleo CrossFitters at the gym before, and they’d droned on and on about all the bacon they ate. But Sisson wasn’t like that.

He has his share of critics, those who say that this style of nutrition is baseless. A 2013 article in Scientific American, “Why the Paleo Diet Is Half-Baked,” attacked Sisson and the archetype he created in his writing: “Grok,” a male hunter-gatherer living 10,000 years ago in the Central Valley of California. The article, by prominent science journalist Feris Jabr, claimed that Sisson and his ilk were wrong to suggest that human evolution hasn’t been able to keep up with changes in the foods we consume. “Grok cannot teach us how to live or eat,” Jabr wrote. “He never existed. Living off the land or restricting oneself to foods available before agriculture and industry does not guarantee good health.”

Even so, I felt drawn to Sisson’s ideas. He’d been a runner—a good one. He quit competing after coming down with osteoarthritis, a series of respiratory infections, and irritable bowel syndrome. He apparently had reverse-engineered his problems and concluded that diet and “chronic cardio”—doing obsessive amounts of cardiovascular exercise—were to blame.

In March of last year, I talked to Sisson on the phone, asking him how I could use his Primal Blueprint diet to get back in shape. “Body composition is 80 percent the result of your diet,” he told me. “The first thing it’s going to do for you, T.J., is get rid of that twenty-pound backpack you’re wearing.”

Stick to a low-carb diet for 21 days and I would begin to transform myself from a sugar burner into a fat-burning beast, as he put it. This gets to the why of paleo: if the human genome has changed little in the past 10,000 years, then we have the hunter-gatherer genes encoded within us. According to Sisson, carb deprivation would switch my genes into fat-burning caveman survival mode. After this “fat-adaptation phase,” I should experiment with intermittent fasting. “Once you’re fat adapted, you’re not chained down by hunger,” he said. It’s no big deal to miss meals when your liver is efficiently breaking down fat into usable energy, including ketones, which your brain can use when glucose levels are low.

Ketones sounded like something out of Dune, and reengineering cells sounded like something out of Philip K. Dick. But everything Sisson told me seemed to be supported by research. Oddly enough, it turned out that the most influential sports scientist in the endurance world, Tim Noakes—an M.D., a professor emeritus at South Africa’s University of Cape Town, and the author of The Lore of Running—had become a proponent of low-carb diets. After talking to Sisson, I Skyped with Noakes, and he confirmed the change of heart.

“For 30 years I was part of the problem,” Noakes said. Then, after failing to control his weight with running, he adopted the low-carb diet and lost 22 pounds in eight weeks. He put his Type 2 diabetes into remission. He’s been on the low-carb warpath ever since, despite calls for his head from colleagues who think he’s promoting a lifestyle that will cause cardiovascular disease.

One reason endurance athletes like to do workouts of two hours and longer is to improve the body’s fat-burning efficiency. The idea is to spend enough time exercising at a low intensity that you burn through muscle glycogen stores and shift to burning fat. But Sisson—as well as Noakes—argues that carb restriction is the superior path.

Paleo advocates point to ultrarunner Tim Olson as evidence that you can achieve elite performance on a low-carb, high-fat diet. Olson broke the 15-hour barrier in 2012 at the prestigious Western States 100-mile run—the first to do so, and he still holds the record—after adopting a high-fat diet devoid of sugar, grain, and wheat. On long training runs, he doesn’t carry any food or bottles. “If I’m thirsty,” he says, “I’ll drink from a stream.”

Sisson suggested I give Sami Inkinen a call if I needed any more convincing. Inkinen is the cofounder of Trulia, the online real-estate network, and a triathlete good enough to win his age group at big races like Wildflower. He used to follow the standard high-carb diet. “Sometimes I bonked in a race,” he told me. “I was always a little sick, always a little sore. I thought it was normal.”

