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How Intermittent Fasting Can Make You Leaner, Less Fit, and Slower

Have you ever tried intermittent fasting, or considered trying it, as a way to improve your endurance performance? Then you’ll be interested in a new study that just came out of UC Davis. Led by nutritionist Ashley Tovar, it aimed to determine the effects of a 16/8 “time-restrictive feeding” (i.e., intermittent fasting) program on body composition and performance in runners.

Twenty-seven male runners between the ages of 21 and 36 participated in the experiment. Each subject completed four weeks of eating within an eight-hour window each day (16/8) as well as four weeks of eating on a normal 12/12 schedule, the order of these two schedules being randomized. The runners were instructed to eat the same types and amounts of food as normal on either schedule, so that only the timing differed.

Before and after each four-week period, the subjects underwent body composition testing and ran a 10K time trial. It was found that body fat decreased slightly, from 16.8 to 15.8 percent, on the time-restricted feeding program, while no change occurred on the normal eating schedule. Improvement in 10K times was about equal on both diets.

It’s easy to draw the wrong conclusion from these findings. In order to draw the right conclusion, we need to understand that it’s not easy to lose body fat and yet fail to improve running performance. That’s because shedding body fat boosts running economy. The fact that time-restrictive feeding failed to improve 10K performance more than normal eating despite triggering fat loss indicates that something about it counteracted the boost in running economy the runners got from getting leaner. In other words, intermittent fasting seems to have made these runners less fit at the same time it made them leaner.

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The authors of the study speculated that reduced carbohydrate availability may have been the main factor neutralizing the expected performance-enhancing effect of fat loss on the time-restrictive feeding program, writing, “Extreme depletions of carbohydrate availability have been shown to limit catecholamine responses, suppressing the effect of epinephrine in inducing glycogenolysis and the formation of lactate. The hypothesis that this effect was demonstrated in this investigation may be further supported in that suppression of catecholamines as a result of a persistent lower carbohydrate availability may reduce oxidizable fuels and lead to decreases in VCO2, as observed in this experiment.”

The authors went on to suggest that the self-sabotaging underfueling effect of intermittent fasting is likely to have even greater negative consequences on fitness and performance at race distances both longer and shorter than the 10K distance used in this study, explaining, “The indication that a [time-restrictive feeding] diet may lower lactate at higher intensities (90% VO2peak) suggests that performance during longer duration events requires a greater total contribution of carbohydrate as a fuel. Therefore, a ≥21.1 km race, and shorter duration events requiring a higher reliance on glycolytic type IIa muscle fibers, such as a 5 km race, may be more affected by the 16/8 diet.”

If all this talk of glycogenolysis and catecholamines is a bit over your head, here’s an analogy: Practicing time-restrictive feeding is like racing with a carbon-plated super shoe on one foot and a hiking boot on the other. In this analogy, the super shoe is the equivalent of the fat-loss effect of intermittent fasting and the hiking boot stands for its glycogen-depleting effect. One makes you faster, while the other makes you slower, and the net result is that you’re neither faster nor slower.

If TRF were the only way to lose body fat, certain athletes might still want to consider it as a way to improve, and least in races of moderate duration. But there are in fact other ways for athletes to lose body fat that do not leave the body underfueled and thus actually improve endurance performance. Improving overall diet quality and practicing intuitive eating are two such methods, which, unlike time-restrictive feeding, are practiced widely by elite athletes to promote a lean body composition without robbing the muscles of precious fuel.

I look forward to the time when the intermittent fasting fad has run its course in endurance sports. In the meantime, as an individual athlete, you can use it to your advantage. While others jump on the time-restrictive feeding bandwagon for reasons they believe are rational but are in fact psychosocial, and are fooled into thinking the diet has benefited them because they’ve lost a bit of body fat when in fact this would-be benefit has been neutralized by a form of semi-starvation, you can do the rational thing and simply emulate the less gimmicky eating habits shared almost universally by the world’s best endurance athletes and get both leaner and faster.