6 Protein in Health and Food Sources

6.1 Introduction to Protein in Foods

Protein in foods provides amino acids, which are used as building materials for body maintenance and repair. When we talk about dietary protein, we are describing the amino acids that each food source provides and how the body uses those amino acids. Foods from both plants and animals supply protein. High biological value protein matches body amino acid needs and is readily digested. There are nine indispensable, six conditionally indispensable, and six dispensable amino acids.¹

Digestion of protein begins in the stomach and continues in the small intestine. Use of newly absorbed amino acids depends upon need. Energy needs are met first, followed by body maintenance and repair, which require the availability of all indispensable amino acids. Unused amino acids are broken down, releasing ammonia and an organic acid that is used to make triglycerides for fat storage. Body protein is in a continuous state of change. As tissues, hormones, enzymes, lipoproteins, albumin, and other body substances age, they are broken down and replaced. In healthy people, breakdown is balanced by buildup.

Many protein substances such as insulin (a hormone) or amylase (an enzyme) have functions that require a specific molecular configuration. If a protein is denatured and its configuration is lost, so is its function. When body protein is broken down or deaminated, ammonia, a toxic substance, is released. The liver detoxifies ammonia by converting it to urea. Urea travels to the kidney, where it is expelled in urine.

Milk and eggs are described as the protein “gold standard.” The amino acid profile in these foods closely matches that needed for body maintenance and repair. Most foods from animal sources provide a protein with the indispensable amino acids that the body needs to carry out protein synthesis or manufacture. Soybean is heralded as a plant food that contains all of them too, while other plant foods are combined throughout the day to optimize protein synthesis in the body.

Objectives

• Investigate plant and animal food sources of protein
• Recognize amino acid classification
• Identify key components of protein metabolism
• Calculate protein needs based upon weight

6.2 Protein in Our Foods

Overview

People living in the US obtain most of their protein from animal foods, and protein typically contributes 14-16 percent of daily calories.² Protein needs are calculated using a factor (0.8) provided by the World Health Organization.³ Individuals who are in athletic training or starting new exercise programs initially experience an increase in protein needs; however, as the body adapts, needs moderate. Protein supplements can be expensive and should be cost compared to milk and eggs.

Key Concepts

• Animal and plant food sources
• Biological value of proteins
• Dispensable or indispensable amino acids
• Amino acid patterns

Animal or Plant Sources

Animal- and plant-based foods provide protein. Most people do not realize that even grains, such as flour and cornmeal, contain protein. The quality of a protein, however, depends upon its source. Proteins from animals, such as those provided by eggs, milk, meats, poultry, and fish, are of higher biological value than those provided by grains, legumes, and nuts. Why the difference?

High biological value proteins closely match your body’s need for certain amino acids and provide them in adequate amounts for your body to begin producing a wide range of substances made from them. Proteins of lower biological value either lack an essential amino acid or contain it in amounts that are too small to support protein synthesis without combining other food sources. Most people eat proteins from both animal and plant foods, thereby consuming a spectrum of different amino acids throughout the day. Combining plant and animal protein sources enhances the quality of meals and reduces the number of animal foods needed to meet protein needs.

Indispensable and Dispensable

Protein in the foods that you eat supplies amino acids, from which your body makes a range of substances. Hormones and enzymes are examples of protein-based substances found in the body.

Amino acids are building blocks, and you need an adequate amount of a variety of different amino acids for healthy body function. These amino acids are classified as indispensable (essential), conditionally indispensable, and dispensable (nonessential). Dietary protein requirements are derived from the body’s need for amino acids that it cannot synthesize or make for itself. Strictly speaking, you should consume all of the indispensable amino acids over a period of a day, while conditionally dispensable and dispensable amino acids intake is more flexible. Your body can transaminate dispensable amino acids or transform one into another to meet protein synthesis needs. As a result, intentionally consuming individual dispensable amino acids is not necessary.

Distribution in the Food Supply

Sometimes people mistakenly believe that only animal foods supply protein, but protein is in fact widely, if unevenly, distributed throughout our food supply. A cup of milk contains about eight grams of protein; an egg, six; a cup of red beans and rice, ten; and a slice of bread, two. If you ate a whole cup of dried pecans, you would garner about nine grams of protein!

It is important to remember, though, that not all proteins are created equal, and dependence on plant proteins requires more awareness and creativity to ensure that all the indispensable amino acids are present and available for the maintenance of a healthy body. Accomplish this by combining plant foods with different amino acid profiles.