Blood tests showed that he was pre-diabetic. Concerned, he started experimenting with changes in his diet and kept a vigilant eye on his blood-glucose levels. He looked at research by Steve Phinney and Jeff Volek, two Ph.D.’s who have focused on using a high-fat, low-carb diet to increase fat-burning capacity in endurance athletes. By simply flipping the carbs and fat intake, Inkinen says, he became a better fat burner. To demonstrate what he gained, he paddled a kayak with his girlfriend for 2,750 miles, from the coast of California to Hawaii, living for six weeks on a diet with a carb content of between 6 and 8 percent.

“The most amazing thing for me was that, after being on a boat for three months, I won a 103-mile bike race that had a lot of heavy climbing,” Inkinen said. “And I’m not a good climber. The high-fat diet is like a performance-enhancing drug.”

In April, I crossed a footbridge over the Colorado River in downtown Austin, Texas, on my way to the annual PaleoFX conference, where Sisson was scheduled to appear. I smiled at the fact that this gathering of leading ancestral-living experts was being held in the Palmer Events Center, a shiny quadrangle with hyperbolic curves and silver solar panels. It looked like a stranded spaceship.

The paleo diet is largely attributed to Loren Cordain, an exercise scientist and professor emeritus at Colorado State. Cordain based his work on a New England Journal of Medicine article from the 1980s called “Paleolithic Nutrition.” His hope was that Stone Age dietary principles could reverse the obesity epidemic that, according to current Centers for Disease Control and Prevention estimates, now affects more than 78 million Americans.

It took only a few minutes of lurking around PaleoFX to be reminded of how the movement has expanded from a cult diet into a lifestyle industry: the first thing that caught my eye was a couple wearing Flintstones costumes. I weaved through a giddy crowd to peek over shoulders at the stuff being sold in booths. I could buy things like Caveman Coffee, MegaSpore Biotic, and protein bars made with crickets. I saw a paleo dentistry booth and a probiotics skin company that used the slogan “Bacteria Is the New Black.” The idea here was that our skin hosts trillions of microorganisms. Because of daily showers and constant hand sanitizing, we don’t have all the microbes we need for healthy skin and hair, so it’s necessary to rub them back on. At one booth, a Mother Dirt product was selling for $49.

Sisson earns a very good living by selling a modified caveman lifestyle to fitness buffs, defined by ten “primal” laws, including “Move around a lot at a slow pace,” “Get lots of sleep,” and “Play.”

I met up with Sisson just before he went on for a talk. He was dressed in Banana Republic casual—slightly faded blue jeans, a cotton polo, and a nice watch on his tanned left arm. He has the body of a soccer player, with a waist I’d estimate to be around 30 inches. In a slightly surreal moment, his face appeared on a JumboTron above a stage adjacent to the exposition space while I was talking to the real thing. On the screen, he was savoring bites of a wrap made with an avocado-oil mayo he sells. The voiceover was his voice, and I would see this commercial about 20 times over the next couple of days.

The conference was one of the few chances that Sisson’s fans got to see him in person. They were here in force, wearing T-shirts bearing an illustration of the fabled Grok, clutching a spear in an outstretched arm while making a Nijinsky leap, with bold-faced words stamped below: LIVE LONG. DROP DEAD. An impromptu line formed in front of Sisson before he took the stage. He graciously accepted people’s thanks and listened to their stories, tales of how a hundred pounds had been lost or Celiac disease had been pushed into remission.

On stage an enormous picture filled the screen: it was Sisson’s son, Kyle, soaring through sea spray to catch a Frisbee above the surf. LIVE AWESOME, the slide read. (Sisson and his wife, Carrie, have been married 25 years and also have a daughter, Devyn.) Most of the pictures I had seen that related to Sisson’s Primal Blueprint were of Sisson himself. His blog is flooded with them: Sisson playing ultimate Frisbee; Sisson, shirtless and showing off a 12-pack, doing an L-sit chin-up under a dock; Sisson smiling, arms outstretched, crossing a slackline strung between two trees in his Malibu backyard. If you’re wondering whether Sisson might have an ego thing going on, he won’t argue. “I’m vain,” he confessed on his blog in 2010. “I want to look good naked.”

