Vitamins and minerals are only needed in small quantities in the body, but their role is essential to overall health and proper functioning of all body systems. And while many vitamins and minerals work together to perform various functions in the body, they are classified based on their independent characteristics. These characteristics impact not only how we obtain them in our diets, but also how we absorb them and store them, as well as how we experience deficiencies or toxicities when too little or too much is consumed. After we review the classifications for vitamins and minerals, we will examine key vitamins and minerals based on their similar functions to further highlight the importance of how these micronutrients work together.
Vitamins
The name “vitamin” comes from Casimir Funk, who in 1912 thought “vital amines” (similar to amino acids) were responsible for preventing what we know now as vitamin deficiencies. He coined the term “vitamines” to describe these organic substances that were recognized as essential for life, yet unlike other organic nutrients (carbohydrates, protein, and fat), do not provide energy to the body. Eventually, when scientists discovered that these compounds were not amines, the ‘e’ was dropped to form the term “vitamins.”1
Classification of Vitamins
Vitamins are essential, non-caloric, organic micronutrients. There is energy contained in the chemical bonds of vitamin molecules, but our bodies don’t make the enzymes to break these bonds and release their energy; instead, vitamins serve other essential functions in the body. Vitamins are traditionally categorized into two groups: water-soluble or fat-soluble. Whether vitamins are water-soluble or fat-soluble can affect their functions and sites of action. For example, water-soluble vitamins often act in the cytosol of cells (the fluid inside of cells) or in extracellular fluids such as blood, while fat-soluble vitamins play roles such as protecting cell membranes from free radical damage or acting within the cell’s nucleus to influence gene expression.
Figure 8.1. Classification of vitamins as water-soluble or fat-soluble.
One major difference between water-soluble and fat-soluble vitamins is the way they are absorbed in the body. Water-soluble vitamins are absorbed directly from the small intestine into the bloodstream. Fat-soluble vitamins are first incorporated into chylomicrons, along with fatty acids, and transported through the lymphatic system to the bloodstream and then on to the liver. The bioavailability (i.e., the amount that gets absorbed) of these vitamins is dependent on the food composition of the diet. Because fat-soluble vitamins are absorbed along with dietary fat, if a meal is very low in fat, the absorption of the fat-soluble vitamins in that meal may be impaired.
Figure 8.2. “Absorption of Fat-Soluble and Water-Soluble Vitamins.”
Fat-soluble and water-soluble vitamins also differ in how they are stored in the body. The fat-soluble vitamins—vitamins A, D, E, and K—can be stored in the liver and the fatty tissues of the body. The ability to store these vitamins allows the body to draw on these stores when dietary intake is low, so deficiencies of fat-soluble vitamins may take months to develop as the body stores become depleted. On the flip side, the body’s storage capacity for fat-soluble vitamins increases the risk for toxicity. While toxic levels are typically only achieved through vitamin supplements, if large quantities of fat-soluble vitamins are consumed, either through foods or supplements, vitamin levels can build up in the liver and fatty tissues, leading to symptoms of toxicity.
There is limited storage capacity in the body for water-soluble vitamins, thus making it important to consume these vitamins on a daily basis. Deficiency of water-soluble vitamins is more common than fat-soluble vitamin deficiency because of this lack of storage. That also means toxicity of water-soluble vitamins is rare. Because of their solubility in water, intake of these vitamins in amounts above what is needed by the body can, to some extent, be excreted in the urine, leading to a lower risk of toxicity. Similar to fat-soluble vitamins, a toxic intake of water-soluble vitamins is not common through food sources, but is most frequently seen due to supplement use.
|
Characteristics of Fat-Soluble Vitamins |
Characteristics of Water-Soluble Vitamins |
|
Protect cell membranes from free radical damage; act within the cell’s nucleus to influence gene expression |
Act in the cytosol of cells or in extracellular fluids such as blood |
|
Absorbed into lymph with fats from foods |
Absorbed directly into blood |
|
Large storage capacity in fatty tissues |
Little to no storage capacity |
|
Do not need to be consumed daily to prevent deficiency (may take months to develop) |
Need to be consumed regularly to prevent deficiency |
|
Toxicity is more likely |
Toxicity is rare |
Table 8.1. Characteristics of fat-soluble and water-soluble vitamins.
Minerals
Similarly to vitamins, minerals are micronutrients that are essential to human health and can be obtained in our diet from different types of food. Minerals are abundant in our everyday lives. From the soil in your front yard to the jewelry you wear on your body, we interact with minerals constantly. Minerals are inorganic elements in their simplest form, originating from the Earth. They can’t be broken down or used as an energy source, but like vitamins, serve essential functions based on their individual characteristics. Living organisms can’t make minerals, so the minerals our bodies need must come from the diet. Plants obtain minerals from the soil they grow in. Humans obtain minerals from eating plants, as well as indirectly from eating animal products (because the animal consumed minerals in the plants it ate). We also get minerals from the water we drink. The mineral content of soil and water varies from place to place, so the mineral composition of foods and water differs based on geographic location.2
Classification of Minerals
Minerals are classified as either major minerals or trace minerals, depending on the amount needed in the body. Major minerals are those that are required in the diet in amounts larger than 100 milligrams each day. These include sodium, potassium, chloride, calcium, phosphorus, magnesium, and sulfur. These major minerals can be found in many foods. While deficiencies are possible with minerals, consuming a varied diet significantly improves an individual’s ability to meet their nutrient needs. We’ll discuss the concern of both deficiencies and toxicities of specific minerals later in this unit.
Trace minerals are classified as minerals required in the diet in smaller amounts, specifically 100 milligrams or less per day. These include iron, copper, zinc, selenium, iodine, chromium, fluoride, manganese, and molybdenum. Although trace minerals are needed in smaller amounts, a deficiency of a trace mineral can be just as detrimental to your health as a major mineral deficiency.
Figure 8.3. The classification of minerals as either major minerals or trace minerals.
Minerals are water-soluble and do not require enzymatic digestion. They are absorbed directly into the bloodstream, although some minerals need the assistance of transport proteins for absorption and transport in blood.
Minerals are not as efficiently absorbed as most vitamins, and many factors influence their bioavailability:
- Minerals are generally better absorbed from animal-based foods. Plant-based foods often contain compounds that can bind to minerals and inhibit their absorption (e.g., oxalates, phytates).
- In most cases, if dietary intake of a particular mineral is increased, absorption will decrease.
- Some minerals influence the absorption of others. For instance, excess zinc in the diet can impair iron and copper absorption. Conversely, certain vitamins enhance mineral absorption. For example, vitamin C boosts iron absorption, and vitamin D boosts calcium and magnesium absorption.
- As is the case with vitamins, mineral absorption can be impaired by certain gastrointestinal disorders and other diseases, such as Crohn’s disease and kidney disease, as well as the aging process. Thus, people with malabsorption conditions and the elderly are at higher risk for mineral deficiencies.
Self-Check:
Attributions:
- University of Hawai‘i at Mānoa Food Science and Human Nutrition Program, “Vitamins: Introduction,” CC BY-NC 4.0
- University of Hawai‘i at Mānoa Food Science and Human Nutrition Program, “Minerals: Introduction,” CC BY-NC 4.0
- Micronutrients Overview Kansas State University Human Nutrition CC BY 3.0
References:
- 1Carpenter, K. J. (2003). A short history of nutritional science: part 3 (1912–1944). The Journal of nutrition, 133(10), 3023-3032.
- 2Linus Pauling Institute-Micronutrient Information Center. (2020). Minerals. https://lpi.oregonstate.edu/mic/minerals
Image Credits:
- “Variety of Fruits in Tray” photo by Danielle MacInnes on Unsplash (license information)
- Figure 8.1. “Classification of vitamins as water-soluble or fat-soluble” by Allison Calabrese is licensed under CC BY 4.0
- Figure 8.2. “Absorption of Fat-Soluble and Water-Soluble Vitamins” by Allison Calabrese is licensed under CC BY 4.0
- Table 8.1. “Characteristics of fat-soluble and water-soluble vitamin” by Heather Leonard is licensed under CC BY 4.0
- “Green-leafed Vegetables” photo by Kenan Kitchen on Unsplash (license information)
- Figure 8.3. “Classification of major and trace minerals” by Allison Calabrese is licensed under CC BY 4.0
When you eat food, the body’s digestive system breaks down dietary protein into individual amino acids, which are absorbed and used by cells to build other proteins and a few other macromolecules, such as DNA. Let’s follow the path that proteins take down the gastrointestinal tract and into the circulatory system.
Eggs are a good dietary source of protein and will be used as our example as we discuss the processes of digestion and absorption of protein. One egg, whether raw, hard-boiled, scrambled, or fried, supplies about six grams of protein.
In the image below, follow the numbers to see what happens to the protein in our egg at each site of digestion.
Fig. 6.17. Protein digestion in the human GI tract.
1 - Protein digestion in the mouth
Unless you are eating it raw, the first step in digesting an egg (or any other solid food) is chewing. The teeth begin the mechanical breakdown of large egg pieces into smaller pieces that can be swallowed. The salivary glands secrete saliva to aid swallowing and the passage of the partially mashed egg through the esophagus.
2 - Protein digestion in the stomach
The mashed egg pieces enter the stomach from the esophagus. As illustrated in the image below, both mechanical and chemical digestion take place in the stomach. The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin, which initiate the chemical digestion of protein. Muscular contractions, called peristalsis, also aid in digestion. The powerful stomach contractions churn the partially digested protein into a more uniform mixture, which is called chyme.
Fig. 6.18. Protein digestion in the stomach
Because of the hydrochloric acid in the stomach, it has a very low pH of 1.5-3.5. The acidity of the stomach causes food proteins to denature, unfolding their three-dimensional structure to reveal just the polypeptide chain. This is the first step of chemical digestion of proteins. Recall that the three-dimensional structure of a protein is essential to its function, so denaturation in the stomach also destroys protein function. (This is why a protein such as insulin can’t be taken as an oral medication. Its function is destroyed in the digestive tract, first by denaturation and then further by enzymatic digestion. Instead, it has to be injected so that it is absorbed intact into the bloodstream.)
Fig. 6.19. In the stomach, proteins are denatured because of the acidity of hydrochloric acid.
Once proteins are denatured in the stomach, the peptide bonds linking amino acids together are more accessible for enzymatic digestion. That process is started by pepsin, an enzyme that is secreted by the cells that line the stomach and is activated by hydrochloric acid. Pepsin begins breaking peptide bonds, creating shorter polypeptides.
Fig. 6.20. Enzymatic digestion of proteins begins in the stomach with the action of the enzyme pepsin.
Proteins are large globular molecules, and their chemical breakdown requires time and mixing. Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion. Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach. Food remains in the stomach longer, making you feel full longer.
3 - Protein digestion and absorption in the small intestine
The chyme leaves the stomach and enters the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juices into the small intestine, and these contain more enzymes to further break down polypeptides.
The two major pancreatic enzymes that digest proteins in the small intestine are chymotrypsin and trypsin. Trypsin activates other protein-digesting enzymes called proteases, and together, these enzymes break proteins down to tripeptides, dipeptides, and individual amino acids. The cells that line the small intestine release additional enzymes that also contribute to the enzymatic digestion of polypeptides.
Tripeptides, dipeptides, and single amino acids enter the enterocytes of the small intestine using active transport systems, which require ATP. Once inside, the tripeptides and dipeptides are all broken down to single amino acids, which are absorbed into the bloodstream. There are several different types of transport systems to accommodate different types of amino acids. Amino acids with structural similarities end up competing to use these transporters. That’s not a problem if your protein is coming from food, because it naturally contains a mix of amino acids. However, if you take high doses of amino acid supplements, those could theoretically interfere with absorption of other amino acids.
Fig. 6.21. Summary of protein digestion. Note that the lines representing polypeptide chains in the stomach consist of strings of amino acids connected by peptide bonds, even though the individual amino acids aren’t shown in this simplified representation.
Proteins that aren’t fully digested in the small intestine pass into the large intestine and are eventually excreted in the feces. Recall from the last page that plant-based proteins are a bit less digestible than animal proteins, because some proteins are bound in plant cell walls.
What happens to absorbed amino acids?
Once the amino acids are in the blood, they are transported to the liver. As with other macronutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Dietary amino acids then become part of the body’s amino acid pool.
Assuming the body has enough glucose and other sources of energy, those amino acids will be used in one of the following ways:
- Protein synthesis in cells around the body
- Making nonessential amino acids needed for protein synthesis
- Making other nitrogen-containing compounds
- Rearranged and stored as fat (there is no storage form of protein)
If there is not enough glucose or energy available, amino acids can also be used in one of these ways:
- Rearranged into glucose for fuel for the brain and red blood cells
- Metabolized as fuel, for an immediate source of ATP
In order to use amino acids to make ATP, glucose, or fat, the nitrogen first has to be removed in a process called deamination, which occurs in the liver and kidneys. The nitrogen is initially released as ammonia, and because ammonia is toxic, the liver transforms it into urea. Urea is then transported to the kidneys and excreted in the urine. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it ideal for transporting excess nitrogen out of the body.
Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 percent of the protein ingested does not get broken down further than the amino acid monomers.
Self-Check:
Attributions:
- Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR, CC BY-NC-SA 4.0
- “Protein Digestion and Absorption”, section 6.3 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
Image Credits:
- Fig 6.17. “Protein digestion in the human GI tract” by Alice Callahan is licensed under CC BY 4.0; edited from "Digestive system diagram edit" by Mariana Ruiz, edited by Joaquim Alves Gaspar, Jmarchn is in the Public Domain
- Fig 6.18. “Protein digestion in the stomach” from “Protein Digestion and Absorption,” section 6.3 from An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
- Fig 6.19. “Denaturation of proteins” by Alice Callahan is licensed under CC BY 4.0; edited from “Process of denaturation” by Scurran is licensed under CC BY-SA 4.0
- Fig 6.20. “Enzymatic digestion of proteins” by Alice Callahan is licensed under CC BY 4.0; edited from “Process of denaturation” by Scurran is licensed under CC BY-SA 4.0
- Fig 6.21. “Summary of protein digestion” by Alice Callahan is licensed under CC BY 4.0; edited from “Process of denaturation” by Scurran is licensed under CC BY-SA 4.0
What Is Protein?
Proteins are macromolecules composed of amino acids. For this reason, amino acids are commonly called the building blocks of protein. There are 20 different amino acids, and we require all of them to make the many different proteins found throughout the body. Proteins are crucial for the nourishment, renewal, and continuance of life.
Just like carbohydrates and fats, proteins contain the elements carbon, hydrogen, and oxygen, but proteins are the only macronutrient that also contain nitrogen as part of their core structure. In each amino acid, the elements are arranged into a specific conformation, consisting of a central carbon bound to the following four components:
- A hydrogen
- A nitrogen-containing amino group
- A carboxylic acid group (hence the name “amino acid”)
- A side chain
The first three of those components are the same for all amino acids. The side chain—represented by an “R” in the diagram below—is what makes each amino acid unique.
Figure 6.1. Amino Acid Structure.
Amino acid side chains vary tremendously in their size and can be as simple as one hydrogen (as in glycine, shown in Figure 6.1) or as complex as multiple carbon rings (as in tryptophan). They also differ in their chemical properties, thus impacting the way amino acids act in their environment and with other molecules. Because of their side chains, some amino acids are polar, making them hydrophilic and water-soluble, whereas others are nonpolar, making them hydrophobic or water-repelling. Some amino acids carry a negative charge and are acidic, while others carry a positive charge and are basic. Some carry no charge. Some examples are given below. For this class, you don’t need to memorize amino acid structures or names, but you should appreciate the diversity of amino acids and understand that it is the side chain that makes each different.
Figure 6.2. Amino acids have different structures and chemical properties, determined by their side chains.
Essential and Nonessential Amino Acids
We also classify amino acids based on their nutritional aspects (Table 6.1 "Essential and Nonessential Amino Acids"):
- Nonessential amino acids are not required in the diet, because the body can synthesize them. They’re still vital to protein synthesis, and they’re still present in food, but because the body can make them, we don’t have to worry about nutritional requirements. There are 11 nonessential amino acids.
- Essential amino acids can’t be synthesized by the body in sufficient amounts, so they must be obtained in the diet. There are 9 essential amino acids.
|
Essential |
Nonessential |
|
Histidine |
Alanine |
|
Isoleucine |
Arginine* |
|
Leucine |
Asparagine |
|
Lysine |
Aspartic Acid |
|
Methionine |
Cysteine* |
|
Phenylalanine |
Glutamic Acid |
|
Threonine |
Glutamine |
|
Tryptophan |
Glycine* |
|
Valine |
Proline* |
|
|
Serine |
|
|
Tyrosine* |
|
*Conditionally essential |
|
Table 6.1. Essential and nonessential amino acids
Sometimes during infancy, growth, and in diseased states, the body cannot synthesize enough of some of the nonessential amino acids and more of them are required in the diet. These types of amino acids are called conditionally essential amino acids.
The nutritional value of a protein is dependent on what amino acids it contains and in what quantities. As we’ll discuss later, a food that contains all of the essential amino acids in adequate amounts is called a complete protein source, whereas one that does not is called an incomplete protein source.
The Many Different Types of Proteins
There are over 100,000 different proteins in the human body. Proteins are similar to carbohydrates and lipids in that they are polymers (simple repeating units); however, proteins are much more structurally complex. In contrast to carbohydrates, which have identical repeating units, proteins are made up of amino acids that are different from one another. Different proteins are produced because there are 20 types of naturally occurring amino acids that are combined in unique sequences.
Additionally, proteins come in many different sizes. The hormone insulin, which regulates blood glucose, is composed of only 51 amino acids. On the other hand, collagen, a protein that acts like glue between cells, consists of more than 1,000 amino acids. Titin is the largest known protein. It accounts for the elasticity of muscles and consists of more than 25,000 amino acids!
The huge diversity of proteins is also due to the unending number of amino acid sequences that can be formed. To understand how so many different proteins can be made from only 20 amino acids, think about music. All of the music that exists in the world has been derived from a basic set of seven notes C, D, E, F, G, A, B (with the addition of sharps and flats), and there is a vast array of music all composed of specific sequences from these basic musical notes. Similarly, the 20 amino acids can be linked together in an extraordinary number of sequences. For example, if an amino acid sequence for a protein is 104 amino acids long, the possible combinations of amino acid sequences is equal to 20104, which is 2 followed by 135 zeros!
Building Proteins with Amino Acids
The decoding of genetic information to synthesize a protein is the central foundation of modern biology. The building of a protein consists of a complex series of chemical reactions that can be summarized into three basic steps: transcription, translation, and protein folding.
Figure 6.3. Overview of protein synthesis. Protein folding happens after translation.
- Transcription - Deoxyribonucleic acid, or DNA, is the long, double-stranded molecules containing your genome—instructions for making all of the proteins in your body. In the nucleus of the cell, the DNA must be transcribed or copied into the single-stranded messenger ribonucleic acid (mRNA), which carries the genetic instructions into the cell’s cytosol for protein synthesis.
- Translation - At the ribosomes in the cell’s cytosol, amino acids are linked together in the specific order dictated by the mRNA. Each amino acid is connected to the next amino acid by a special chemical bond called a peptide bond (Figure 6.4). The peptide bond forms between the carboxylic acid group of one amino acid and the amino group of another, releasing a molecule of water. As amino acids are linked sequentially by peptide bonds, following the specific order dictated by the mRNA, the protein chain, also known as a polypeptide chain, is built (Figure 6.5).
Figure 6.4. Peptide bond formation
Figure 6.5. A polypeptide chain
3. Protein folding - The polypeptide chain folds into specific three-dimensional shapes, as described in the next section.
VIDEO: “DNA Transcription,” by DNA Learning Center, YouTube (March 22, 2010), 1:52 minutes.
VIDEO: “mRNA Translation,” DNA Learning Center, YouTube (March 22, 2010), 2:04 minutes.
Protein Organization
Protein’s structure enables it to perform a variety of functions. There are four different structural levels of proteins (Figure 6.6.):
- Primary structure - This is the one-dimensional polypeptide chain of amino acids, held together by peptide bonds.
- Secondary structure - The polypeptide chain folds into simple coils (also called helices) and sheets, determined by the chemical interactions between amino acids.
- Tertiary structure - This is the unique three-dimensional shape of a protein, formed as the different side chains of amino acids chemically interact, either repelling or attracting each other. Thus, the sequence of amino acids in a protein directs the protein to fold into a specific, organized shape.
- Quaternary structure - In some proteins, multiple folded polypeptides called subunits combine to make one larger functional protein. This is called quaternary protein structure. The protein hemoglobin is an example of a protein that has quaternary structure. It is composed of four polypeptides that bond together to form a functional oxygen carrier.
Figure 6.6. A protein has four different structural levels.
VIDEO: "What is a Protein," by RCSBProteinDataBank, YouTube (September 4, 2016), 2:38 minutes. This video gives an overview of the structure of amino acids, the four different structural levels of protein, and examples of different types of proteins in the body.
A protein’s structure also influences its nutritional quality. Large fibrous protein structures are more difficult to digest than smaller proteins and some, such as keratin, are indigestible. Because digestion of some fibrous proteins is incomplete, not all of the amino acids are absorbed and available for the body to utilize, thereby decreasing their nutritional value.
The specific three-dimensional structure of proteins can be disrupted by changes in their physical environment, causing them to unfold. This is called denaturation, and it results in loss of both structure and function of proteins. Changes in pH (acidic or basic conditions) and exposure to heavy metals, alcohol, and heat can all cause protein denaturation. The proteins in cooked foods are at least partially denatured from the heat of cooking, and denaturation in the stomach is an important part of protein digestion, as we’ll discuss later in this unit. We can see everyday examples of denaturation in cooking techniques, like how egg whites become solid and opaque with cooking, and cream becomes fluffy when it’s whipped. Both of these are examples of denaturation leading to physical changes in protein structure, and because protein structure determines function, denaturation also causes proteins to lose their function.
Shape Determines Function
An important concept with proteins is that shape determines function. A change in the amino acid sequence will cause a change in protein shape. Each protein in the human body differs in its amino acid sequence and consequently, its shape. The synthesized protein is structured to perform a particular function in a cell. A protein made with an incorrectly placed amino acid may not function properly, and this can sometimes cause disease. An example of this is sickle cell anemia, a genetic disorder. Below is a picture of hemoglobin, a protein with a globular three-dimensional structure. When packed in red blood cells to deliver oxygen, this structure gives red blood cells a donut shape.
Figure 6.7. Structure of hemoglobin
In people with sickle cell anemia, DNA gives cells the incorrect message when bonding amino acids together to make hemoglobin. The result is crescent-shaped red blood cells that are sticky and do not transport oxygen like normal red blood cells, as illustrated in the figure below.
Figure 6.8. Difference in blood cells and blood flow between normal red blood cells and sickle shaped blood cells.
Self-Check:
Attributions:
- Lindshield, B. L. Kansas State University Human Nutrition (FNDH 400) Flexbook. goo.gl/vOAnR, CC BY-NC-SA 4.0
- “Defining Protein,” section 6.1 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
Image Credits:
- Figure 6.1. “Amino acid structure” from “Defining Protein”, section 6.1 from the book An Introduction to Nutrition (v. 1.0) is licensed under CC BY-NC-SA 3.0
- Figure 6.2. “Amino acids diagram” from Section 3.4 of Biology by OpenStax is licensed under CC BY 4.0
- Table 6.1. “Essential and nonessential amino acids” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
- Figure 6.3. “Protein synthesis diagram” from “Intro to gene expression (central dogma)” by Khan Academy is licensed under CC BY-NC-SA 4.0
- Figure 6.4. “Polypeptide chain” by the NIH is in the Public Domain
- Figure 6.5. “Peptide bond formation” by Yassine Mrabet is in the Public Domain
- Figure 6.6. “Structural levels of protein” from “Defining Protein”, section 6.1 from the book An Introduction to Nutrition (v. 1.0) is licensed under CC BY-NC-SA 3.0
- Figure 6.7. “Hemoglobin” by Richard Wheeler is licensed under CC BY-SA 3.0
- Figure 6.8. “Blood flow and red blood cell shape of normal and sickle cell shaped hemoglobin” by The National Heart, Lung, and Blood Institute (NHLBI) is in the Public Domain
In this section, we’ll discuss how to determine how much protein you need and your many choices in designing an optimal diet with high-quality protein sources.
How Much Dietary Protein Does a Person Need?
Because our bodies are so efficient at recycling amino acids, protein needs are not as high as carbohydrate and fat needs. The Recommended Dietary Allowance (RDA) for a sedentary adult is 0.8 g per kg body weight per day. This would mean that a 165-pound man and a 143-pound woman would need 60 g and 52 g of protein per day, respectively. The Acceptable Macronutrient Distribution Range (AMDR) for protein for adults is 10% to 35% of total energy intake. A Tolerable Upper Intake Limit for protein has not been set, but it is recommended that you not exceed the upper end of the AMDR.
Protein needs are higher for the following populations:
- growing children and adolescents
- women who are pregnant (they’re using protein to help grow a fetus)
- lactating women (breast milk has protein in it for the baby’s nutrition, so mothers need more protein to synthesize that milk)
- athletes
The Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine recommend 1.2 to 2.0 grams of protein per kilogram of body weight per day for athletes, depending on the type of training.1 Higher intakes may be needed for short periods during intensified training or with reduced energy intake.
Nitrogen Balance to Determine Protein Needs
The appropriate amount of protein in a person’s diet is that which maintains a balance between what is taken in and what is used. The RDAs for protein were determined by assessing nitrogen balance. Nitrogen is one of the four basic elements contained in all amino acids. When amino acids are broken down, nitrogen is released. Most nitrogen is excreted as urea in urine, but some urea is also contained in feces. Nitrogen is also lost in sweat and as hair and nails grow. The RDA, therefore, is the amount of protein a person should consume in their diet to balance the amount of protein used by the body, measured as the amount of nitrogen lost from the body. The Institute Of Medicine used data from multiple studies that determined nitrogen balance in people of different age groups to calculate the RDA for protein.
- Nitrogen Balance- A person is said to be in nitrogen balance when the nitrogen consumed equals the amount of nitrogen excreted. Most healthy adults are in nitrogen balance. If more protein is consumed than needed, this extra protein is used for energy, and the nitrogen waste that results is excreted. The lowest amount of protein a person can consume and still remain in nitrogen balance represents that person’s minimum protein requirement.
Figure 6.14. People are in nitrogen balance when they excrete as much nitrogen as they consume.
- Negative Nitrogen Balance- A person is in negative nitrogen balance when the amount of excreted nitrogen is greater than that consumed, meaning that the body is breaking down more protein to meet its demands. This state of imbalance can occur in people who have certain diseases, such as cancer or muscular dystrophy. Someone who is eating a low-protein diet may also be in negative nitrogen balance as they are taking in less protein than they actually need.
Figure 6.15. People are in negative nitrogen balance when they excrete more nitrogen than they consume, usually because they are not eating enough protein to meet their needs.
- Positive Nitrogen Balance- A person is in positive nitrogen balance when a person excretes less nitrogen than what is taken in by the diet, such as during pregnancy or growth in childhood. At these times the body requires more protein to build new tissues, so more of what gets consumed gets used up and less nitrogen is excreted. A person healing from a severe wound may also be in positive nitrogen balance because protein is being used up to repair tissues.
Figure 6.16. People are in positive nitrogen balance when they excrete less nitrogen than they consume, because they are using protein to actively build new tissue.
Dietary Sources of Protein
Although meat is the typical food that comes to mind when thinking of protein, many other foods are rich in protein as well, including dairy products, eggs, beans, whole grains, and nuts. Table 6.3 lists the grams of protein in a standard serving for a variety of animal and plant foods.
|
Animal Sources |
Grams of Protein per Standard Serving |
|
Egg White |
3 g per 1 large white |
|
Whole Egg |
6 g per 1 large egg |
|
Cheddar Cheese |
7 g per 1 oz. (30 g) |
|
Milk, 1% |
8 g per 1 cup (8 fl oz) |
|
Yogurt |
11 g per 8 oz |
|
Greek Yogurt |
22 g per 8 oz |
|
Cottage Cheese |
15 g per ½ cup |
|
Hamburger |
30 g per 4 oz |
|
Chicken |
35 g per 4 oz |
|
Tuna |
40 g per 6 oz can |
|
Plant Sources |
Grams of Protein per Standard Serving |
|
Almonds, dried |
6 g per 1 oz |
|
Almond milk |
1 g per cup (8 fl oz) |
|
Soy milk |
8g per cup (8 fl oz) |
|
Peanut butter |
4 g per 1 tbsp |
|
Hummus |
8 g per ½ cup |
|
Refried beans |
6 g per ½ cup |
|
Lentil soup |
11 g per 10.5 oz |
|
Tofu, extra firm |
11 g per 3.5 oz |
|
Enriched wheat bread |
1 g per slice (45 g) |
|
Whole Grain Bread |
5g per slice (45 g) |
|
Grape Nuts |
7 g per ½ cup |
Table 6.3. Protein in common foods2
Notice in the table above that whole foods contain more protein than refined foods. When foods are refined—for example, going from a whole almond to almond milk or whole grain to refined grain—protein is lost in that processing. Very refined foods like oil and sugar contain no protein.
The USDA provides some tips for choosing your dietary protein sources. The overall suggestion is to eat a variety of protein-rich foods to benefit health. Examples include:
- Lean meats, such as round steaks, top sirloin, extra lean ground beef, pork loin, and skinless chicken.
- 8 ounces of cooked seafood every week (typically as two 4-ounce servings).
- Choosing to eat beans, peas, or soy products as a main dish. For example, chili with kidney and pinto beans, hummus on pita bread, and black bean enchiladas.
- Enjoy nuts in a variety of ways. Put them on a salad, in a stir-fry, or use them as a topping for steamed vegetables in place of meat or cheese.
Protein Quality
While protein is contained in a wide variety of foods, it differs in quality. High-quality complete proteins contain all nine essential amino acids. Lower-quality incomplete proteins do not contain all nine essential amino acids in proportions needed to support growth and health. 
Foods that are complete protein sources include animal foods such as milk, cheese, eggs, fish, poultry, and meat. A few plant foods also are complete proteins, such as soy (soybeans, soy milk, tofu, tempeh) and quinoa.
Most plant-based foods are deficient in at least one essential amino acid and therefore are incomplete protein sources. For example, grains are usually deficient in the amino acid lysine, and legumes are low in methionine and tryptophan. Because grains and legumes are not deficient in the same amino acids, they can complement each other
in a diet. When consumed in tandem, they contain all nine essential amino acids at adequate levels, so they are called complementary proteins. Some examples of complementary protein foods are given in Table 6.4. Mutual supplementation is another term used when combining two or more incomplete protein sources to make a complete protein. Complementary protein sources do not have to be consumed at the same time—as long as they are consumed within the same day, you will meet your protein needs. Most people eat complementary proteins without thinking about it, because they go well together. Think of a peanut butter sandwich and beans and rice; these are examples of complementary proteins. So long as you eat a variety of foods, you don’t need to worry much about incomplete protein foods. They may be called “lower quality” in terms of protein, but they’re still great choices, as long as they’re not the only foods you eat!
|
Foods |
Lacking Amino Acids |
Complementary Food |
Complementary Menu |
|
Legumes |
Methionine, tryptophan |
Grains, nuts, and seeds |
Hummus and whole-wheat pita |
|
Grains |
Lysine, isoleucine, threonine |
Legumes |
Cornbread and kidney bean chili |
|
Nuts and seeds |
Lysine, isoleucine |
Legumes |
Stir-fried tofu with cashews |
Table 6.4. Complementary protein sources
The second component of protein quality is digestibility, as not all protein sources are equally digested. In general, animal-based proteins are more fully digested than plant-based proteins, because some proteins are contained in the plant’s fibrous cell walls and these pass through the digestive tract unabsorbed by the body. Animal proteins tend to be 95 percent or more digestible; soy is estimated at 91 percent; and many grains are around 85 to 88 percent digestible.3
Self-Check:
Attributions:
- “Proteins, Diet, and Personal Choice”, section 6.4 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
References:
- 1Thomas, D. T., Erdman, K. A., & Burke, L. M. (2016). Position of Dietitians of Canada, the Academy of Nutrition and Dietetics and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics, 116(3), 501-528.
