You can get energy from protein, but it’s not your best choice. Protein has other jobs to fill that take priority over using it as an energy source, such as building muscles and producing the protein-based substances that make muscles contract. It also takes your body longer to turn protein into energy compared to the quick boost you can get from carbohydrates.
From Protein to Amino Acids
When the proteins you eat are digested, they’re broken down into individual amino acids so that cells in your body have access to whichever ones they need for the job at hand. About 75 percent of amino acids are used to synthesize new proteins. These proteins help build and repair tissues, including muscles, bones and skin. They're also used to produce enzymes that digest food and activate your metabolism. Amino acids that aren't reassembled into proteins help make neurotransmitters and hormones.
Amino Acids Turned into Energy
Amino acids are chemically similar to glucose except that they contain nitrogen. This means that even after protein is digested into amino acids, they must go through more steps to have the nitrogen removed. Once the nitrogen is gone, the amino acids are converted into glucose or fatty acids. Either way, they give you energy. Due to the extra steps, protein provides a slower but longer-lasting source of energy than carbohydrates, according to the Merck Manual Home Health Handbook.
Your Body's Energy Preferences
When it's turned into energy, protein provides 4 calories of energy for every gram of protein you consume. This is the same amount you'll get from carbohydrates, but fats deliver 9 calories per gram. Any extra calories you consume are stored as fat because it's such a concentrated source of energy. When it needs energy, your body first uses glucose from carbohydrates, then fatty acids. As long as you consume enough calories from other sources, protein is not turned into energy.
Downside for Muscles
Your muscles can use fatty acids for energy when they’re resting, but when your activity level increases, they depend on glucose. The conversion of amino acids into glucose can fill the gap when you’re low on other sources of energy, but when protein is used for energy, ammonia is produced as a byproduct. During intense or extended exercise, ammonia can accumulate in your muscles and cause fatigue. You'll also deplete protein needed to repair and restore muscles, reports Idaho State University.