Converting Fat to Energy During Exercise

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Fat is an important component of a diet designed to fuel exercise. One gram of dietary fat equals nine calories, and one pound of stored fat provides approximately 3,600 calories of energy. This calorie density (the highest of all nutrients), along with our seemingly unlimited storage capacity for fat, makes it our largest reserve of energy.

These calories are less accessible to athletes performing quick, intense efforts like sprinting or weight lifting. But fat is essential for longer, slower, lower intensity and endurance exercise, such as cycling and walking.

Understanding Dietary Fat

Everything we eat is made up of macronutrients (protein, carbohydrates, and fat) and micronutrients (vitamins and minerals). These are converted to energy inside the body, helping to fuel all of our bodily functions.

Dietary fat has been blamed for many health problems, but it is actually an essential nutrient for optimal health. Adipose tissue (stored fat) provides cushion and insulation to internal organs, protects nerves, moves certain vitamins (A, D, E, and K) throughout the body, and is the largest reserve of stored energy available for activity.

Stored body fat is different from dietary fat. Body fat is only stored in the body when we consume more calories than we use, from any and all foods we eat, not just from dietary fats. There is an optimal level of body fat for health and for athletic activity.

How the Body Uses Fat for Fuel

Fat provides the main fuel source for long-duration, low- to moderate-intensity exercise (think endurance sports, such as marathons). Even during high-intensity exercise, where​ carbohydrate is the main fuel source, the body needs fat to help access the stored carbohydrate (glycogen). Using fat to fuel exercise, however, is not a simple process. It has three key parts:

  • Digestion: Fat is slow to digest and be converted into a usable form of energy. This process can take up to six hours.
  • Transportation: After the body breaks down fat, it needs time to transport it to the working muscles before it can be used as energy.
  • Conversion: Converting stored body fat into energy takes a great deal of oxygen, which requires decreased exercise intensity.

So athletes need to carefully time when and how much fat they eat. In general, it’s not a great idea to eat foods high in fat immediately before or during intense exercise. First, the workout will be done before the fat is available as usable energy. And second, doing so can cause uncomfortable gastrointestinal symptoms, such as nausea, vomiting, and diarrhea.

Popular High-Fat Diets

Popular low-carbohydrate and high-fat diets, such as the ketogenic diet and Paleo diet, all work on the same premise. The theory is that lower carbohydrate intake, coupled with high fat and moderate to high protein intake, leads to burning body fat as the main fuel source while exercising.

There is, in fact, some scientific evidence that long-term low-carb/high-fat diets are safe and may help improve metabolic risk factors for chronic disease. In studies, these diets have shown to be beneficial for performance in ultra-endurance sports—but at least several months of adaptation to a low-carb/high-fat diet are required for metabolic changes to occur.

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  1. Chang CK, Borer K, Lin PJ. Low-carbohydrate-high-fat diet: Can it help exercise performance?J Hum Kinet. 2017;56:81-92. doi:10.1515/hukin-2017-0025

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