Converting Fat to Energy: Understanding the Process

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Fat is an essential component of a diet designed to fuel exercise. One gram of dietary fat equals nine calories. This calorie density (the highest of all nutrients) and our seemingly unlimited storage capacity for fat make 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 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.

Types of Body Fat

Scientists are learning more and more about body fat (or adipose tissue) and its roles in our bodies. Two well-known types of adipose tissue are white fat and brown fat. White fat is largely responsible for energy storage and metabolic functions like insulin sensitivity. Brown fat helps regulate body temperature.

Beige fat is another type of adipose tissue that scientists are still understanding. White fat can transition to brown fat under certain circumstances, like cold temperatures.

What Happens When Fat Is Burned?

You might wonder what exactly happens to fat cells as they are used for energy. When you use fat as fuel, the fatty acids inside the fat cell are broken down and released into your system as water and carbon dioxide. The carbon dioxide is exhaled through your lungs. Your body uses the water for hydration.

The remaining fat cell shrinks in size, now depleted of its fatty acids. This energy conversion also produces heat.

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 much oxygen, requiring decreased exercise intensity.

So athletes need to carefully time when and how much fat they eat. Generally, eating foods high in fat immediately before or during intense exercise is not a great idea. 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.

Optimizing Fat Loss

If you are trying to alter your body fat composition, the most important thing is to adopt a safe and effective exercise routine and to eat a balanced diet of nutrient-dense foods that provide you with adequate amounts of macronutrients, including dietary fat.

Macronutrient Balance

Popular low-carbohydrate and high-fat diets, such as the ketogenic and Paleo diets, all work on the same premise. The theory is that lower carbohydrate intake, high fat, and moderate to high protein intake lead to burning body fat as the primary 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.

High-Intensity Interval Training

One of the most efficient ways to convert fat to energy is through high-intensity interval training, also known as HIIT. In a study, overweight participants could convert body fat to energy in half the time using HIIT vs. aerobic activity alone. HIIT specifically converts visceral fat, typically white adipose tissue, and is often found in the midsection.

HIIT has another benefit of helping increase muscle mass, boosting resting metabolism. However, any exercise regimen that helps increase muscle mass has this beneficial effect, whether those gains are made through HIIT or another method.

Bottom Line

When working to improve health and fitness, balance is key. Focus on eating foods that make you feel well-nourished and making changes to improve your fitness level. As you progress, your body will likely change in ways that demonstrate your progress. If you have concerns about your eating plan or exercise, speak with a healthcare provider.

7 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Elizabeth Quinn, MS
Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics.