Ramping up fat-burning seems like a good idea in theory, but it can hurt your ability to burn carbs.
Ramping up fat-burning seems like a good idea in theory, but it can hurt your ability to burn carbs. (Photo: Kaare Iverson/TandemStock)
Sweat Science

The Latest on Low-Carb, High-Fat Diets

We've finally got the data

pounds of butter lined up in row on a table.  The butter is being warmed up to room temperature for a commercial baker.
Kaare Iverson/TandemStock(Photo)

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In December, racewalker Evan Dunfee notched a huge personal best at a 50-kilometer race in Australia, smashing the Canadian record and punching his ticket to the Rio Olympics this summer. Just a few weeks earlier, he’d completed a three-week block of intense training on a diet of 75 to 80 percent fat, fueled by pre-workout boiled eggs and nutballs—“nuts, cocoa, and I’m not sure what else to hold them together,” he recalls, “but they were alright”—and mid-workout cheese and birthday cake.

Yet another anecdote in the ongoing low-carb, high-fat (LCHF) wars, you might say. But Dunfee is actually part of an ambitious multi-month study at the Australian Institute of Sport featuring an international group of Olympic-bound race walkers randomized to different diets. Along with the recent publication of a new study on the metabolism of fat-adapted ultra-runners and triathletes, the research marks the beginning of a new phase in the raucous debate over whether it’s possible, or even preferable, to compete as a high-level endurance athlete on a LCHF diet. In short: we’re finally getting some data.

Louise Burke, the head of nutrition science at AIS, is leading the Australian study. She and her colleagues spent more than a decade studying LCHF diets in search of a competitive advantage before concluding, in 2006, that the approach was a dead-end. But as the concept’s popularity surged across the blogosphere over the past few years, she took notice. “I think it is important to keep your finger on the pulse of what athletes are talking about or trying, and what new theories and ideas are being promoted,” she says.

The basic logic of the high-fat approach is simple. An average person can store about 2,500 calories of carbohydrates, enough to last a couple of hours; in contrast, even the slimmest of athletes has something like 50,000 calories of fat ready and waiting. If, by subsisting on a high-fat diet, you can train the body to burn primarily fat rather than carbohydrate, then you’ll never run out of fuel, and you’ll be freed from the need to suck down nausea-inducing quantities of gels and sports drinks.

By ramping up fat burning, you also curb carb-burning. That’s a problem.

The theory is fine. But in practice, what Burke and her colleagues found in the 1990s and 2000s was that by ramping up fat burning, you also curb carb-burning. That’s a problem, because carbohydrates provide the rapid fuel needed for high-intensity bursts. A 2006 study at Tim Noakes’s lab in South Africa had fat-adapted cyclists ride a 100-kilometer time trial interspersed with 4-kilometer surges and 1-kilometer sprints, simulating the thrust and parry of a Tour de France stage. While the overall times were similar, the high-fat cyclists were significantly slower on the race-defining sprints. That’s the study that Burke initially dubbed the “nail in the coffin” for LCHF.

There are a number of possible counter arguments to this objection. One is that the subjects hadn’t spent enough time adapting to a high-fat diet to fully reap the benefits. Another is that some people—ultrarunners, for example, particularly those whose primarily goal is simply to complete the distance—would gladly sacrifice the ability to charge up a hill or sprint to the finish in exchange for easier in-race refueling. But without data, it’s hard to draw firm conclusions either way.

To address the problem of adaptation time, a team led by Jeff Volek of Ohio State University recruited 20 elite ultra runners and Ironman triathletes, half of whom had voluntarily switched to a LCHF diet months or years earlier, and brought them to the lab for testing. The results, published this month in the journal Metabolism, showed that the fat-adapted runners were able to burn fat twice as quickly as the non-fat-adapted control group. During a three-hour treadmill run at a moderate pace, they relied on fat for 88 percent of their energy, compared to 56 percent for the controls. “The rates of fat burning are extraordinary based on conventional wisdom,” Volek says.

