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Protein Sources, Digestion and Digestions

best protein sourcesLesson Outcomes:

  • Understand the role of protein and amino acids in the diet
  • Know the characteristics and structure of proteins and amino acids
  • Be able to describe how proteins are complete or incomplete
  • Be familiar with protein intake guidelines

Protein is a nutrient that is critical to both the structure and function of the body. It is more appropriate to use the term “proteins” than “protein,” since there are a multitude of proteins in the human body. The three-dimensional shape and sequence of amino acids determine the functional role of any particular protein within the body.

Proteins and Amino Acids

Protein is an important part of clients’ eating profiles due to its role in repairing tissue after bouts of PA. Protein is used for building, maintaining and repairing muscle, skin and blood. Protein supplies very little energy to the body (approximately 5 to 15%) during resting conditions.

What is Protein?

Protein was the first substance to be recognized as a vital part of living tissue. In fact, the word protein comes from the Greek word proteos, which means “primary” or “taking first place”, indicating the importance of this nutrient in the function of the body. Accounting for 20 percent of our body weight, proteins perform a wide variety of functions throughout the body as vital components of body tissues, enzymes, and immune cells.

Proteins are complex molecules comprised of a combination of different amino acids, compounds that contain carbon, oxygen, hydrogen, nitrogen and sometimes sulfur. Amino acids link together in specific numbers and unique combinations to make each different protein.

Protein is an essential component of the diet, because it provides the amino acids the body needs to synthesize its own proteins.    In traditional nutrition textbooks, we typically learn the two types of amino acids: essential amino acids and non-essential amino acids.

Essential amino acids are amino acids that our body cannot synthesize on its own. Essential amino acids must, therefore, be obtained from our diet. As traditionally defined, the eight essential amino acids are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Nonessential amino acids have traditionally been defined as those that the body can manufacture on its own. It is therefore not necessary to obtain these amino acids from the diet. As traditionally defined, the nonessential amino acids include glutamate, alanine, aspartate, and glutamine, as well as arginine, proline, serine, tyrosine, cysteine, taurine, and glycine.

This traditional separation of amino acids into the categories of “essential” and “non-essential” seems complex. While it is true that the human body has the potential to manufacture all non-essential amino acids, this potential is not the same as actually making them. There are many circumstances in which the body cannot make nearly enough of the non-essential amino acids it needs

Classification of Amino Acids


Essential (Indispensable)


Nonessential (Dispensable) Amino Acids




Aspartic acid


Glutamic acid




Amino acids in bold italic font are conditionally essential.


*Histidine was initially believed to be essential only in infants, however, some long-term studies have shown it to be essential in adults as well.


For example, when a person is exposed to large amounts of environmental toxins and pollutants, the amount of glycine (a non-essential amino acid) made by the body may be far from adequate. For this reason, it may be more constructive to think about all non-essential amino acids as “conditionally essential.” This classification would point out that under certain physiological circumstances, the body is unable to manufacture enough of these amino acids so they would have to be obtained through diet (or supplementation).

This concept of “conditionally essential amino acids” tells us that all of the amino acids can be equally important when it comes to our diet, and that it’s worthwhile for us to pay attention to all amino acids when thinking about the nourishment we get from our food. There are 19 different amino acids required by the body.

The body is only able to make the proteins it needs when there are sufficient quantities of all the necessary amino acids in the so-called “amino acid pool.” If we are deficient in essential amino acids, the body is unable to make proteins and will have to break down muscle proteins to obtain the amino acids it needs.

As a result, it is imperative that our daily intake of food contains each of the essential amino acids, which is easily accomplished by eating a variety of vegetables, beans, whole grains, nuts, seeds, and meat and animal products, if desired.

What Are the Functions of Protein?

Protein, providing four calories per gram, is an important source of energy for the body, when carbohydrates and fats are not available. In addition to using protein to generate energy for cellular function whenever necessary, the body uses the amino acids contained in the protein we eat to manufacture its own proteins. The proteins synthesized by the body perform a variety of important physiological functions:

Production and maintenance of structural proteins

The body manufactures several structural proteins, such as myosin, actin, collagen, elastin, and keratin that maintain the strength and integrity of muscles, connective tissues (ligaments and tendons), hair, skin, and nails.

Production of enzymes and hormones:

All of the enzymes, which are compounds that catalyze chemical reactions in the body, are made from protein. In addition, the hormones involved in blood sugar regulation (insulin and glucagon) as well as the thyroid hormones are synthesized from proteins.

