Do Low-Carb Diets Improve Endurance Performance?

Noted South African endurance sports expert Tim Noakes, M.D., is nothing if not an out-of-the box thinker and researcher. In the mid-1970s he was part of the group that first showed marathoners could suffer a heart attack, disproving a popular theorem of the time. In the 1980s, he was the first to recognise athletic hyponatremia, or excessive fluid consumption.

Now he has turned his attention to carbohydrate consumption. Noakes, author of the popular book Lore of Running, isn’t simply joining the swell of voices warning about too many sugary, over-processed foods. No, he’s questioning the marathon world’s favourite pasttime, carboloading for peak performance.

In an essay that appears in the online version of the British Journal of Sports Medicine, Noakes, Jeff Volek, and Stephen Phinney argue that the field of low-carb sports performance is woefully underinvestigated. They claim, in fact, that only 11 low-carb performance studies have been reported. Of those, nine found that the subjects performed better, or the same, on a low-carb diet as on a high-carb diet.

Moreover, only one of the 11 studies used subjects who were adapted to a low-carb diet. In other words, just as a minimalist runner needs time to adapt to thin-soled shoes, a low-carb eater needs time to adapt to a switch from the typical high-carb diet practiced by most runners. “Studies of elite athletes chronically adapted to low-carbohydrate diets have uncovered one unexpected finding – their extraordinary ability to produce energy at very high rates purely from the oxidation of fat,” Noakes and colleagues write.

In fact, Phinney published an endurance exercise paper in Metabolism in 1983. He found that, after a three-week low-carb diet, cyclists burned significantly more fat than they had on a typical carb-rich diet, and also exhibited a “four-fold reduction in muscle glycogen use.”

Humans store vast quantities of fat-energy in the body, but limited quantities of muscle glycogen, often referred to as “the preferred fuel source for vigorous exercise.” This glycogen is generally depleted after 32 kilometres of running, hence the dreaded “wall” in marathon races.

In Phinney’s 1983 study, cyclists endured the same amount of time on both the typical diet and the low-carb diet. It must be noted that they were exercising at just 62 to 64 per cent of their Vo2 max, compared to the 75 to 80 per cent that many runners maintain. As Vo2 max increases, so does the body’s dependence on glycogen as fuel, at least in classical exercise physiology.

Noakes et al base their hypothesis on the Paleo outlook.

“Because they live and train with chronically low blood insulin concentrations,” they write of low-carb eaters, “they have instantaneous access to those fat reserves at all times. Just as should occur in a metabolism crafted by our evolutionary history as predatory hunters.”

They conclude: “Now is the time to determine whether the conclusion that ‘there is very little or no evidence to support the use of high-fat diets’ is true.”

The preceding quote within a quote comes from Asker Jeukendrup, Ph.D., the highly regarded Dutch exercise physiologist and sports nutritionist who is global senior director of the Gatorade Sports Science Institute.

The battle lines have been drawn. “We have tested some athletes with peak fat-burning closer to 80 per cent of Vo2 max rather than the more traditional 65 per cent,” Volek told Runner’s World. “There is a lot of variation among athletes, and so many questions to be answered since there has been so little low-carb research relative to the thousands of studies done on high-carb diets. We hope to balance those out with well-designed low-carb experiments in different athletes.”


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