The name protein is derived from the Greek term proteos, which means “primary” or “to take place first.” Protein was first identified well over a century ago, at which time scientists described it as a nitrogen-containing part of food that is essential to human life.
While protein has long been the darling of the sport, bodybuilding and fitness community, over the past few decades more attention has focused on the importance of protein as it relates to weight loss, advancing age and general health. One of the key objectives for the International Protein Board (iPB) is to foster a greater global understanding and alignment of the key aspects and applications of dietary protein, including how much we should consume, different protein types in foods and supplements, and the relevance of protein to people with different lifestyles and throughout their lives. Or more simply stated, Protein Matters!
Protein as a Diet Macronutrient
The term “macronutrient” describes a nutrient’s major contribution (i.e. grams) to the human diet. Macronutrients also include carbohydrates and lipids. Protein is unique from the other energy-providing macronutrients because its elemental composition goes beyond carbon, oxygen, and hydrogen to include nitrogen (N), as well as sulfur (S).
From an energy standpoint, protein provides approximately 4 kilocalories (16 kilojoules) per gram, which is roughly the same energy content of carbohydrate and a little less than half of fat. Protein is more concentrated in animal-derived foods vs those derived from plants and other sources.
Protein as a Body Macro-component
From a body composition standpoint about 16% of a lean adult’s body mass is protein. About one-half of the dry weight of human cells is protein, while skeletal muscle tissue in general is closer to 90%. Some of the most significant roles for proteins in the body are structural- and functional-based, serving as connective tissue proteins, muscle contraction filaments, antibodies for immune responses, transporters, hormones, enzymes and more.
Roughly two-thirds of body protein can be found in skeletal muscle in the form of contractile fibers (actin and myosin), as well as connective tissue proteins, transporters, pumps, receptors, and enzymes. For people engaging in regular exercise, muscle protein (mass and/or %) can be a little higher and the opposite is likely true for inactive people. Elsewhere in the body, protein serves as the basis of bone (30-40% of bone mass), as well as the structural and functional basis of skin, hair, organs and other tissue.
Protein is somewhat unique from other energy-contributing macronutrients in that for the most part it is not stored or primarily intended as an energy reserve for the body. Roughly 99% of body protein is in the form of protein and only about 1% is free amino acids in the blood and inside cells. This is in constant flux between protein production, breakdown and what might be coming in from the diet. Teleologically, body protein is not meant to be a significant, readily available fuel, while nearly all the body carbohydrate and the majority of fat can be viewed as readily available fuel. (see figure below). However, during times of either limited energy intake, stress or disease, body protein can be more involved in energy requirements.
Amino Acids are the Building Blocks for Protein
There are hundreds of different proteins in the human body and thousands in nature; however, all proteins are based on links of smaller molecules called amino acids. Said differently, amino acids are the “building blocks” of proteins. Although the final functional form of proteins may include minerals or other nonprotein components, the structural basis for these proteins is still amino acids linked together by peptide bonds in a specific sequence as defined in the genes within human DNA.
All amino acids have the same basic design (see amino acid figure below). There is both a nitrogen-containing amine (amino) portion and carboxylic acid portion attached to a central carbon atom, hence the term amino acid. There is also always a hydrogen atom attached to the central carbon, as well as a unique “R” group. It is the latter (R group) that makes one amino acid different from the next and determines the reason why it is targeted to be added into a protein at a specific location in the sequence.
There are numerous amino acids found in nature, but only 20 are significant to human protein, as coded in our DNA as well as other living things. This is a very important point as it allows us to derive all amino acids used to make human protein from the diet and eating other life forms whether it be animals, plants, yeast, fungi, insects or microbes.
Not all of the amino acids found in the human body are part of protein or even used to make protein for that matter. Examples of amino acids important in our body, but not used to make protein include taurine, citrulline, ornithine, homocysteine and beta-alanine. Meanwhile there are several other important “amino” molecules made from amino acids like carnitine, sarcosine, creatine and GABA. Some of these amino acids and derivatives will be discussed latter.
Peptides and Proteins
Some proteins contain just a few amino acids linked together and are referred to as peptides. Meanwhile others contain hundreds of amino acids. Scientists often refer to the links of amino acids in the following manner:
- Peptides are two to ten amino acids including dipeptides and tripeptides
- Polypeptides are 11 to 100 amino acids
- Proteins are > 100 amino acids.
Meanwhile, scientists often describe protein size based upon the weight of the protein molecule (molecular weight) and use terms Daltons as a unit of weight.
Proteins in Our Body
Most of structure and function of the human body is based on proteins. Thus, protein and individual amino acids must function in our body in a number of ways. For instance, proteins can function as: enzymes (regulate chemical reactions), structural proteins (yield form to cells and tissue), contractile proteins (provide basis for muscle contraction), antibodies (help protect us from foreign entities), transport proteins (help transport substances in our blood), protein hormones (i.e., insulin, glucagon, and growth hormone), clotting factors (allow our blood to clot to stop a hemorrhage), and receptors (allow hormones and neurotransmitters to function).
Additionally, certain individual amino acids can be used to make certain hormones and neurotransmitters such as epinephrine, serotonin, norepinephrine and thyroid hormone. In fact, most neurotransmitters are derived from amino acids. Amino acids are also used to make other important substances such as creatine, choline, carnitine, nucleic acids and the vitamin niacin. Lastly, amino acids can be used by some tissue as an energy source or can be converted to glucose or fat depending upon our current nutritional/metabolic state (i.e., fasting, fed, exercise).
Dr Don Layman presents on the important of protein at meals and getting enough protein throughout the day. Protein Choices. It’s all about Meal Choices. Nutrition Forum – Dr. Donald Layman, PhD
Dr Heather Leidy discusses the importance of starting the day with protein Breakfast in the Classroom for Teachers. Discussion overviews the positive benefits of the protein-rich breakfast for children and teens in school.
Dr Heather Leidy presents on Protein, Particularly at Breakfast, on Appetite, Satiety, and Weight Management Adolescents. Protein-rich breakfast can have a positive impact on weight management for adolescents.