In This Chapter

  • What is a protein?
  • The function of amino acids in the body
  • The process of protein digestion and absorption
  • Complete and incomplete proteins
  • How not eating enough protein affects health
  • What happens if you eat more protein than your body needs?

Protein is one of the three macronutrients along with fat and carbohydrates. The word protein comes from the Greek term for “primary,” proteios. All macronutrients are “energy” nutrients that provide calories. Protein, just like carbohydrates, provides 4 calories per gram.

Each cell in your body contains protein and it is involved in the development, maintenance, and repair of the human body. In this chapter, we'll look at what protein is made of, how much you need each day, and examine its role in your body. We’ll also discuss why consumption of complete proteins is important.

The Building Blocks of the Body

Proteins are large molecules made up of strands of amino acids linked together. Variations of proteins are made depending on the order of amino acids. This order is predetermined by the genetic code in the DNA. Proteins don’t live forever; their lifespan varies from a few minutes to a few years. Eventually they need to be replicated.

Protein is present in every single cell in our bodies. Proteins have many jobs and work together to perform many functions. Some of the functions of proteins include acting as enzymes, neurotransmitters, and structural components of the cells. Inside each cell there’s genetic information called DNA that provides instructions for making every type of protein needed by the body. DNA contains the genetic information for how cells will grow, absorb nutrients, act, and the job they will do. When new proteins are needed, RNA comes into play.


Deoxyribonucleic acid (DNA) is a molecule in your cells that contains the genetic information on how to build and maintain an organism—basically instructions for building a human—you. Ribonuleic acid (RNA) is a molecule present in your cells that acts as a messenger to translate the information received from DNA to build proteins.

Inside the center or nucleus of each cell is a system set up like a tiny factory line whose job is to replicate DNA. This system can read the genetic information from DNA, then use this information to make an exact copy of the DNA in the form of messenger RNA. The messenger RNA (mRNA) carries all the instructions needed for making new proteins. Once formed, the mRNA leaves the center of the cell and moves to the outer part of the cell, called the cytoplasm.

Located in the cytoplasm is another tiny factory system called ribosome whose job is to get all the instructions from the mRNA to make new proteins. Transfer RNA (tRNA) then gathers amino acids and brings them to the ribosome. Ribosomal RNA (rRNA) then uses the amino acids (from the tRNA) and instructions (from the mRNA) to make new proteins. These new proteins exactly match the specifications dictated by the mRNA. This new protein then is released from the ribosome and travels to its predetermined workplace.

Protein serves many functions in the body, including building bones, repairing and replacing damaged cells, and making enzymes, antibodies, and hormones. It also helps maintain bodily fluid levels and pH balance. Additionally, protein aids in the transportation of nutrients and molecules throughout the body.

Due to protein’s important role in the body, it has been given the nickname the “ultimate building block” of the body. After all, it makes up 18 to 20 percent of our body weight, with about 40 percent of all the body’s protein in muscle.

Now that we’ve discussed the origin of protein synthesis at the cellular level, let’s look at the assembled proteins. Proteins are a sequence of 50 to 20,000 amino acids all linked together. Each sequence is distinct, and one mishap could alter a protein’s structure and cause an error to occur, which could translate into a disease or, in other words, a genetic defect.

An amino acid is similar in structure to a carbohydrate, but it contains an amino group (NH2) and an additional carboxyl group (COOH). There are 9 essential amino acids and 11 nonessential amino acids. They’re categorized as such because essential amino acids must be obtained from food due to the fact that the body doesn’t have the capacity to make them. Nonessential amino acids can be made from other amino acids.

The 9 essential amino acids are tryptophan, threonine, isoleucine, leucine, lysine, methionine, phenylalanine, valine, and histidine. Tryptophan is the most recognizable essential amino acid on the list. It’s found in turkey and milk and is conducive to making one feel tired or sleepy.

Key Functions of Essential Amino Acid


Aids in the manufacture of brain chemicals used as neurotransmitters


Aids in muscle growth


Maintains the immune system


Aids in sleep


Required in the formation of hemoglobin a molecule in blood cells that helps carry oxygen throughout the body


Maintains skin


Makes hormones


Makes enzymes, antibodies, and hormones


Involved in the synthesis and production of blood cells

In total, the body needs 20 amino acids, but from the 9 essentials it can synthesize the remaining 11 amino acids, which are alanine, arginine, aspartic acid, asparagine, glutamic acid, glutamine, glycine, proline, serine, cysteine, and tyrosine.


