Endurance athletes who 'go against the grain' become incredible fat-burners
Elite performance on a diet with minimal carbs represents a paradigm shift in sports nutrition
Elite endurance athletes who eat very few carbohydrates burned more than twice as much fat as high-carb athletes during maximum exertion and prolonged exercise in a new study -- the highest fat-burning rates under these conditions ever seen by researchers.
The study, the first to profile elite athletes habitually eating very low-carbohydrate diets, involved 20 ultra-endurance runners age 21-45 who were top competitors in running events of 50 kilometers (31 miles) or more.
"These low-carb athletes were spectacular fat burners," said lead researcher Jeff Volek, professor of human sciences at The Ohio State University. "Their peak fat burning and the amount of fat burned while running for three hours on a treadmill was dramatically higher than what the high-carb athletes were able to burn.
"This represents a real paradigm shift in sports nutrition, and I don't use that term lightly," he said. "Maybe we've got it all backwards and we need to re-examine everything we've been telling athletes for the last 40 years about loading up on carbs. Clearly it's not as straightforward as we used to think."
The 10 low-carb athletes ate a diet consisting of 10 percent carbs, 19 percent protein and 70 percent fat. Ten high-carb athletes got more than half their calories from carbs, with a ratio of 59 percent carbs, 14 percent protein and 25 percent fat.
In all other respects, the athletes were similar: elite status, age, performance, training history and maximum oxygen capacity. "They all had the same engine, so to speak," Volek said.
Scientists measured gas exchange repeatedly during a test determining the athletes' maximum oxygen intake to gauge carb- and fat-burning rates. On average, the low-carb runners' peak fat-burning rate was 2.3-fold higher than the rate for high-carb athletes: 1.5 versus .67 grams per minute.
The research is published online in the journal Metabolism: Clinical and Experimental.
Volek has been studying the effects of low-carb eating -- and ketogenic diets specifically -- for years, particularly in the context of obesity and diabetes. But he has always been interested in how such a diet might augment physical performance and recovery. Ketogenic diets are those that reduce carbohydrates enough to allow the body to access its fat stores as the primary source of fuel. Lowering carbs and increasing fat intake leads to the conversion of fat into ketones, molecules that can be used by cells throughout the body, especially the brain, as an alternative to glucose.
It can take weeks or longer for the human body to fully adjust to a ketogenic diet, so the low-carb athletes in the study were eligible only if they had been restricting carbs for at least six months. Their average time on a ketogenic diet was 20 months.
"The goal was to characterize their metabolic response to a standardized exercise test," Volek said. "This is the first time we've had the opportunity to peek under the hood at what a long-term low-carb, fat-adapted athlete looks like."
Over two days, researchers subjected the athletes to tests to determine peak fat burning during a brief high-intensity workout and metabolic characteristics during prolonged exercise.
On day one, the athletes ran on a treadmill to determine their maximum oxygen consumption and peak fat-burning rates. On day two, the athletes ran on a treadmill for three hours at an intensity equal to 64 percent of their maximum oxygen capacity. During this test, they drank water but took in no nutrition -- before the run, athletes consumed either low- or high-carb nutrition shakes consisting of about 340 calories.
During the endurance run, the two groups did not differ significantly in oxygen consumption, ratings of perceived exertion or calorie expenditure. However, fat-burning rates during prolonged exercise were again about twice as high in the low-carb athletes, and the average contribution of fat during exercise in the low-carb and high-carb groups was 88 percent and 56 percent, respectively.
"The low-carb guys go beyond what you can achieve with good genetics and extensive training," Volek said. "The high-carb runners were very healthy, and were awesome fat burners by conventional standards -- yet their peak fat burning is less than half that of endurance athletes eating low-carb diets. This shows that we have far underestimated how much fat humans can burn. There is a large reserve capacity that can only be tapped if carbs are restricted.
"So far, this has been a grassroots movement. Athletes on their own have been going against the grain, so to speak, and experiencing a lot of success. I think it's mainly taken off in the ultra-endurance world because the self-perceived benefits are so high there, but many other athletes competing in a variety of events and various sports teams are experimenting with carb restricting," Volek said.
Another key finding: Despite their low intake of carbs, these fat-burning athletes had normal muscle glycogen levels -- the storage form of carbohydrates -- at rest. They also broke down roughly the same level of glycogen as the high-carb runners during the long run, and synthesized the same amount of glycogen in their muscles during recovery as the high-carb athletes.
"This was completely unexpected, but now that we have observed it we have some novel ideas why this is the case. We can only speculate on the mechanism behind it," Volek said.
Muscle glycogen was discovered in the 1960s to be a critical energy source for athletes, which led to decades of emphasis on high-carb diets to support energy needs during intense exercise. But Volek said the body has an elegant system to support glycogen levels even when carbohydrates are limited in the diet.
"The blue print for becoming 'fat- or keto-adapted' is hard wired into our genetic code. However, traditional 'healthy' diets with carbohydrates as the dominant nutrient prevent this alternative metabolic operating system from ever booting up.
"Restricting carbs allows the program to reboot and enable many athletes to achieve improved levels of health and performance" he said.
The study itself is open access.
The bit about glycogen is interesting. From earlier talks by Phinney and Volek, I would have expected the low carb group to have conserved their glycogen during the run. The usual scenario described was that burning more fat, keto runners would conserve glycogen, the suggestion was that what would happen is that they'd begin the race with lower glycogen levels, due to their diet, but would deplete glycogen more slowly, making them less likely to bonk by the end of the race. Now I'm not sure if this was based on actual data or extrapolation... they had less data points to work from, but I know they had some data on endurance athletes on a keto diet having a higher peak fat oxidation than high carb athletes. And in short term studies, certainly people on a low carb diet will have lower glycogen levels... put the two together, and the "usual scenario" seems like a reasonable hypothesis.
But maybe we gain something, where we lost something. The bonk-proofing of a ketogenic diet was supposed to work because glycogen was preserved--an unfortunate corrollary of that was that maximal effort was supposed to be compromised. Maybe it's not so compromised. These athletes were able to dip into their glycogen as necessary, just fine--and their maximal fat burning was quite high. The glycogen actually measured in the study was muscle glycogen, liver glycogen might have been a different story.
The test was 65 % of VO2 max for a three-hour treadmill run--it would be interesting to see differences in actual distance run.
Other studies have shown
that a low-carbohydrate/high-fat diet decreases resting glycogen and the rate of glycogen use
during submaximal exercise (25,26). The duration of the LC diet was shorter (4 wk) in the work
by Phinney (10), suggesting that complete adaptations in glycogen homeostasis and kinetics may
take several months. The different glycogen responses could also be due to lower carbohydrate
intake, which was <10 g/day in cyclists (10) versus 86 g/day in the LC runners. A short-term
glycogen loading effect is unlikely since food logs were recorded for the two days leading up to
testing and indicated the average carbohydrate intake was 64 g/day in LC athletes
(Supplemental Table 1). The small relative contribution of carbohydrate to energy expenditure
in LC athletes, but similar use of glycogen as HC athletes, indicates a decreased reliance on
circulating glucose in the keto-adapted athlete.
Okay, there was a somewhat higher carbohydrate intake than in the studies showing a different glycogen response. This is in the range that Ben Greenfield was in when he was doing keto, he was around a hundred grams of carbohydrate a day if I remember right.