- 2USDA National Nutrient Database for Standard Reference, December 2018.
- 3Tome, D. (2012). Criteria and markers for protein quality assessment – a review. British Journal of Nutrition 108, S222–S229.
Image Credits:
- Fig 6.14. “Nitrogen balance” by Tamberly Powell is licensed under CC BY-NC-SA 2.0 with “Dancing Exercises” by Forum Danca is licensed under CC BY-NC 2.0
- Fig 6.15. “Negative nitrogen balance” by Tamberly Powell is licensed under CC BY-NC-SA 2.0 with “Indian Prisoners of War” by Chris Turner is licensed under CC BY 2.0
- Fig 6.16. “Positive nitrogen balance” by Tamberly Powell is licensed under CC BY-NC-SA 2.0 with “Child at Seoul” by Philippe Teuwen is licensed under CC BY-SA 2.0
- Table 6.3. “Protein in common foods” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
- “Meat!” by Chris Suderman is licensed under CC BY-NC-ND 2.0
- “Mexican-Rice-and-Beans-2” by Meg H is licensed under CC BY 2.0
- Table 6.4. “Complementary protein sources” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
Proteins are the “workhorses” of the body and participate in many bodily functions. As we’ve already discussed, proteins come in all sizes and shapes, and each is specifically structured for its particular function. This page describes some of the important functions of proteins. As you read through them, keep in mind that synthesis of all of these different proteins requires adequate amounts of amino acids. As you can imagine, consuming a diet that is deficient in protein and essential amino acids can impair many of the body’s functions. (More on that later in the unit.)
Figure 6.9. Examples of proteins with different functions, sizes, and shapes.
Major types and functions of proteins are summarized in the table below, and the subsequent sections of this page give more detail on each of them.
|
Protein Types and Functions |
||
|
Type |
Examples |
Functions |
|
Structure |
Actin, myosin, collagen, elastin, keratin |
Give tissues (bone, tendons, ligaments, cartilage, skin, muscles) strength and structure |
|
Enzymes |
Amylase, lipase, pepsin, lactase |
Digest macronutrients into smaller monomers that can be absorbed; performs steps in metabolic pathways to allow for nutrient utilization |
|
Hormones |
Insulin, glucagon, thyroxine |
Chemical messengers that travel in blood and coordinate processes around the body |
|
Fluid and acid-base balance |
Albumin, hemoglobin |
Maintains appropriate balance of fluids and pH in different body compartments |
|
Transport |
Hemoglobin, albumin, protein channels, carrier proteins |
Carry substances around the body in the blood or lymph; help molecules cross cell membranes |
|
Defense |
Collagen, lysozyme, antibodies |
Protect the body from foreign pathogens |
Table 6.2. Protein types and functions
Structure
More than one hundred different structural proteins have been discovered in the human body, but the most abundant by far is collagen, which makes up about 6 percent of total body weight. Collagen makes up 30 percent of bone tissue and comprises large amounts of tendons, ligaments, cartilage, skin, and muscle. Collagen is a strong, fibrous protein made up of mostly glycine and proline amino acids. Within its quaternary structure, three protein strands twist around each other like a rope and then these collagen ropes overlap with others.
Figure 6.10. Triple-helix structure of collagen
This highly ordered structure is even stronger than steel fibers of the same size. Collagen makes bones strong but flexible. Collagen fibers in the skin’s dermis provide it with structure, and the accompanying elastin protein fibrils make it flexible. Pinch the skin on your hand and then let go; the collagen and elastin proteins in skin allow it to go back to its original shape. Smooth-muscle cells that secrete collagen and elastin proteins surround blood vessels, providing the vessels with structure and the ability to stretch back after blood is pumped through them. Another strong, fibrous protein is keratin, an important component of skin, hair, and nails.
Enzymes
Enzymes are proteins that conduct specific chemical reactions. An enzyme’s job is to provide a site for a chemical reaction and to lower the amount of energy and time it takes for that chemical reaction to happen (this is known as “catalysis”). On average, more than 100 chemical reactions occur in cells every single second, and most of them require enzymes. The liver alone contains over 1,000 enzyme systems. Enzymes are specific and will use only particular substrates that fit into their active site, similar to the way a lock can be opened only with a specific key. Fortunately, an enzyme can fulfill its role as a catalyst over and over again, although eventually it is destroyed and rebuilt. All bodily functions, including the breakdown of nutrients in the stomach and small intestine, the transformation of nutrients into molecules a cell can use, and building all macromolecules, including protein itself, involve enzymes.
Figure 6.11. Enzymes are proteins. An enzyme’s job is to provide a site for substances to chemically react and form a product, and decrease the amount of energy and time it takes for this to happen.
VIDEO: “Enzymes,” by Amoeba Sisters, YouTube (August 28, 2016), 5:46 minutes. This video demonstrates the action of enzymes.
Hormones
Proteins are responsible for hormone synthesis. Hormones are the chemical messengers produced by the endocrine glands. When an endocrine gland is stimulated, it releases a hormone. The hormone is then transported in the blood to its target cell, where it communicates a message to initiate a specific reaction or cellular process. For instance, after you eat a meal, your blood glucose levels rise. In response to the increased blood glucose, the pancreas releases the hormone insulin. Insulin tells the cells of the body that glucose is available and to take it up from the blood and store it or use it for making energy or building macromolecules. A major function of hormones is to turn enzymes on and off, so some proteins can even regulate the actions of other proteins. While not all hormones are made from proteins, many of them are.