In his talk, Sisson’s mission was a bit different. Everyone in attendance was a believer, so there was no need to sell them on the gospel itself. Rather, his keynote was designed to rein in extremists.

“We embraced the movement so much we became almost militant about it,” he said, pacing the stage, ticking off the ways in which some paleo fanatics had gone too far. “We became a pain in the ass to dine with.”

Sisson emphasized the need for flexibility and perspective. If you decide to fast every time you can’t find the perfect “grass-fed, wild, line-caught, pasture-raised” food you want, then you’re missing the point. The point, he said, is to enjoy life, great health, and good food, not to adhere to rules at all costs. You’re at a kid’s birthday party and somebody offers you a piece of cake? Eat it.

“We stopped having fun,” Sisson said. “We got too attached to the numbers.” Too many blood tests, too many wearable devices, too much data. “We’re starting to see what happens if you take the dogma and push it too far.” The paleo obsessed were even becoming unhealthy, not to mention miserable. Sisson suggested an alternative: rather than judge the quality of your life by the degree to which you obey an arbitrary doctrine, stay loose and see what you can get away with. “Make choices that will allow you to enjoy your life,” Sisson said. “Do it.”

Back home, loaded up with info and inspiration, I focused on the basics. The Primal Blueprint starts with food but expands into other dimensions of Grok’s life, like the various ways he got exercise. (Walking long distances, lifting heavy objects, occasionally sprinting away from tigers.) You’re advised to get lots of rest and sunlight and avoid things that are poisonous or otherwise might kill you.

I got going on the food, loading my kitchen with vegetables, grass-fed beef, olive oil, berries, and butter. (Unlike many paleos, Sisson is not automatically anti-dairy.) My running injury was healing, but I was still too fat, so I tried my version of the prison workout. I joined a sullen, broken-down gym, Blast Fitness in Medford, on the outskirts of Boston. The place smelled iffy and the music was bad, but it cost just $10 a month.

Per Sisson’s advice, I tested myself to establish baseline numbers: maximum pull-ups (a good measure of strength) and max burpees (stamina). I managed five pull-ups, which wasn’t bad, but the burpee test was ugly. I set a countdown timer for 15 minutes and started. Before long I felt like I was going to throw up. I could only do 50.

Sisson's fans wore T-shirts bearing an illustration of the fabled Grok, clutching a spear in an outstretched arm while making a Nijinsky leap, with bold-faced words stamped below: LIVE LONG. DROP DEAD.

Mark’s Daily Apple is loaded with nutrition strategies, recipes, tips, hacks, and supplements. It was overwhelming. I zoomed in on what Sisson says makes the biggest difference: carbs. Every day I ate as many calories as I wanted, with olive-oil dressing, macadamia nuts, and avocados providing plenty of fat. But I tried to consume no more than 100 grams of carbs—in Sisson’s world, this is the basic secret to losing body fat and weight, with or without exercise.

The diet wasn’t especially difficult. I ate meat, fish, vegetables, and berries and avoided high-carb stuff like pasta, bread, and beans. (No more Dogfish Head IPA for me, alas.) Sisson’s plan allows for some flexibility, too. With a happy shrug, he puts heavy cream and a teaspoon of sugar in his coffee every morning. At night I enjoyed red wine and dark chocolate. I just kept an eye on the overall carb count.

The only annoying thing about Primal Blueprint was the flood of marketing offers that began hitting my inbox once Sisson’s company knew that I existed. I got suckered into a deal that sounded too good to be true: $1,033.97 worth of books, referred to as “the bodyweight bundle,” for just $37. All I really wanted was Amazing Feets!, Sisson’s book on how to switch over to a barefoot-dominant lifestyle. But damn, what a bargain.