Amino Acid Patterns

The indispensable, or essential, amino acids are as follows: (1) histidine, (2) isoleucine, (3) leucine, (4) lysine, (5) methionine, (6) phenylalanine, (7) threonine, (8) tryptophan, and (9) valine. These amino acids must be consumed in the foods that you eat, as your body cannot make them from other amino acids. See Table 6.1.

The conditionally indispensable amino acids include (1) arginine, (2) cysteine, (3) glutamine, (4) glycine, (5) proline, and (6) tyrosine. Stress, injury, and illness can increase needs, shifting these amino acids to indispensable when the body is unable to synthesize enough to meet its needs. The dispensable amino acids are (1) alanine, (2) aspartic acid, (3) asparagine, (4) aspartic acid, (5) glutamic acid, and (6) serine. While these amino acids are considered nonessential, their synthesis by the body depletes supplies of indispensable amino acids.4

Summary

Protein is an important nutrient that provides both energy and building materials for body maintenance and repair. In the United States, the consumption of protein is more than adequate to meet needs. Few people are at risk of nutritional deficiency. Even vegetarians who rely on plant foods consume enough protein to maintain their health, particularly if they eat a variety of foods in adequate quantity or include eggs and milk in their diets.

6.3 Protein Digestion and Function

Overview

All protein in the body is in a state of continuous change. Even as DNA in the cell directs the linking of amino acids to make “native” or natural proteins, other processes are dismantling proteins to add amino acids to the “pool,” or short-term supply that supplements newly absorbed protein from digested foods.

Key Concepts

• Process of digesting and absorbing protein
• Utilization of amino acids
• Protein turnover and functionality

Protein Digestion

Digestion of protein begins with the action of pepsin in the stomach. As you chew foods, the stomach is stimulated to secrete acid, which activates pepsinogen to make pepsin. Protein in foods exists as a large macromolecule, and pepsin breaks this macromolecule into small amino acid units.

These small amino acid units travel to the small intestine, where pancreatic secretions and enzymes break them into even shorter units that are available for  uptake by absorptive cells. Inside the absorptive cells lining the small intestine, these units are completely broken down into discrete amino acids. Subsequently, individual amino acids travel to the liver via the bloodstream.

Use Depends on Need

The destiny of newly absorbed amino acids is determined based on need. Energy for body function is a primary need that must be met before any other is addressed. Once energy needs are met, ideally with carbohydrates, amino acids are used for the manufacture of glucose, new proteins, or fat for storage.

When body energy needs are not met with dietary carbohydrate or stored glycogen, available amino acids in plasma and body proteins are cleaved to release an organic acid from which glucose is made. If a meal contains carbohydrates and energy needs are met, amino acids are utilized to build new proteins, and finally, if all other needs are met, they are deaminated and used in the manufacture of triglycerides for storage. First use. Energy when dietary and stored carbohydrate (glycogen) is inadequate. Second use. Building protein substances when required amino acids are available. Third use. Storage when amino acids are used to make triglycerides for storage

Value of Carbohydrate in the Diet

Did you know that your body sometimes uses amino acids to maintain a steady supply of glucose for the brain and red blood cells? During the night when you are fasting or when more than five hours have passed since your last meal, normal blood glucose levels are maintained by the release of glucose from the liver.

Your body does this by using recently absorbed nutrients, such as amino acids, as well as body resources to produce glucose when blood levels fall. The red blood cells and the brain require a continuous supply of glucose, and if you do not consume enough carbohydrates, the liver breaks down amino acids to produce it.

When a person routinely fails to eat an adequate amount of carbohydrates, amino acids are used to generate glucose, and the building of body proteins is postponed until blood glucose needs are met.

The presence of an adequate amount of carbohydrates in the diet “spares” amino acids for the task of providing energy, making them available for protein building. Conversely, a diet that is too high in calories or that supplies more than needed to sustain function results in the deposition of fat, even if those extra calories are derived from protein.

The All-or-None Principle

Protein synthesis is halted if any of the essential amino acids are missing. This is the “all-or-none principle.” In a particular food, the essential amino acid available in the lowest amount limits the usefulness of that protein food source for body function.

When evaluating the usability of a plant protein, the essential amino acid present in the lowest amount is called the “limiting amino acid.” In combining plant protein sources, foods with different limiting amino acids should be combined so that the result is a complete protein. The process of combining different foods to make a complete protein is complementation, and the combined foods are said to be complementary.