Fluid and Acid-Base Balance
Adequate protein intake enables the basic biological processes of the body to maintain homeostasis (constant or stable conditions) in a changing environment. One aspect of this is fluid balance, keeping water distributed properly in the different compartments of the body. If too much water suddenly moves from the blood into a tissue, the results are swelling and, potentially, cell death. Water always flows from an area of high concentration to an area of low concentration. As a result, water moves toward areas that have higher concentrations of other solutes, such as proteins and glucose. To keep the water evenly distributed between blood and cells, proteins continuously circulate at high concentrations in the blood. The most abundant protein in blood is the butterfly-shaped protein known as albumin. The presence of albumin in the blood makes the protein concentration in the blood similar to that in cells. Therefore, fluid exchange between the blood and cells is not in the extreme, but rather is minimized to preserve homeostasis.
Figure 6.12. The butterfly-shaped protein, albumin, has many functions in the body including maintaining fluid and acid-base balance and transporting molecules.
Protein is also essential in maintaining proper pH balance (the measure of how acidic or basic a substance is) in the blood. Blood pH is maintained between 7.35 and 7.45, which is slightly basic. Even a slight change in blood pH can affect body functions. The body has several systems that hold the blood pH within the normal range to prevent this from happening. One of these is the circulating albumin. Albumin is slightly acidic, and because it is negatively charged it balances the many positively charged molecules circulating in the blood, such as hydrogen protons (H+), calcium, potassium, and magnesium. Albumin acts as a buffer against abrupt changes in the concentrations of these molecules, thereby balancing blood pH and maintaining homeostasis. The protein hemoglobin also participates in acid-base balance by binding hydrogen protons.
Transport
Proteins also play vital roles in transporting substances around the body. For example, albumin chemically binds to hormones, fatty acids, some vitamins, essential minerals, and drugs, and transports them throughout the circulatory system. Each red blood cell contains millions of hemoglobin molecules that bind oxygen in the lungs and transport it to all the tissues in the body. A cell’s plasma membrane is usually not permeable to large polar molecules, so to get the required nutrients and molecules into the cell, many transport proteins exist in the cell membrane. Some of these proteins are channels that allow particular molecules to move in and out of cells. Others act as one-way taxis and require energy to function.
Figure 6.13. Molecules move in and out of cells through transport proteins, which are either channels or carriers.
VIDEO: “The Sodium-Potassium Pump,” by RicochetScience, YouTube (May 23, 2016), 2:26 minutes. This tutorial describes how the sodium-potassium pump uses active transport to move sodium ions (Na+) out of a cell, and potassium ions (K+) into a cell.
Immunity
Proteins also play important roles in the body’s immune system. The strong collagen fibers in skin provide it with structure and support, but it also serves as a barricade against harmful substances. The immune system’s attack and destroy functions are dependent on enzymes and antibodies, which are also proteins. For example, an enzyme called lysozyme is secreted in the saliva and attacks the walls of bacteria, causing them to rupture. Certain proteins circulating in the blood can be directed to build a molecular knife that stabs the cellular membranes of foreign invaders. The antibodies secreted by white blood cells survey the entire circulatory system, looking for harmful bacteria and viruses to surround and destroy. Antibodies also trigger other factors in the immune system to seek and destroy unwanted intruders.
VIDEO: “Specific Immunity, Antibodies,” by Carpe Noctum, YouTube (December 11, 2007), 1 minute. Watch this video to observe how antibodies protect against foreign intruders.
Energy Production
Some of the amino acids in proteins can be disassembled and used to make energy. Only about 10 percent of dietary proteins are catabolized each day to make cellular energy. The liver is able to break down amino acids to the carbon skeleton, which can then be fed into the citric acid or Krebs cycle. This is similar to the way that glucose is used to make ATP. If a person’s diet does not contain enough carbohydrates and fats, their body will use more amino acids to make energy, which can compromise the synthesis of new proteins and destroy muscle proteins if calorie intake is also low.
Not only can amino acids be used for energy directly, but they can also be used to synthesize glucose through gluconeogenesis. Alternatively, if a person is consuming a high protein diet and eating more calories than their body needs, the extra amino acids will be broken down and transformed into fat. Unlike carbohydrate and fat, protein does not have a specialized storage system to be used later for energy.
Self-Check:
Attributions:
- “Protein Functions”, section 6.4 from the book An Introduction to Nutrition (v. 1.0), CC BY-NC-SA 3.0
Image Credits:
- Fig 6.9. ”Enzyme , antibody, and hormone” from “Protein Functions”, section 6.4 from the book An Introduction to Nutrition (v. 1.0), is licensed under CC BY-NC-SA 3.0
- Table 6.2. “Protein types and functions” by Tamberly Powell is licensed under CC BY-NC-SA 2.0
- Fig 6.10. “Collagentriplehelix” by JWSchmidt is licensed under CC BY-SA 3.0
- Fig 6.11. “Enzyme activity” from “Protein Functions”, section 6.4 from the book An Introduction to Nutrition (v. 1.0), is licensed under CC BY-NC-SA 3.0
- Fig 6.12. “Albumin” by Jawahar Swaminathan and MSD staff is in the Public Domain
- Fig 6.13. “Protein carriers in cell membranes” by LadyofHats, Mariana Ruiz Villarreal is in the Public Domain
