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A regimented nutrition strategy for marathoners

Some marathon runners go by “feel” when it comes to fluid and carb intake, which may worsen performance.

Study under review: Improved marathon performance by in-race nutritional strategy intervention

Introduction

As a reader of the NERD, you probably already appreciate the fact that most “hacks” for improved performance have very little effect. The paper behind this review describes the surprisingly large impact of in-race carbohydrate supplementation during marathons.

These days, it has become more and more normal for runners to participate in marathons or triathlons. In addition to requiring high oxygen uptake and high levels of musculoskeletal fitness (to withstand running impacts), these types of sports are very metabolically challenging.

During rest, your body burns 60-70% fat and 30-40% carbohydrates (CHO); during exercise, the relative amount of energy derived from fat decreases as intensity increases. This is because glycolytic (sugar-burning) pathways can deliver energy much faster than fat-burning pathways. As energy production requirements increase, metabolism gradually shifts from fat to CHO. This is relevant because depletion of CHO stores is a major issue in endurance sports. Depletion of muscle glycogen stores[1] in particular may be the single most important energy-related contributor to fatigue in endurance sports.

The problem of CHO depletion has typically been handled in two ways from a sports nutrition perspective: The first is to try to improve fat utilization at submaximal intensities, a phenomenon also known as improved metabolic fitness or “fat adaptation.” If you could change your fuel source from 50% fat/50% CHO to 55% fat and 45% CHO, this would, in principle, reduce the impact of running out of glycogen. Efforts to improve fat utilization involve doing massive amounts of low-intensity training (in which relative utilization of fat is highest), doing fasted exercise[2], and eating very little carbs, in some cases little enough to reach nutritional ketosis[3]. Although some of these strategies may hold promise, there hasn’t been much research testing them in different competition endurance sport formats, and these strategies often diverge enough from a typical athlete’s strategy that they aren’t considered.

The second way is trying to increase the availability of CHO in the body. The body typically holds 350-400 grams of CHO in the form of glycogen in the muscles, approximately 100 grams of glycogen in the liver, and 25 grams of blood glucose at any given time. For good runners, marathon pace corresponds to 70-80% of maximal oxygen uptake. At this intensity, the relative contributions of substrates are 60-80% CHO and 20-40%[4] from fat, depending on the metabolic fitness of the athlete[5]. Better-trained athletes tend to burn relatively more fat and are thus less reliant on carbohydrates.

Increasing CHO availability can be accomplished by increased pre-workout CHO delivery (so-called “carb loading”), glycogen supercompensation strategies (glycogen is depleted and then reloaded prior to racing, leading to overall greater storage), and peri-workout CHO delivery, which is the method the authors of the study under review examined.

Who and what was studied?

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What were the findings?

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What does the study really tell us?

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The big picture

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Frequently Asked Questions

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What should I know?

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Other Articles in Issue #05 (March 2015)