Turns out it was too good to be true. I paid, received a bundle of e-files, and discovered that I couldn’t even find Amazing Feets!, which was buried in the folder somewhere. One of the so-called “books” was only seven pages long.

So, yes, that was irritating. But the low-carb program? That started working right off. Weight sizzled away.

In May, a work trip took me to Southern California, and I dropped by to meet with Sisson at his clifftop house in Malibu. I passed a shiny black Maserati on the way to his doorstep, circled a fountain, and brushed past a half-dozen palm trees in the front yard. Sisson greeted me, smiling, and asked me in. A baby grand piano sat in the foyer. (Sisson’s bucket list includes learning to play well enough to earn tips at a lounge bar.) Just past the piano was his office. I could see a stand-up desk and a computer.

We made our way to the back patio, passing his kitchen, freshly remodeled with white marble. On the far side of the center island, Sisson had installed refrigerated drawers, something I never knew existed. On the base of the island itself was a shelf crammed with paleo cookbooks. “I don’t cook,” Sisson confided. “I use the books to point out to the housekeeper what I’d like, and she makes it.”

The backyard had a pool, a whirlpool, and a fireplace filled with glass crystals. There was a canyon view. I walked near the edge and could see what looked like an equestrian ranch. “Laird Hamilton lives just over there,” Sisson said, pointing.

I asked Sisson about his path to Malibu dreamland. He opened an old folder full of yellowed clippings from his glory days as a runner and triathlete, starting back in the late 1970s. Sisson was elite in an era when the U.S. was awash with sub-2:20 marathoners—the golden age of Bill Rodgers and Alberto Salazar. In 1980, Sisson finished fifth in the U.S. National Marathon Championships. In 1982, he finished fourth in the Hawaii Ironman. Then the high mileage started to get to him.

Sisson was a serial entrepreneur from the beginning. After a stint when he painted houses for a living, he started making money as a trainer, using his Ironman cred to charge $100 per hour for his services. He signed on with the governing bodies of triathlon, ultimately serving for 15 years as the anti-doping chairman for the International Triathlon Union, during a time when triathlon had made it into the Olympic Games. Watching young triathletes brutalize their health in pursuit of Olympic gold got Sisson into the business of selling antioxidant supplements. He started Mark’s Daily Apple in October of 2006, eventually developing a large online following. In 2009, he self-published The Primal Blueprint. He has also published a line of cookbooks.

Later that day, we buzzed down Highway 1 in his Maserati. Sisson drove us to a private beach entry, and we walked down wooden steps to the white sands of Paradise Cove. We headed west, arriving at the spot where Charlton Heston falls to his knees in the original Planet of the Apes.

We plodded along in the bright sunshine, into a light, salty breeze, the rocky bluffs to our right and the blue-green Pacific to our left. I was thinking about a post I had read on LetsRun.com, where a discussion of Sisson was lit up by the following topic: Is he a legitimate athlete?

A correspondent using the handle Whirled Pees bypassed the question and attacked Sisson directly. “He’s in the business of selling you his ideas, so you will buy his products, thereby increasing his income,” he wrote. “Twenty years ago when he was poor, and only good at running, he probably went to a Tony Robbins seminar … and the rest is history.”

There’s a point to the snark: Who are you supposed to trust? On one hand, Sisson is offering all sorts of advice on his blog, but how can you not observe that much of it is directly tied in to what he sells? Walking on the beach, Sisson told me about a new restaurant chain he’s working on and various angel investments he has going in the paleo universe. If paleo doesn’t fade quietly away, like Atkins and Zone, then Sisson may be the reason why.

You can certainly connect the dots and follow the money—Sisson’s evangelism on behalf of paleo could sustain and maybe even grow the movement, increasing profits. But if what he says actually works, what’s wrong with that?

