Protein: A Major Macro


Protein often doesn’t get as much attention as fats or carbohydrates. This is because protein is generally readily available in most typical eating patterns in the United States, as well as other developed countries. Indeed, if you follow a diet that includes meat, eggs, or dairy (which many Americans do,) you likely get a naturally sufficient amount of protein every day.

Protein has many crucial functions within the body that are often overlooked; it plays an important role in weight management, bone and muscle maintenance, and risk reduction of many diseases. 


The most basic units that comprise what we commonly refer to as “proteins” are amino acids. The body uses about twenty unique amino acids as building blocks to form the raw materials which compose every part of our bodies. As an analogy, amino acids function very much like the letters of the alphabet, which can be arranged and rearranged in thousands of combinations to form words in many different languages. Each word, then, is a unique protein.

Everything in our bodies from our hair and nails, our blood cells, organ systems, connective tissues, stomach acid, eye pigments….all of these are coded for and formed using the combined efforts of thousands of proteins, each with unique structures.

Nine of the twenty amino acids used by the body are classified as essential. This means that the body does not have the capacity to form these amino acids on its own. Therefore, they must be obtained from the diet.


As discussed previously, protein is integral to every major bodily organ system. The body maintains an internal “pool” of amino acids, which are on call to form whatever structures or cells that are needed. The functions of proteins can be broadly separated into three major categories:

Growth and Repair
Proteins are essential for building body tissues, especially during periods of growth, such as infancy, adolescence, or pregnancy and nursing. Body cells – especially muscle cells – have a high rate of turnover. They are constantly being formed, broken down, and remade.

Proteins are also crucially important when the body experiences an injury or trauma; healing from a burn or replacing blood loss are two obvious examples.

Children and adolescents need more protein per bodyweight than adults do, in order to build things like hormones and enzymes related to puberty.

Bodily processes – especially those related to metabolism – are very complex. However, proteins step up in many ways to ensure speed and efficiency.

Proteins function as enzymes, co-factors, and hormones within the body to speed up metabolic reactions and send chemical signals between organ systems. Everything from glucose levels, blood vessel constriction, body temperature, and fluid balance are regulated by protein messengers.

The skin, which is composed primarily of protein, is the body’s first line of defense against disease or injury. Antibodies within the blood stream are coded for and built using protein, which seek out and destroy bacterial or viral invaders.

Proteins function in other ways, too. Hemoglobin transports the oxygen carried in our blood into our body cells. Actin and myosin allow muscles to contract. Neurotransmitters such as epinephrine and norepinephrine transmit signals within the nervous system.

Protein’s final role, though not its preferred one, is as an energy source. Like fat and carbohydrates, protein can be broken down to provide calories to the body if needed. The body will avoid using protein in this way if possible, seeking to first break down other macronutrients for energy. By doing so, proteins are saved to perform all the other functions listed above, which cannot be carried out by any other nutrient.


Protein consumed in excess of the body’s needs will be broken down and stored as fat. The average person consumed adequate protein through a normal diet to meet all needs. Individuals seeking to build muscle mass (athletes, especially bodybuilders,) or maintain it (the elderly, who naturally begin to gradually lose muscle mass due to sarcopenia,) have higher protein needs.

Spreading protein out through the day by consuming some at each meal is the most efficient way to promote protein synthesis within the body. This also helps you to feel full throughout the day, as foods which are high in protein increase satiety, or the feeling of fullness. This is also important for individuals seeking to lose weight, as it can prevent overeating.

The recommended intake range for men and women aged 19 and older is 10-35% of daily total calories. For an individual following a 2000 calorie/day diet, this equates to 50-175 grams of protein per day. Why such a wide range? Because, as stated previously, protein intake is highly dependent on individual factors, the most important of which being physical activity level and muscle growth/maintenance requirements.

As a rule of thumb, the average individual need not consume more than 0.8 grams of protein per pound of bodyweight, if they are not actively seeking to build lean muscle mass. For a 150lb individual, this comes to around 120 grams of protein per day.


Protein is absorbed most efficiently from animal sources, such as meat, eggs, and dairy. Animal sources of protein also provide all nine essential amino acids.

With a few exceptions, including soy, plant-based sources of protein such as nuts, seeds and legumes are incomplete sources, meaning at least one of the essential amino acids are missing. Because of this fact, those who follow a diet that does not include animal products should take care to pair protein sources in order to cover their amino acid bases. Examples of this include eating peanut butter on toast, or a pasta dish with a side of vegetables.


Collene, A.L., Smith, A. M., & Wardlaw, G. M. (2013). Contemporary Nutrition: A Functional Approach - 3rd Ed. New York, NY: The McGraw-Hill Companies, Inc.

Duyff, R.L. (2017). Academy of Nutrition and Dietetics: Complete Food & Nutrition Guide. New York, NY:  Mifflin Harcourt.

U.S. Department of Agriculture. (2015). Dietary Guidelines for Americans 2015-2020. Accessed from:

Yi Zhang