Protein, along with carbohydrates and fats, supplies the energy every body needs to maintain and repair itself. Most people need around 0.4 milligrams of protein per pound of body weight, or about 8 g of protein for every 20 lbs. of body weight, according to the Harvard School of Public Health. Unlike carbohydrates and fat, protein isn't stored in the body, so you need a fresh supply every day. Protein supplies 4 calories per gram of energy, the same as carbohydrates and less than the 9 calories per gram supplied by fats.
Primary Energy Source
Carbohydrates are the primary source for energy in your body because they break down quickly into sugars that are easily absorbed into the bloodstream. Simple sugars such as glucose and sucrose are absorbed most quickly; complex carbohydrates made up of long chains must break down into simple sugars before they can be absorbed. Even complex carbs break down more quickly than proteins, which is why they're used as a primary energy source before proteins.
Protein Breakdown
Unlike carbs, protein breaks down into amino acids, which are a longer-lasting source of energy than carbs. Protein is normally not used as an energy source unless you're not getting enough calories from carbs or fats. If you take in too much protein, your body breaks the protein down and stores it as fat, not as protein.
Energy Needs
Everyone has a unique energy requirement depending on their age, sex, body type, activity level and metabolism. Energy requirements are measured in kilocalories, which are usually just called calories. A calorie is the amount of energy required to raise the temperature of water 1 degree Celsius. The average basal metabolic rate of a person doing absolutely nothing but lying in one place for 24 hours is half of a kilocalorie per hour per 1 pound of weight. For most people, a diet of between 45 to 65 percent carbs, 10 to 35 percent protein and 20 to 35 percent fat is ideal, according to the Mayo Clinic.
Protein-Energy Undernutrition
Amino acids, the building blacks of protein, are essential for repairing old tissue and building new tissue. If nutrient intake is inadequate for energy, protein-energy undernutrition, or PEU, occurs. After fat stores are depleted, protein sources such as muscle and organs such as the intestines and liver are broken down. The heart and kidneys can also become damaged. Skin turns cold, pale, thin and inelastic, and muscles shrink, causing weakness and fatigue. The person with PEU has increased susceptibility to infection. Severe breakdown of protein for energy can cause death in 8 to 12 weeks, the Merck Manual says.
References
- Harvard School of Public Health: Protein -- Moving Closer to Center Stage
- Mayo Clinic: Healthy Diet -- End the Guesswork with these Nutritional Guidelines
- Merck Manuals Online Medical Library: Protein -- Overview of Nutrition
- Merck Manuals Online Medical Library: Protein-Energy Undernutrition
- Springfield Technical Community College: Daily Intake
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