I talked about this with an M.D. friend of mine, Leon Chang, an anesthesiologist who believes in Sisson’s diet. “I learned absolutely nothing about nutrition in medical school,” he told me. “You have to do your research and consider the source, whether it’s from a doctor or not. And make sure they look the part and walk the walk. Sisson checks out.”

My results were certainly checking out. At three the next morning, I drove down to San Diego. A friend had decided not to race in the Rock ’n’ Roll Marathon and gave me his number. The only “training” I had done was carb restriction, Primal Blueprint style, along with my Blast workouts and a couple of runs to test out the Achilles. My weight was down to 188.

That day I ran slow, in the back of the pack, but I made a point of not drinking any Gatorade or eating any gels the entire race. If I had failed to make progress with Primal Blueprint, I would surely bonk. But as I descended Pershing Drive into Balboa Park, with just a couple of miles left of the 13-mile run, I hadn’t felt so much as a hitch in my energy. I finished with ease.

People like me—there are 29.8 million runners in the U.S.—are a big part of Sisson’s newest commercial wave. His next book is aimed at changing the minds of stubborn sugar addicts in the endurance crowd.

In June, I hit a snag. Progress had been steady—I was getting fitter from my prison workouts and decided to add sprinting to my schedule. Sprinting is a Primal Blueprint specialty; Sisson says it can turbocharge your training. But during the second session, I pushed it too hard and limped home.

Oddly, the best part of my experience in Primal—or paleo or whatever you call it—happened in early July. I dropped my son off at day care and went to Blast. Movers were carting the TVs out to a truck. “We’re closed,” a woman at the desk said with a glare. Closed for good.

On the drive home, I pulled over near a public soccer field, which had brittle, dying grass and hard ground. July in Boston. Hot and humid. I mashed together a prison workout—sets of burpees, air squats, and push-ups. The sun was punishing and good.

I followed up with a Grok stretch, and I felt great. A bit jarred and sick to my stomach, but great. I couldn’t fathom why I’d been doing my workouts in a junky fitness center to the sound of Pandora playlists. I told Sisson about it. “You’re starting to get the idea, T.J.,” he said. “You’re being intuitive.”

By the end of August, my weight had dropped to 179 pounds—24 pounds lighter than when I started. During my max pull-up test, I did 12—more than I’d ever done in my life. Then I did 182 burpees, another personal best, and this on a day when I ate less than 50 grams of carbs. According to the conventional wisdom I’ve always lived by, I should have lost steam because of the low carb intake. Granted, I really did want to throw up, and I felt light-headed, but my engine ran full throttle the whole way. Along with the half-marathon experience, this workout convinced me that the low-carb, high-fat protocol worked. Energy was not only plentiful but consistent. Even if my training program sagged, I stayed lean. My wife told me that I was a different person than that grumpy dude I had been last winter. “Night and day” was how she put it.

There were parts of Sisson’s program I struggled with. For one thing: minimalist shoes, which he recommends, just don’t work for me. I get injured. Maybe I’m too beat up or too old or too impatient, but there are times when it’s best to stick with what you know. In my case, a pair of heavy-duty trainers allowed me to run without pain.

Running reminded me of the most important lesson I got from Sisson: Let go, have fun. I had been freed from a rigid structure that limited what I got out of exercising. Along with being liberated from a gym, my running had been unboxed. It used to be that a one-hour run was a one-hour run—something I simply did, robotically recording the distance and pace in a logbook.

But the Sisson way changed me. I went out for a run on a wooded trail and spotted a pipe that supported a telephone pole. I jumped up, grabbed it, and did some pull-ups. On the trail, I started dropping down every two minutes for a set of ten push-ups. Later I did sets of step-ups onto a chunk of exposed granite. I wasn’t leaping through the air in a grass skirt with a spear in my hand, but it was a start.

T. J. Murphy (@Burning_Runner) wrote about Sealfit camp in January 2015.