The hair on your head is made up entirely of dead compacted protein cells called keratin. Just beneath your skin is the hair bulb, which holds the root and anchors the hair. The cells are alive and growing at the root but as they extrude out of the hair follicle through the scalp where you can see it, the cells are no longer living.

The body needs a large pool of amino acids to pick and choose from in order to recombine them to make the type of protein required by the body. It keeps about 100 grams of amino acids available for this purpose. Proteins in the body are constantly being made and broken down and repackaged to the tune of about 300 to 400 grams of body protein per day, which are more grams than you would consume in a day.

Unlike carbohydrates, protein is not stored in the body even though it’s an energy nutrient. After the molecule is broken apart, some components are saved or converted to fat, but the majority is released into your urine.

How Proteins Are Broken Down

When you eat protein, the chewing action of the teeth actually begins to break apart some of the long chains of protein. When protein enters the stomach, hydrochloric acid and the enzyme pepsin begin to uncoil the long strands of protein and break them up into smaller strandlike molecules.

Next, these shorter strands move into the small intestine where pancreatic enzymes are released along with additional enzymes, which break the molecules into even smaller particles like single amino acids. These amino acids are transported through the intestinal cells by a carrier protein or by active transport, which requires energy to travel into the mucosal cells. After absorption, they’re distributed through the bloodstream to wherever they are needed in the body.

Where to Find Protein

Protein is the building block for more than the human body in this universe. Proteins make up cells in animals and plants alike, albeit in different amounts. The highest amounts per ounce come from animal sources such as meat, poultry, eggs, fish, and shellfish; it’s also present in plant foods such as beans, peas, and whole grains, although in lesser amounts. For example, 1 ounce of lean meat contains about 7 grams, whereas 1 ounce of pinto beans contains 2½ grams of protein.

Not all proteins are made equal in nature. When learning about the bioavailability of proteins and their quality in our diet, it’s helpful to view humans as animals. Then it makes sense that the proteins found in other animals such as beef and chicken are more like the proteins in our bodies. The proteins in plants are less similar. This means that animal protein is more bioavailable, or easier for our bodies to digest and absorb.


Some proteins take longer to digest than others. For example, whey protein found in dairy can be digested in 1½ hours, whereas the protein in a steak may take 24 to 72 hours. A factor in the protein absorption rate is the nutrients it’s accompanied by. An increase in fat or fiber will delay the time it takes for proteins to reach cells in your body. This makes sense in the steak example and is why whey protein shakes are popular among fitness enthusiasts. The protein is extracted from milk and has little to no fat, which allows the protein to be more quickly digested and absorbed and to reach the muscle tissue faster.

Animal-Based Protein

Proteins present in animal foods and animal byproducts contain all of the essential amino acids your body needs, and are considered to be high-quality proteins. However, along with some of those high-protein foods come a higher amount of fat and cholesterol that your body does not benefit from, such as fatty cuts of meat and cholesterol. There are plenty of lean meats, fish, and low-fat options available. The leanest and most tender cuts of beef and pork come from the tenderloin. These have about 1-1½ grams of fat per ounce. Shellfish such as shrimp are also low in fat, and 1 ounce of shrimp contains less than 0.1 gram of fat.

Animal Sources of Protein


Protein in grams

3 oz. beef fillet


3 oz. ground bison


3 oz. pork tenderloin


3 oz. lamb chops


3 oz. turkey breast


3 oz. chicken breast


3 oz. wild salmon fillet


3 oz. cod fish


3 oz. bluefin tuna


3 oz. shrimp


3 oz. scrambled eggs


3 oz. 1% fat cottage cheese


It’s easy to meet your protein needs with animal-based protein foods. Most people exceed their needs daily.


Collagen is a protein that forms connective tissues. It’s used to make gelatin-type desserts and also used to make marshmallows.

Plant-Based Protein

The majority of plant-based proteins contain limited amounts of the essential amino acids, or they’re present in very low amounts to meet your body’s requirements for protein synthesis. The exceptions to this are soybeans, quinoa, and spinach, which are considered to be complete protein foods, meaning they contain all nine essential amino acids.

In order for the body to use all the amino acids from noncomplete protein foods, it must combine them with other foods to create complementary proteins. For example, rice and beans are a staple combination in many cultures around the world. They might not have known it under these terms, but ancient civilizations discovered combining rice and beans resulted in healthier people. The limiting amino acid in rice is lysine, which means it’s the essential amino acid not present in rice. Beans and legumes contain this amino acid, but they lack methionine. Rice contains plenty of methionine. By combining the two foods, you have a complete protein with all essential amino acids necessary for good health. Complementary proteins don’t have to be consumed at the same meal as long as they’re eaten during the course of the day.

The following chart shows vegetarian sources of proteins.

Vegetarian Foods High in Protein


Protein in grams

1 cup black beans


1 cup amaranth


1 cup lentils


1 cup chickpeas


1 cup wild rice


1 cup buckwheat


3 tsp. hemp seed


1 artichoke


1 cup arugula


1 cup brown rice


1 cup brussels sprouts


¼ cup pecans


¼ cup almonds


1 cup asparagus


½ cup tempeh


Meeting your daily protein requirements can be easily attainable using vegetarian plant-based foods.


Tempeh is a soy product made from fermented soybeans with the addition of a mold (Rhizopus oligosporus) and formed into a shape. It has a firm and chewy texture with a slightly nutty taste.

How Much Do I Need?

The Recommended Dietary Allowance (RDA) for protein ranges from 0.8 grams of protein per kilogram (kg) of body weight to 1.3 grams/kg per day depending on your age, sex, and whether you’re pregnant or breastfeeding. The acceptable macronutrient distribution range (AMDR) for protein is 10 to 35 percent of your total calories. Most people require about 0.8 grams of protein per kilogram of body weight per day.

Calculate your protein needs by first converting your body weight from pounds to kilograms (1 pound = 2.2 kilograms). Next multiply your weight in kg × 0.8 grams protein. Let’s look at an example:

176-pound woman

176 pounds/2.2 = 80 kilograms

80 kilograms × 0.8 grams protein = 64 grams protein per day

Pregnant and nursing mothers need more protein to support the growth of a baby as well as additional protein to produce milk. It’s recommended that they consume between 10 and 20 grams more daily.

Athletes need more protein as well: about 1.4 to 1.8g/kg/day for strength training and 1.2 to 1.4g/kg/day for endurance runners. Overall, most athletes need about 1.0 to 1.8g/kg/day.

What If I Can’t Get Enough?

The average American diet provides well over the recommended daily intake. According to the 2011-2012 National Health and Nutrition Examination Survey, males aged 20-49 average protein intake was 102-104 grams per day and women aged 20-49, 66-72 grams. Protein deficiencies are almost exclusive to developing countries.

When children don’t get enough protein in their diet, it’s referred to as Protein Energy Malnutrition (PEM), meaning there are either not enough calories or protein to support proper growth and overall bodily functions. Two of the most common PEM conditions are marasmus and kwashiorkor.

Marasmus is a lack of protein and calories in which the infants and small children affected are bony in appearance. Kwashiorkor is a condition resulting from a lack of protein, although calorie levels might be sufficient. This happens when an older child has a high-carbohydrate diet and consumes enough calories throughout the day, but is insufficient in protein. This child has a distended belly from water retention, misleading some people to believe the affected child is full of food or overweight. Since protein plays a role in regulating bodily fluid levels and pH, a child with kwashiorkor has temporarily lost the ability to complete this function successfully.

Although these two PEM conditions exist mostly in developing countries, first-world countries such as the United States are not exempt. Eating disorders such as bulimia, anorexia, and even overeating could be the cause for mild to severe protein deficiencies.

When a person is experiencing a protein deficiency in the diet, the body will begin using the protein in muscles by tearing them down and converting existing proteins to the particular kind that the body is in need of to function. Many factors effect protein needs: stress levels, injuries, increase/decrease in activity level, or breastfeeding, for example.


Amino acid supplementation is not recommended as it can disrupt your body’s normal processes, and leads to an abnormal demand for some amino acids while creating a deficiency for others.

Protein supplements are one method of including more protein in the diet, and there are many available in the marketplace, especially in the form of powders to make protein shakes. There are also protein bars and nutritional ready-to-drink shakes. Animal-based protein powder is made from whey (dairy protein), while vegetarian protein powders are made from ingredients such as soy, brown rice, peas, and artichokes. Most mix easily with water, milk, or milk alternatives such as rice milk and soymilk. Athletes may benefit from supplementing their diets with protein powders for fast digestibility. It’s also a good solution for some elderly and the disabled, who are unable or less likely to cook and eat a balanced diet due to mobility or chewing issues.

Is More Protein Better?

Eating more protein than your body needs typically won’t harm you if you’re in good health. Your body will break down the protein and use whatever it needs, then dispose of the excess and waste byproducts in your urine. However, you may need to consume a little more water to aid in this process.

Many untrained athletes believe that more protein is better because protein helps build muscle. While this is true, eating more won’t build more muscle. When a muscle is stressed by lifting weights, it causes small tears in the muscle fibers. The body will use its sources of amino acids to rebuild and repair the protein tears in the muscle, and thus build more muscle.

Some studies have suggested there’s a link between high-protein diets and chronic diseases such as heart disease, cancer, and osteoporosis, although research on this topic is limited. In some instances, overconsumption of protein could cause serious problems such as undiagnosed kidney disease, in which case it would place extra stress on the kidneys.

Errors in Protein Metabolism

A rare inherited disorder called phenylketonuria (PKU) occurs when a person has a defect in a gene that helps make the enzyme required to break down phenylalanine. The inability to digest this amino acid leads to a buildup in the body, which causes brain damage along with damage to the central nervous system.

All newborns in the United States are tested for PKU disease. Symptoms can be mild or severe and include delayed development, psychiatric disorders, neurological disorders, and being born with a small head.

Treatment includes following a diet that’s low in phenylalanine, along with the addition of vitamin and mineral supplements. Phenylalanine is found in milk, eggs, and other foods. The artificial sweetener aspartame also contains phenylalanine and should be avoided. It’s imperative that the diet be strictly followed, as PKU can cause brain damage in the first year of life.

High-Protein Recipe

The following recipe is a great source of high protein. Protein is an indispensable nutrient. Its functions are numerous throughout the body, and it really is the ultimate building block for the body and for good health.

Buffalo Chicken Stuffed Mushrooms

This is a great game-time appetizer with a spicy kick to share with a crowd.

16 oz. baby bella mushrooms

2 TB. olive oil

¼ tsp. kosher salt

⅛ tsp. ground black pepper

12 oz. chicken breast, roughly chopped

¼ cup buffalo sauce

2 cloves garlic

⅔ cup Panko (Japanese bread crumbs)


¼ cup light sour cream

2 tsp. buffalo sauce

1. Preheat oven to 400°F.

2. Quickly rinse mushrooms and let drain. Make certain the mushrooms are fairly dry and remove any excess water by patting down with a paper towel.

3. Gently remove stems from the mushroom caps and place the stems into a food processor fitted with a chopping blade. Set aside.

4. Place the caps into a medium-size mixing bowl. Drizzle olive oil over the mushroom caps and add salt and pepper. Toss gently until well coated.

5. Place mushroom caps onto a greased sheet pan.

6. Add to the food processor chicken breast, ¼ cup buffalo sauce, garlic, and Panko. Pulse about 45 seconds or until well combined.

7. Spoon a heaping teaspoon of the chicken mixture into each mushroom cap and place onto sheet pan. Repeat until all caps are stuffed.

8. Bake for about 15 minutes.

9. Combine sour cream and 2 teaspoons buffalo sauce in a small bowl. Mix well. Dollop each mushroom cap with sauce. Serve warm.

The Least You Need to Know

  • Protein is found in both plant and animal products.
  • There are 9 essential amino acids and 11 nonessential amino acids used as the building blocks of protein by your body.
  • Protein works as the body’s repair kit.
  • Complementary proteins don’t need to be consumed at the same time to provide benefits, just over the course of the day.
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