Production of Transport Proteins and Lipoproteins:

Certain proteins are used by the body to carry various substances to body tissues. These transport proteins include hemoglobin (carries oxygen), transferrin (carries iron), ceruloplasmin (carries copper), retinol-binding protein (carries vitamin A), albumin and transthyretin (both carry other proteins). Lipoproteins participate in the transportation of fat and cholesterol.

Production of Antibodies:

Antibodies, which are proteins, play an important role in the immune system by attaching to antigens (viruses, bacteria, or other foreign invaders), thereby inactivating the antigens and making them more visible to the immune cells (called macrophages) that destroy antigens.

Maintenance of proper fluid balance: 

Proteins participate in the maintenance of osmotic pressure, which controls the amount of water that is found inside of cells.

Average protein consumption for an adult in the U.S. is 100 grams/day, but your clients will most likely need a little more than that, depending on the individual.

Protein Requirements

  • 10-20% of the diet should be protein for sedentary individuals.
  • 40 grams/day for
  • 55-70 grams/day or 0.8 grams/kg body weight for

Recommended Protein Intake for Athletes or Physically Active People

Protein requirements for endurance athletes are greater than weight-trained athletes, since endurance athletes are more likely to deplete carbohydrate sourcing for glucose and the body then turns to make glucose from protein (typically, lean body mass)

In many popular low-carbohydrate diets, protein requirements increase when calories are insufficient from other macronutrients. The idea behind this is that if carbohydrates are cut from the diet, the difference needs to come from one of the other macronutrients. Since fat is not an ideal choice, the only other option is protein.

Athletes and Physically Active Individuals

It has been shown that the protein requirements for athletes may well exceed that suggested by the (USRDA) .80 g/kg/day. If an individual’s protein requirement increases in response to exercise, then changes in protein metabolism will become apparent. When the body is in a homeostatic state, protein synthesis is equal to protein degradation and the protein requirement of the body for tissue maintenance is satisfied. The most common way to detect changes in protein metabolism is to assess nitrogen balance of the body.

Positive nitrogen balance occurs when the total nitrogen excreted in the urine, feces, and sweat is less than the total nitrogen ingested. Positive nitrogen balance must exist for new tissue to be synthesized. When dietary protein intake or total energy intake is inadequate to maintain tissues’ total nitrogen balance, negative nitrogen balance occurs and new tissue is unable to be synthesized. When the body is in nitrogen balance, protein and energy intake is sufficient to maintain tissue protein needs and nitrogen entering and exiting the body is equal.

The results of nitrogen balance studies on endurance athletes indicate that these athletes have protein requirements that exceed the USRDA of 0.8 g/kg/day. A study found that endurance athletes (defined as training for at least 12 hours per week for at least 5 years) require 1.37 g/kg/day of protein to maintain nitrogen balance compared to g/kg/day for sedentary individuals.

It now appears that weight training can also lead to a daily protein requirement that exceeds the current USRDA. It has been found that 2.0 to 2.2 g/kg/day of protein was barely sufficient to maintain nitrogen balance during moderate intensity weight training. Furthermore, weightlifters’ protein requirements increased proportionally to training intensity.

Research has shown that 2.0 to 2.6 g/kg/day of protein are required for periods of very intense weight training, whereas protein intakes of 2.0 g/kg/day maintained a positive nitrogen balance during periods of less intense weight training.

Protein Gram Requirements

currently RDA for

.4g per .8 g per
sedentary adults pound of bodyweight kilogram of bodyweight


.5 – .7 g


.8 – 1.5 g


athlete pound of bodyweight kilogram of bodyweight
adult endurance athletes .6 – .7 g

per pound of


1.2 – 1.6 g

per kilogram of bodyweight


adult strength trained athletes

.7 – .8 g

per pound of


1.5 – 1.7 g

per kilogram of bodyweight


It is clear that athletes need to consume more protein than the current USRDA for 0.8 g/kg/ day in order to maintain nitrogen balance. Conversely, since the requirements of carbohydrates and overall calories also increase with physical activity, the recommended proportion of calories from protein does not change significantly. With a calorie-sufficient diet, protein requirement values needed to maintain positive nitrogen balance of both weight trained and endurance trained athletes constitutes intakes of 12% to 20% of total daily calories.

