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Daily Nutrition & Health

Why You Need Protein: How Amino Acids Work in Your Body

Susan Bowerman M.S., R.D., CSSD, CSOWM, FAND – Sr. Director, Worldwide Nutrition Education and Training 19 July 2023

You know it’s important to consume protein every day, but do you really know why? What exactly is protein and what does it do for your body? Let’s break it down.

Protein is a macronutrient that’s a vital component of a healthy diet. Ever since you were a kid, you’ve probably heard that protein would make you “big and strong.” And, while there’s some debate as to who actually created the word “protein” (it first appeared in the scientific literature in 1838), there’s no disagreement that it was derived from the Greek word “protos” – meaning “first rank or position” – in recognition of how important protein is to life.

What Are Amino Acids? How Many Are There?

The proteins you eat (and the proteins already in your body) are all made up of small units called amino acids. You often hear amino acids described as “building blocks” because these small individual units are assembled in various ways to build proteins.

It may help to think of amino acids in the same way as letters of the alphabet. In English, we use just 26 letters to make up all the words that we write and speak. Some words are short, some are long – but we create millions of words from just 26 letters. The final sequence of the letters is what gives each word its sound and its meaning.

Similarly, there are 20 amino acids that can be strung together to make proteins – the ones you eat and the ones that are made by your body. And just as we don’t use all 26 letters to make every word, most proteins don’t contain all 20 amino acids, either.

But – just as letters are strung together to make words – amino acids can be strung together in different sequences and in different lengths (from just a few amino acids to several thousand) to make different proteins. The sequence of amino acids gives each protein its “meaning” – because the final structure of the amino acid chain determines specifically what that protein is and what it does.

What Are the Different Types of Proteins in Food?

Maybe you never thought about it, but not all food proteins are the same. The sequence of amino acids that creates the white of an egg is much different from the arrangement of amino acids that creates the protein in a glass of milk.

When you eat foods that provide protein, then, it should make sense that different foods contain different proteins (and usually more than one) – even though they’re all made up of amino acids.

For example:

  • When you have milk or yogurt, you’re getting proteins called casein and whey.
  • When you eat meat, fish or poultry, you get proteins called collagen and myosin.
  • Beans have proteins called legumins.
  • Eggs contain a number of different proteins, including avidin and ovalbumin.

Each of these proteins is unique because it’s made up of a unique sequence of amino acids. Once the proteins are digested and absorbed, those amino acids can then be used as building blocks for the proteins within your body.

How Do Amino Acids Form Proteins in Your Body?

As protein foods travel through the digestive tract, they’re broken down into their individual amino acids and absorbed into the bloodstream. Your body uses these building blocks to manufacture some 50,000 different body proteins, each with a specific structure (and function) based upon its arrangement of amino acids.

As long as your body has all the necessary “raw materials” in the form of the amino acid building blocks, it can manufacture these important body proteins – from the enzymes that speed up chemical reactions in the body to hormones that act as chemical messengers. Other proteins support your immune function or transport nutrients in your body – and, of course, you have proteins that provide structure to your bones, skin, hair, nails and muscles, too.

Once the amino acids enter your bloodstream, there’s no way to tell whether they were derived from a bowl of lentils or a steak; they all end up as an amino acid “pool” in your body’s tissues and fluids – a pool that can be tapped into as needed. To ensure a steady supply, it’s important to consume adequate amounts of protein every day.

Why You Need Protein Daily

Eating the right amount and types of protein every day is important because if you consistently have a shortage of protein in your diet, your body would have no choice but to start breaking down proteins within your body to provide the amino acids needed to produce the most vital body proteins.

While this process of building up and breaking down happens in your body all the time, the system only works as long as there are adequate amino acids coming from the diet to keep the two processes in balance.

Complete and Incomplete Proteins

The types of protein you eat matter, too. Of the 20 amino acids that your body uses to manufacture body proteins, nine of them are called “essential” – they have to come from your diet because your body cannot make them (although it can manufacture the remaining 11 amino acids).

Complete Proteins

Proteins from animal sources contain all the essential amino acids, so they’re referred to as “complete.” These include:

  • Meats
  • Fish
  • Poultry
  • Eggs
  • Milk and milk products

Soybeans and protein foods derived from soy - such as tofu, tempeh, soy milk or soy protein powders – are also considered complete proteins.

Incomplete Proteins

With the exception of soybeans, plant proteins lack one or more essential amino acids, so they’re considered “incomplete.” These include:

  • Beans
  • Lentils
  • Nuts
  • Whole grains

Strict vegetarians work around this by consuming a wide variety of foods to ensure they get their full complement of essential amino acids in their diet.