Protein is a macronutrient, as are fats and carbohydrates. The U.S. Food and Drug Administration categorizes macronutrients as components of food that provide energy or calories. Eating more calories than you need for normal body functions and daily activities can lead to weight gain. Knowing the calories in a gram of protein may help keep your weight under control.
Proteins are made up of 20 amino acids. Eleven can be synthesized in the body and nine are classified as essential amino acids because they cannot be synthesized in the body and must be obtained from food.
Sources of Protein
Protein is derived from animal and plant sources. Animal sources include meat, poultry, eggs, fish and dairy products. These contain the nine essential amino acids and are classified as complete proteins. Plant sources include vegetables, beans, nuts and legumes. They lack the essential amino acids.
Function of Protein
Protein is responsible for cellular maintenance and growth. It is the building block for all body tissue, including internal organs, muscle tissue, ligaments and tendons.
Need for Protein
A study by William M. Rand, Peter L. Pellett and Vernon R. Young, reported in the American Journal of Clinical Nutrition, suggests that healthy adults require between 0.65 g and 0.83 g of protein for every 2.2 lb of body weight. Athletes and people involved in resistance training require between 1.2 g and 2.2 g of protein per 2.2 lb of body weight, as suggested in a study by Jacob Wilson and Gabriel J. Wilson in the Journal of the International Society of Sports Nutrition.
Calories in Protein
Calories indicate and measure the energy available in food. Fat is the most calorie-dense food and carbohydrates are the body's preferred energy source. According to Ucla.edu, protein and carbs each contain 4 calories per gram and fat contains 9 calories per gram.