So while we know that client who regularly engages in endurance activities needs dietary protein intake of 0.55 to 0.64 g/lb body weight per day (1.2-1.4 g/ kg body weight per day), this is far beyond intake guidelines for non-active adults.

Intake recommendations for clients who regularly engage in strength exercise

are to consume dietary protein intake of 0.73 to 0.77 g/lb body weight per day (1.6-1.7 g/kg body weight per day; this is nearly double the current adult RDA).

Since protein turnover is an energy-requiring process, optimal use of dietary protein by the body requires that the energy needs of the individual be met. Increasing protein intake while energy intake is adequate and constant does not improve nitrogen balance or protein usage. Consumption of protein in excess of what is needed for maintenance, synthesis, or repair of proteins is of little benefit to the body, since this leads to an increase in the oxidation of protein as a fuel source (Bolster et al. 2005). As a result, energy balance, or the consumption of adequate calories to meet those expended, is important to protein metabolism so that amino acids are spared for protein synthetic processes and not oxidized to assist in meeting energy needs. Since exercise training contributes to energy expenditure, participation in routine exercise programs challenges this relationship. Attention to energy intake is therefore of particular importance with regard to training-specific nutritional strategies focused on optimal protein utilization.

Protein Sources – Plants and Animals

Animal protein and vegetable protein seem to have the same effects on health. It’s the protein type that’s likely to make a difference. A six-ounce steak would be a great source of protein—38 grams worth. But it also contains 44 grams of fat, 16 of them saturated. That’s almost three-fourths of the recommended daily intake for saturated fat. An equal amount of wild salmon gives you 34 grams of protein and 18 grams of fat, four of them saturated. A cup of cooked lentils has 18 grams of protein, but less than one gram of fat. So there ARE differences and choices that are better than others. Unlike animal proteins, plant proteins may not contain all the essential amino acids, however, a varied vegetarian diet means a mixture of proteins are consumed so that both essential and non-essential amino acids are consumed daily.

Coach your clients so that when choosing protein-rich foods, they pay attention to what comes along with the protein – as in, how much fat is present or how much of it is saturated?

Vegans or vegetarian clients will have different needs. In general, if your client fits this description, ensure that enough kcals are consumed on a regular basis. There must also be sufficient variety in the diet while paying special attention to protein intake values with detailed planning.

Vegetable sources of protein, such as beans, nuts, and whole grains, are excellent choices, and they also provide healthier fiber benefits, vitamins, and minerals. These items should be staples in your client’s diet; it would be ideal for us to tell your client’s needs and how much of these items to consume, but you are not providing only detailed information so that the client’s nutritional needs are met. You are also personalizing your service, since every client you serve will have different needs depending on their current metrics and their goals.

Protein Digestion

Protein digestion breaks down the ingested proteins into simple amino acids, dipeptides, and tripeptides for absorption across the intestinal mucosa.  This process (protein hydrolysis) takes place within the stomach and small intestine and depends on specific protein-digesting enzymes (proteases) and the acidity of the stomach. Specific cells produce and secrete hydrochloric acid into the stomach.

HCl has various functions; among other things it activates the protease enzyme pepsin, kills many pathogenic organisms and increases the absorption of iron and calcium.  Pepsin from the stomach inactivates hormones of consumed plant and animal origin and denatures (breaks down) food proteins, making them more vulnerable to enzyme action. Proteases are often stored in the form of an inactive precursor, but as soon as it is released into the stomach or small intestine it becomes active. This mechanism prevents the digesting of the cells in which the proteases are produced and stored.


This lesson has discussed the important role of protein in supporting exercise, while ensuring adequate recovery from exercise. Studies of nitrogen balance show that strength athletes and endurance athletes need more protein than most people—but only about 1.4 to 2.0 times the current adult RDA.  These totals are already lower than the protein level many athletes already consume and within the AMDR for protein. By consuming adequate energy and a varied diet, most clients should be able to consume dietary protein in adequate amounts, without needing to supplement protein.  Excessive protein intake is actually harder for the kidneys to clear and clients who increase protein intake levels should also drink extra fluids to avoid becoming dehydrated since excess removal requires water; clients with known kidney disease should avoid excessive protein.  On a separate note, we hope it is obvious that this type of client would also need a medical clearance before training.

You have choices for nutrition education and certifications:

NESTA Lifestyle and Weight Management Specialist Certification
NESTA Sports Nutrition Specialist Certification
NESTA Fitness Nutrition Coach Certification