The body uses all three macronutrients, carbohydrates, fat and protein, to handle all bodily functions. When exercising, the fuel your body burns depends on what it has available. You'll burn fat before protein burn begins, but fat isn't the first to burn.
Fat burns before protein, but carbohydrates burn first. How your body breaks each down depends on dietary intake, along with type and duration of exercise.
Macronutrients and Metabolism
According to Mayo Clinic, metabolism is the biological process the body uses to convert food into energy. The body constantly uses energy, even at rest, to address functions such as breathing, adjusting hormone levels and circulating blood. Many factors influence your metabolism, such as body size and composition, gender and age. The more muscle you have, the more calories you'll burn. As you age, muscle mass tends to decrease and slow down calorie burn.
About 10 percent of calories from carbohydrates and protein you eat are used to digest and absorb the nutrients.
According to MedlinePlus, carbohydrates are the primary source of energy for the body. An October 2013 study in Clinical Biochemistry describes how carbohydrates are broken down into glucose and distributed to cells for use. At a healthy weight, the skeletal muscles and liver cells store the sugars as glycogen to fuel the muscles during a workout. In a person living with obesity, the extra glucose converts to triglycerides and is stored as excess fat.
When the body runs out of glycogen stores, it turns to fat as the main energy source. Because we have a decent amount of fat storage for times when food is scarce, the body rarely uses protein as the sole fuel source.
Read more: What Happens to Body Fat When You Lose Weight?
High-Protein Diet for Weight Loss
In a normal diet, the USDA recommends eating 0.8 grams of high quality protein for every kilogram (0.36 grams for every pound) of body weight. For every 20 pounds of body weight, this is about 7 grams of protein.
In an April 2015 review of multiple studies, the American Journal of Clinical Nutrition showed a high protein diet that contains anywhere from 1.2 to 1.6 grams of protein for every kilogram of body weight helped to keep the appetite at bay, increased satiety and improved weight loss along with fat loss.
Your high protein diet should consist of a variety of protein foods that are rich in nutrients, yet low in saturated fats and calories. According to the USDA, options include:
- Lean meats
- Nuts and seeds
- Low-fat dairy
For the most nutritious intake, aim for a mix of plant and animal protein sources. It's also important to make sure you're eating healthy fats. It may seem counterintuitive to eat fat to burn fat, but dietary fat is more energy dense than carbohydrates or protein. The MSD Manual says that 1 gram of fat provides nine calories, whereas carbohydrates and protein each provide four calories per gram.
Healthy fats, such as olive oil, avocado and those found in dark chocolate and eggs, will fill you up faster than high-protein and high-carb foods. Though fat is the most energy efficient, it is the slowest source of energy.
Even with a high protein diet, you'll still need to consume carbohydrates. The key is to focus efforts on complex carbohydrates, or those found in whole grains, beans and root vegetables. By staying away from simple carbohydrates like sugar and highly processed food, you can avoid spikes in blood sugar and storing excess carbohydrates as fat.
Read more: Does Water Help to Burn Fat?
When Does Protein Burn?
Protein only burns as the sole energy source when the body lacks carbohydrates and fats. Otherwise, the body uses protein for a number of bodily functions, including building muscle. If your daily protein intake isn't adequate, you may still lose weight but will struggle to build muscle mass. Consuming a higher amount of protein while trying to lose weight will help prevent muscle loss.
- Mayo Clinic: "Metabolism and Weight Loss: How You Burn Calories"
- Clinical Biochemistry: "A Quick Look at Biochemistry: Carbohydrate Metabolism"
- American Journal of Clinical Nutrition: "The Role of Protein in Weight Loss and Maintenance"
- Harvard Health Publishing: "How Much Protein Do You Need Every Day?"
- USDA: "Look for Lean Protein Foods"
- MedlinePlus: "Carboyhydrates"
- MSD Manual: Consumer Version: "Carbohydrates, Proteins, and Fats"