Protein Turnover

While most people know that protein is used in the building of muscle tissue, they may not realize that it is integral to lipoproteins that transport fats through the bloodstream and the manufacture of hormones, neuropeptides, antibodies, and enzymes.

Some of these substances exist for very short periods of time before they begin to degrade in a process called catabolism. The circular process of protein loss through breakdown (catabolism) and protein deposition through synthesis (anabolism) is called “protein turnover.” In healthy adults, protein turnover is balanced between catabolism and anabolism.

Protein Functionality

The DNA in a cell directs the linking of amino acids in a specific sequence or pattern to create a unique protein. The function of this protein is determined by the relationship between these linked amino acids and the unique three-dimensional structure that results from their interactions.

A protein is “denatured” and loses its function when its three-dimensional structure or shape is altered. When this happens, the interactions between amino acids change.

For example, a denatured enzyme no longer functions as an enzyme, although all the same amino acids are still present in the molecule. During digestion stomach acids denature or uncoil proteins, rendering them more susceptible to enzymes and negating their function.

The process of cooking our foods heats proteins and denatures them. Preparing foods with acid (vinegar), alkaline (baking soda), or agitation (whipping an egg white) does so as well. For example, although the cooked protein in a fried egg still contains the same amino acids as a raw egg, the shape of the protein has permanently changed. While the white of a raw egg can be poured, in a cooked egg, it is solid and rubbery. This change is irreversible.

Protein Deamination

Both the liver and muscles are capable of deaminating amino acids to produce energy. This process of converting amino acids to glucose results in an increase in blood ammonia levels. Why? The breakdown of proteins, or the deamination of amino acid molecules, removes ammonia, leaving an organic acid that is used to manufacture glucose.

The liberated ammonia is toxic to cells and must be quickly converted to a less damaging substance. Fortunately, the liver efficiently converts ammonia into urea, which the bloodstream transports to the kidney for disposal.

Measurement of blood ammonia level provides information about liver health. How can this be? If the liver is unable to convert ammonia to urea, blood ammonia levels rise. This is an indication of liver disease.

When kidney function is compromised, blood urea levels rise. Physicians test blood urea levels to measure the kidney’s ability to concentrate and excrete urine. If levels are too high, it is likely that kidney health is compromised.

A diet high in protein and excessively low in carbohydrates places an added burden on the liver and kidney, as both organs participate in the detoxification of the ammonia waste product of amino acid catabolism. Under normal circumstances, however, the body efficiently manages ammonia, and problems are not encountered.

Summary

Protein digestion begins in the stomach as pepsinogen is activated by hydrochloric acid and converted to pepsin. Large protein molecules are broken down by pepsin into smaller units that move on to the small intestine for final simplification in preparation for absorption. Protein use depends on needs, with energy production taking priority over maintenance when carbohydrate intake is low. The functions of a protein molecule are determined by its shape. When a molecule is denatured or loses its shape, it also loses its function.

6.4 Protein and a Healthy Diet

Overview

While protein can be found in both plant- and animal-based foods, the quality of a protein differs based upon its source. Proteins from animal origins, such as those found in eggs, milk, meats, poultry, and fish, are considered “complete” or of higher biological value, whereas proteins from plants, such as those found in grains, legumes, and nuts, are “incomplete” or of lower biological value. The World Health Organization provides a formula for calculating your daily protein needs.

Key Concepts

• Protein food sources
• Planning a healthy diet with adequate protein
• Calculating protein needs

The Protein Gold Standard

Milk and eggs are sometimes called the protein “gold standard.” Why? These foods provide indispensable and dispensable amino acids in quantities closely matched to the needs of the body. Proteins from animal sources also tend to be more easily digested, which enhances their value. Compare proteins found in other foods to eggs and milk to see how well they meet needs.

Proteins that provide essential amino acids in adequate amounts and in the correct proportion are complete or high quality; those that do not are considered incomplete or low quality. While almost all animal-based foods provide a complete protein, soybean is the only plant food that is considered complete and does not require combination with another plant or animal food source. All other plant foods must be eaten in combination to build a complete protein. For example, grains should be combined with legumes and nuts to provide enough essential amino acids.

Building a Healthy Diet

To overcome the amino acid limitation of plant proteins, most vegetarian diets as well as many traditional ethnic eating patterns combine different types of plant foods to enhance protein value. A well-balanced plant-based diet combines legumes (beans, peas, and lentils) with whole grains (rice, corn, wheat, oats) and fruits/vegetables. Past recommendations stressed combining different types of plant foods in a single meal, creating a total protein intake similar to that from eggs, milk, or meats. Recent advice is more relaxed.

