April 18, 2021

Macronutrients Series: Let’s Break Down Protein

We have arrived at the last post in our Macronutrients Series! Today, it’s all about protein. Warning: another science-heavy post! If you’re into it, please enjoy :) if not, you can scroll down to the bottom line message for the key takeaways.

If you missed it, we have previously covered the other 2 macronutrients, carbohydrates and fat. As a reminder, macronutrients (carbs, fat, protein) are what give us energy (i.e., calories) from foods. Think of them as the building blocks of our food. We need all 3, and all 3 have different functions in our body. A well-balanced meal has all 3 macronutrients. This is what helps us 1) get the various nutrients we need, 2) feel satisfied and full after eating.

Table of Contents

What is protein?

Proteins are built from amino acids held together by peptide bonds.


Wait, back up, what is an amino acid?

We have talked about how carbs, fat and protein are like the building blocks of our food. Similarly, amino acids are the building blocks of proteins. There are hundreds of amino acids found in nature, but only about 20 amino acids are needed to make all of the proteins in the human body.

Chemically speaking, amino acids are composed of a central carbon, an amino group (contains nitrogen), a carboxyl group, and a side chain. The side chain piece is what makes each amino acid unique. There are lots of folding and bonds involved that hold peptides (basically short chains of amino acids) together, ultimately determining the structure and function of each protein. Of the 20 or so amino acids used to make proteins in the human body, 9 are categorized as essential. This means our bodies cannot make them and we must get them from food.


Food Sources of Protein

Protein foods are sources of essential amino acids and nitrogen that we need to make other non-essential amino acids. We find these proteins in animal products, like meat, poultry, fish, eggs, milk, yogurt. Proteins can also be found in plant products like legumes, nuts, seeds, soy, whole grains, and even in vegetables to some extent.

Legumes as protein source

What’s this complete vs incomplete nonsense I’ve heard of?

If you’ve heard of the terms “complete” or “incomplete” proteins before, that is referring to whether or not a food source of protein provides all 9 of the essential amino acids. Animal proteins are typically considered complete proteins because they provide all of the essential amino acids, whereas many plant proteins are incomplete in that they provide some or most, but not all, of the essential amino acids. 

This does not mean you HAVE to eat animal foods if that’s not your thing. Variety is the spice of life as they say, so as long as you are getting in a variety of foods and including a protein source with each meal/snack, it is likely that you are getting all of the essential amino acids you need.


Why do we need protein?

Proteins are found ALL throughout our bodies. Our very existence depends on proteins, which make up over half of the solid content of cells. All enzymes in our bodies are proteins, meaning all metabolic pathways are catalyzed by proteins. Some hormones are peptides like insulin and glucagon, which regulate our blood sugar. Proteins are structural elements like actin and myosin in our muscle, or collagen and keratin in our skin, bone, tendons, blood vessels, hair, nails. Antibodies that work as part of our immune system are all proteins. Proteins help transport substances through the bloodstream and in and out of cells. Proteins act as buffers to help with regulation of acid-base balance, and they help regulate fluid balance. 

Are you impressed yet?! Proteins are wildly busy keeping our bodies intact and functioning.


Quick run-down on digestion

Protein digestion begins in the stomach with our stomach acids, which act to unfold the proteins a bit and make it easier for enzymes to start breaking up the peptide bonds. Now we have a mix of components of proteins that are emptied into the small intestine.

In the small intestine, enzymes continue to work on further breaking down these protein components into smaller and smaller pieces until they can be absorbed through the cells of our small intestine to be used by those cells or to travel through the bloodstream to other parts of our body. One key destination is the liver, which can make ALL sorts of proteins for use by other organs throughout the body. 


Bottom line: Protein is essential! We can get proteins from animal products, plant products, or a mix of the 2. Aim to eat a variety of protein sources, and include a protein source with each meal/snack.


Photo by Emerson Vieira on Unsplash

Photo by Shelley Pauls on Unsplash

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About the Author: Kelly LaVoie, MS, RD, LDN

I'm a Registered Dietitian (RD) based in Chicago, IL. I have worked in both inpatient and outpatient settings, including dialysis, ICU, as well as one-on-one nutrition counseling and groups.