That was the good news. The more puzzling finding was that the fat-adapted runners still seemed to consume glycogen—the form in which muscles store carbohydrate—at exactly the same rate as the non-fat-adapted runners. “We can only speculate as to the reason they bother breaking down glycogen when it is not terminally oxidized,” Volek says. On the surface, that seems like a problem: If you’re burning the same amount of carbohydrate, do you really have an advantage? But observers like Paul Laursen, an exercise scientist with High Performance Sport New Zealand who was not involved in the study, see the results as further evidence of the adaptations that take place: “Things in the LCHF athlete work differently,” he says, “but better.”

Of course, no Olympic medals are awarded for having the most interesting metabolism. Actual endurance performance was not measured in the study, and the diets of the subjects were estimated by having them keep food logs for three days. These are not trivial details. One widely circulated article about elite endurance athletes turning to high-fat diets cited triathlete Simon Whitfield and Tour de France cyclist Dave Zabriskie as converts. But Whitfield, on his blog, estimated his dietary breakdown as 50 percent carbohydrate, 30 percent protein, and just 20 percent fat—far from the 75 percent theoretically required for LCHF territory. Zabriskie, meanwhile, says that his high-fat experiment was interesting but hardly ergogenic: “For long easy training, it’s good. For day-after-day racing like the Tour, you have to eat the carbs.”

That’s the gap that Burke and her colleagues at AIS hope to fill with their study of race walkers. While race walking is a relatively obscure sport, it may be a good fit for LCHF: the 50-kilometer race is the longest track-and-field event, lasting just under four hours, and the rules prevent anyone from breaking into an all-out sprint. The community of walkers is also tightly knit, which enabled Burke to recruit Olympic-caliber walkers from around the world to come to Canberra for several months and train together during the study.

The walkers were randomized to three-week training blocks under three different conditions: a traditional high-carbohydrate diet; a LCHF diet with 75 to 80 percent fat, 15 percent protein, and less than 50 grams a day of carbohydrate; and a “periodized” group that did some workouts with full carbohydrate stores, others while carb-depleted, and sometimes slept without refueling carbohydrate stores after an evening workout, a tactic recently shown to boost endurance performance. All the food was provided—and weighed to the ounce—by AIS staff, ensuring that the athletes followed their dietary plans exactly.

The results? Of the four athletes randomized to the first session of LCHF, Dunfee set his national record two weeks later, and South African walker Marc Mundell broke the African record in the same race. Still, Dunfee is hesitant to draw a direct link between the diet and his performance. After all, he was training hard with world-class peers. “It’s also clearly an individual thing,” he adds: a third member of the group felt terrible throughout the three-week period and dropped out of the 50-kilometer race. Dunfee also notes that his heart rate was higher and his times were slower during training, and that he switched back to carbs two weeks before the race. “In that time I was crushing my workouts,” he says. “I don’t know whether training on [the LFCH] diet was an additional stressor that made training seem like lower quality than it actually was, and I physiologically adjusted to the extra stress.”

The actual results won’t be tabulated until the runners have tried the different diets, and will be based on metabolic data and controlled 10-kilometre time trials. They’ll be available in time for Dunfee and his peers to decide which approach they want to use heading into this summer’s Olympics. Of course, they won’t settle the debate. Laursen, for example, believes it takes months, not weeks, to adapt to a high-fat diet and get over initial problems like the loss of high-intensity surging ability.

Whatever happens, we’ll soon know more about how the body responds to different diets than we do right now. “Nutrition is a cyclical science,” Burke says. “You’d be surprised at how many ‘new ideas’ are simply old ideas reimagined. So there is always the chance that it’s simply ‘hula hoop season’ again, and it will be a craze until it’s not. But there’s also a chance that new science will emerge.” 

Lead Photo: Kaare Iverson/TandemStock