Everything you eat can be divided into three main macronutrient (macros) categories: carbohydrates, fat and protein.
For the most part, protein is used to build and repair tissues and for enzyme and hormone production while carbohydrates and fats can be used as energy sources, per Washington State University.
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Carbohydrates in the form of sugar and starch are broken down into glucose, which is a readily available source of energy for the body. Carbs are easy for the body to burn and convert into useable energy, and they're easy to store and use later. (Cells, especially those in your brain and muscles, love carbs.)
This trifecta of ease and effectiveness causes the body to burn carbs first, before the other macros, in most cases. Let's look a little closer at each reason the body typically burns carbohydrates before fat for energy.
1. Carbs Are Easy to Use
When you eat carbohydrates, they're broken down from starch and sugars into the simplest sugar that your body can use: Glucose.
"Glucose is the sugar that can be utilized most quickly (and with fewer resources) by the body. It is commonly described as the quick and easy energy source for the body," says Reilly Beatty, RD, CSSD of Reilly Beatty Sports Nutrition.
2. Carbs Are Easy to Store
When you eat carbohydrates, your body breaks them down into the simple and useable sugar glucose that runs through your bloodstream and fuels your cells. Once your cells have enough energy, it then converts the extra glucose into glycogen through a process called glycogenesis, according to the Cleveland Clinic.
The glycogen is stored in the liver and muscle until it's needed, where it's converted back to glucose through the process of glycogenolysis. This process allows your body to tap into stored carbohydrates even if it's been several hours since you've last eaten.
But, your muscles and liver can only store so much glycogen at one time. Any excess carbs that are not used for energy and can't be stored are converted into fat and stored for future energy use, according to the Oklahoma State University Extension.
When Does the Body Use Fat for Fuel?
When all of your glucose and glycogen stores have been used up, your body can convert fat back into glucose to be used as energy. But your metabolism has to work harder to achieve this.
"To burn fat, oxygen has to be present; and when enough oxygen is available, fat will be utilized to provide energy for the body," says Beatty. Low-intensity exercise is an optimal time for the body to burn fat, whereas in high-intensity exercise, where oxygen levels are low, carbs are the only available source for energy, Beatty says.
3. Carbs Are Available Anytime
"Your body burns carbohydrates in the presence of oxygen but can also burn them when oxygen is not available. This is due to the molecular makeup of carbohydrates and their easy digestibility," says Beatty.
So whether you're sitting on the couch, lifting weights in the gym or sprinting to the finish line, carbohydrates can be used as an effective energy source.
4. Your Cells Love Carbs
Your brain needs a significant amount of energy to function. At least 20 percent of all of your body's energy needs go towards keeping the brain fueled, according to a May 2015 article in Neuron.
Although your brain can use fat for energy, it's not an efficient process. Fat must first be broken down into ketones for your brain to use them, and it doesn't use ketones as well as it uses glucose.
While your central nervous system needs glucose to operate, all of your other cells can efficiently use it for energy as well.
When Does the Body Break Down Protein?
In the event of starvation, when the body doesn't have enough carbohydrates or fat stored, it will start to break down protein stored in the muscles for energy.
- FAO Corporate Document Repository: Carbohydrate Food Intake and Energy Balance
- Washington State University: "Nutrition Basics"
- Cleveland Clinic: "Glycogen"
- Oklahoma State University Extension: "Carbohydrates in the Diet"
- Neuron: "A Cellular Perspective on Brain Energy Metabolism and Functional Imaging"