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Low-carbing for endurance: the oxygen problem

You might have seen more low-carb endurance athletes popping up in the past few years. This trial tested a ketogenic diet in world-class athletes, compared to two different carb regimens.

Study under review: Low Carbohydrate, High Fat diet impairs exercise economy and negates the performance benefit from intensified training in elite race walkers

Introduction

Among the most contentious topics currently being discussed in the world of sports nutrition is the idea of fat adaptation for improving performance of endurance sports. If carbohydrate depletion is one of the main causes of fatigue during extended exercise and our bodies have a very large reserve of fat stores waiting to be used, it would make sense to try and tap into fat stores as much as possible. On the other hand, a compromise in energy production (in the form of ATP) efficiency needs to be taken into consideration, when performance is an important aspect, such as during competition.

During exercise, our muscles are primarily fueled by a mix of carbohydrates and fat, with protein contributing in minor amounts under certain circumstances. Generally speaking, carbohydrate becomes the predominant source of fuel as exercise intensity increases. If depletion of carbohydrate can impair exercise performance, it would seem logical that interventions aimed at increasing fat utilization over carbohydrate would be beneficial for endurance performance. However, it has been known for over 100 years[1] that more ATP can be generated from carbohydrate than from fat, per given volume of oxygen. Basically, from an oxygen cost standpoint, it’s cheaper to burn carbs.

This doesn’t matter much when we’re sitting or walking around, but when exercising at near-maximal intensities, the demand for ATP production increases significantly, which in turn pushes our energy producing systems (namely glycolysis for carbohydrates and Beta-oxidation/Krebs cycle for fatty acids) to their maximum capacity. Importantly, because fats have a pathway to generating ATP that requires oxygen, it becomes apparent that a shift in fuel utilization from carbohydrate to fat for high-output energy production may not be as great of an idea as it would appear to be on the surface.

In light of studies conducted in the 60’s showing a metabolic advantage of carbohydrate, as well as muscle biopsy techniques to measure [2]muscle glycogen levels[2], sports nutrition [3]guidelines[3] in the 1980s and 90s placed a very large emphasis on carbohydrate consumption before, during, and after exercise sessions. More recently, recommendations have shifted toward [4]fueling for the work required[4], meaning consuming a higher carbohydrate intake around higher intensity training sessions and a lower carbohydrate intake around lower intensity workouts that would rely more on fat as a fuel. This method of manipulating carbohydrate availability to the muscles has gained popularity in part due to [5]research[5] over the past 15 years showing [6]greater adaptations[6] to exercise deliberately undertaken in a state of depleted glycogen.

At the same time, a growing number of [7]athletes[7] and [8]researchers[8] have been looking into boosting the contribution of fat as a fuel during exercise in order to optimize endurance performance by following very low-carb, ketogenic diets. While it is clear that following low-carb diets can increase rates of fat oxidation during exercise, evidence of real-world performance improvements in trained athletes has yet to be shown.

The purpose of this new study was to determine the effects of a three-week adaptation to a low-carb, high-fat diet during an intense block of training for world-class endurance athlete race-walkers. This was compared to a traditional high-carbohydrate diet, as well as a diet that used a carbohydrate intake periodized around training sessions, with some sessions undertaken with high glycogen stores and others with low glycogen. This study is unique because these dietary interventions have yet to be compared against each other in an elite athletic population.

Endurance exercise is fueled by a mix of fat and carbohydrate. Dietary interventions to improve an athlete’s ability to burn fat have been postulated to improve endurance performance, although research on the subject is sparse. The purpose of this study was to determine the effects of a three-week adaptation to a low-carb, high-fat diet compared with two variations of high-carb diets on exercise metabolism and performance in elite level race-walkers.

Many people may be unfamiliar with the sport of race-walking. To give you an idea of how fast these athletes are moving, imagine this: the world record 20 km performance is 76:31, which is an average pace of 6:09 per mile (or 9.7 miles per hour). The qualifying standard for the Rio Olympics was 84 minutes (8.9 miles per hour pace).

Who and what was studied?

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Other Articles in Issue #27 (January 2017)