If you regularly consume a good mixture of different plant proteins in adequate amounts, the benefits of a complete protein can be achieved. Eating a variety of foods high in different dispensable proteins helps to moderate needs for indispensable proteins. A well-balanced diet rich in grains, legumes, fruits, and vegetables is the best approach.²

A Varied Diet Example

The following is an example of how a varied diet improves amino acid availability: Grains are high in methionine but low in lysine, and legumes are low in methionine but high in lysine. Each food provides adequate amounts of different amino acids. To achieve an optimum benefit from plant proteins, foods from the three different groups (grains and nuts, legumes, and vegetables) should be combined.

How Much Protein Do We Eat?

The USDA What We Eat in American Survey (NHANES 2015-2016) reported that men ages twenty to twenty-nine consumed, on average, 107 grams of protein each day, and women ages twenty to twenty-nine consumed 73 grams.² This equaled about 17 percent of total calories for men and 16 percent for women.³

The protein intake in the United States today is surprisingly similar to that of the early 1900s; however, where the citizens of the past relied heavily on plant proteins to round out their diets, Americans today obtain close to 70 percent of their protein from animal sources. The ramifications of this shift include a more refined diet featuring less fiber and more animal or saturated fat.^4,5

Calculating Your Protein Needs

You can determine the number of grams of protein you need each day by using the following procedure:

1. Calculate your weight. Determine your body weight in kilograms by dividing your weight in pounds by 2.2. For example, if you weighed 154 pounds, 154 pounds / 2.2 pounds per kilogram = 70 kilograms.
2. Use the protein factor. Multiply your weight in kilograms × 0.8 grams. For example, 70 kilograms × 0.8 grams/kilogram = 56 grams of protein per day.

A 2,000 calorie diet for a sedentary adult would include fifty grams of protein if 10 percent of calories were derived from protein or seventy-five grams if 15 percent of calories were from protein. This is generally accepted as more than adequate for a healthy individual.

World Health Organization

The WHO established a protein requirement for adults eating a Western diet at 0.75 grams (rounded to 0.8 grams) of protein for each kilogram of body weight  For example, a 185-pound (84.1 kilogram) adult male would have a protein need of about 67 grams, and a 135-pound (61.4 kilogram) adult female would require 49 grams.

Training and Protein Needs

What impact does training have on protein needs? During athletic training or when beginning a new exercise program, the body is adjusting to new demands, and as a result, protein requirements increase. As adaptation takes place, needs moderate, and it is no longer necessary to consume extra protein.

Typically, only 1.6-1.7 grams of protein per kilogram of body weight are sufficient for most people in training. Although the body has an increased need for protein during high intensity and endurance activities, excess dietary protein increases the risk of dehydration. Surprisingly, a well-trained individual uses protein more efficiently and may actually need less than someone just starting a training program. See Table 6.2

Summary

Proteins from animal sources are more easily digested, which enhances their value. If you regularly consume a good mixture of different plant proteins, adequacy can be achieved. You can determine the number of grams of protein you need each day by dividing your weight in pounds by 2.2 pounds/kilogram and multiplying the result by 0.8 grams of protein per kilogram.

References

1. Amino Acids. MedlinePlus. US National Library of Medicine. https://medlineplus.gov/ency/article/002222.htm. Updated May 7, 2020. Accessed January 16, 2021.
2. WWEIA Data Tables, 2015-2016. US Department of Agriculture, Food Survey Research Group. https://www.ars.usda.gov/northeast-area/beltsville-md-bhnrc/beltsville-human-nutrition-research-center/food-surveys-research-group/docs/wweia-data-tables/. Accessed January 16, 2021.
3. World Health Organization. Protein and Amino Acid Requirements in Human Nutrition: Report of a Joint FAO/WHO/UNU Expert Consultation (WHO Technical Report Series 935). ­https://www.who.int/nutrition/publications/nutrientrequirements/WHO_TRS_935/en/. Published 2007. Accessed January 16. 2021.
4. Berryman CE, Leiberman HR, Fulgoni VL III, Pasiakos SM. Protein intake trends and conformity with the Dietary Reference Intakes in the United States: Analysis of the National Health and Nutrition Examination Survey, 2001-2014. Am J Clin Nutr. 2018;108(2):405-413. doi:10.1093/ajcn/nqy088.
5. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies of Science Engineering Medicine, Health and Medicine Division. doi:10.17226/10490. Accessed January 16, 2021.