Each single gram of protein = 4 calories. Approximately 20% of our body weight is protein. Protein is the main component of structural elements like our muscles, hair, nails, skin, eyes, and internal organs such as the heart and brain. Protein is also what makes up the building blocks of our DNA. Each protein molecule is complex, made up of various arrangements and combinations of 22 different naturally occurring amino acids. Some amino acids are considered essential; we must obtain them from our diet because our body cannot produce them, while other amino acids are considered non-essential; our body can produce them if all the systems within the body are functioning properly. Protein synthesis is the term for creation of protein from amino acids.

Protein Sourcing: Carnivores, Omnivores, Vegans & Vegetarians

When a food item is referred to as a complete protein, it implies that the food contains all of the essential amino acids. Meats, fish, poultry, dairy foods and eggs are all almost always considered complete proteins. For this reason, people who eat dairy and/or meat seldom need to consider the quality of their protein intake. Vegans, or people who do not eat any meat or dairy need to be a little more careful, as most plant food sources are not complete proteins. Eating a balanced variety of vegetables, grains, nuts and legumes is optimal for vegetarian based diets, in order to obtain all the essential amino acids for protein synthesis. Relatively new research suggests that eating a balance of amino acids for protein combining within each individual meal is not necessary for optimal nutrition. This is also true over the course of a few meals or a full day as well, however, a vegan should consider the variety of their food intake over the course of a few days to ensure they are getting adequate supplies of all the essential amino acids.

Protein Functions

The name protein comes from the Greek word proteos, meaning “primary” or “first place.” Protein has a diverse set of functions within the body that make it crucial to good health. It is responsible for tissue growth and maintenance of muscle, nails, hair, and other connective tissues. Protein also functions to heal and repair damage after an illness or injury, or even strenuous exercise. When we exercise hard, we are actually breaking down our tissues on a cellular level. This is one of the reasons why you can only lift a heavy weight so many times before no longer having the strength to pick it up again. It’s during recovery that protein builds the muscle tissues back up, stronger and more robust than before the exercise, so that next time you can lift heavier or for longer. Muscle gains are thanks (in part) due to the protein synthesis that occurs between training sessions.

Proteins in the form of enzymes, also aid in biochemical reactions that take place within and outside your cells. An enzyme will combine with molecules within a cell to catalyze reactions that are essential to metabolism, while digestive enzymes like lactase and sucrose, work outside the cells to help you digest food. Body functions that depend on various proteins in the form of enzymes include digestion, blood clotting, muscle contraction and energy production.

Some proteins are also hormones acting as chemical messengers aiding communications between cells, tissues and organs within your body. Examples include Insulin, which works to regulate sugar within the blood stream; Glucagon, which signals the breakdown of stored glucose from the liver; and hGH (human growth hormone), which stimulates growth of various tissues including bone.

Protein also provides structure to your body. Some proteins, such as keratin, collagen and elastin are fibrous and as such provide cells and tissue stiffness and rigidity. Collagen for example, is the most abundant protein in your body and is the structural component of your bones, tendons, ligaments and skin.

Protein has many other roles within your body including nutrient transport and storage, pH regulation, fluid balance, and immune function. And as a last resort, protein with 4 calories per gram, may also function as fuel through gluconeogenesis, to provide your body with energy if sources of carbohydrates and fats are low.

Protein & The Thermic Effect of Food (TEF)

The body must use energy to process and store food. The amount of energy that is required for digestion is known as the Thermic Effect of Food (TEF) or Dietary Induced Thermogenesis (DIT). Of all of the macronutrients protein has the highest TEF, which means it requires the most energy to digest. The TEF of protein is approximately 20-35% because protein is difficult to break down. The TEF of fat is approximately 5-15% because fat is easy to break down. And the TEF of carbohydrates is approximately 5-15% because carbohydrates are moderately difficult to break down. A diet higher in protein expends more energy during digestion than a diet higher in the other macronutrients. By default, if you replace some of your fat or carbohydrate intake with protein instead, you will be eating the same number of calories while burning more calories off simply through digestion. In effect this may make losing weight easier.

When we hear of negative calorie foods like raw celery or grapefruit, it is in reference to the TEF of these foods: digesting them requires more energy than they contain. It’s important to note that there is no research to substantiate the hypothesis of negative calorie foods so while negative calorie foods are possible in theory; there is no actual evidence of this.