How Endurance Athletes Can Reap the Benefits of a Ketogenic Diet

STACK Expert Erick Avila explains ketone metabolism and expounds the benefits of a ketogenic diet for endurance athletes.

Ketosis is a process by which the body uses its internal fat stores for energy instead of glucose. Traditionally, nutritionists have recommended higher carbohydrate diets for athletic performance, but it appears that ketones constitute an alternative that can produce significant improvements in endurance performance.

The U.S. Army recently spent over $10 million researching an effective food to fuel soldiers on the battlefield, and officials were impressed by the effects ketone supplementation had on endurance performance.

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Here's a guide to the relationship between ketone metabolism and athletic performance.

Ketone Metabolism

Ketosis is a metabolic state in which people produce ketone bodies from their fat source for use as fuel instead of relying on carbohydrates. Ketone bodies are molecules produced in the liver from fatty acids. Their production can be stimulated by fasting, carbohydrate restrictive dieting, prolonged exercise or untreated Type I diabetes. Ketone bodies can be oxidized and converted for use as sources of energy for body tissue and brains.

Respiratory Exchange Ratio (RER) is a ratio of carbon dioxide production to oxygen consumption in the body. During low intensity exercise, we predominately use fat as our energy substrate, and this is reflected in an RER of around 0.70. The closer our RER moves to 1.0, the more carbohydrates we are using.

For higher intensity exercise, we prefer carbohydrates as a fuel source, because we're able to convert it more efficiently and quickly into usable energy. The glucose is typically derived from our circulating blood glucose, muscle glycogen and our livers.

Although it is a preferred fuel source, glucose is available in limited amounts in the body. As a result, once we deplete these energy stores, we're unable to perform at a high level.

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The take home message for endurance performance is the longer we can use fats as a fuel source, the more time we have before we start using the more finite source of glucose. When we are in a state of ketosis, our bodies become efficient at burning fat for energy. Ketogenic diets make our bodies more efficient at using fat instead of carbohydrates. For endurance athletes, this efficiency at fat oxidation results in better utilization of this substrate as a fuel source.

In one study, elite ultra-marathoners and Ironman triathletes were separated into groups following a high carbohydrate diet and a ketogenic diet. The athletes performed graded exercise tests and 180-minute sub-maximal runs at 64% VO2 Max. The athletes in the ketogenic group displayed higher rates of fat oxidation, and it occurred at a higher percentage of their VO2 Max (70.3% vs 54.9%). This is a significant adaptation, which had a direct impact on their RER.

The longer athletes are able to sustain performance from using glucose as a fuel source during endurance events, the longer it takes them to fatigue. In this particular study, the athletes were able to reach a higher percent of their maximal oxygen uptake before they started using glucose as a fuel source instead of fat. This has a direct carryover for endurance athletes, because it means they're able to exercise for a longer period at a relatively higher sub maximal intensity.

Typically, research on ketones and performance has been associated with achieving ketosis by maintaining a low caloric or high fat/low carbohydrate state. Ketone supplementation can be an alternative fuel for exercise by changing the heirarchy of substrate utilization. Ketone bodies can spare us from initially using our glucose reserves or muscle protein as sources of energy. Supplementation with ketone bodies has been shown to have a strong influence on suppressing glycolytic flux. A higher glycolytic flux means a greater accumulation of pyruvate, leading to increased acidity in the blood and the inhibition of fatty acids used as a substrate. The decreased usage of fatty acids means an increased reliance on glucose as a fuel source much earlier in a workout.

In a study of ketone supplementation by highly trained cyclists, the cyclists were able to add 400 meters of distance to their workouts. This was achieved by switching to ketone bodies as the primary source of cellular energy rather than using carbohydrates or fats, the energy substrates that more typically fuel exercise. The study found what previous research has shown—that glycolysis can be inhibited in the presence of ketone bodies.

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Types of Ketogenic Diets

There are many ketogenic diets and strategies for using them. Standard macronutrient ranges are around 60-75% of calories from fat, 15-35% calories from protein, and 5-10% calories from carbohydrates.

Some athletes cycle days when they follow a ketogenic diet. This is usually done by having 4-5 ketogenic days and 2-3 high carbohydrate days.

Others follow a targeted ketogenic diet where they add carbohydrates around their workouts.

When choosing how to fuel yourself, it's important to take into account the energy system demands of the sport you participate in.

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