Although water counts for 45 – 70% of the weight of an adult human body and is the most essential nutrient for survival, protein seems to me to be the critical macronutrient to keep the body functioning. Our muscles, organs, blood cells, enzymes, hormones, and other bodily components are made of protein. Given this fact, it’s just as well that most people accept protein as a necessary part of the diet. Whether you are a power lifter or a yogi, someone who follows a paleo diet or a vegan, you likely recognize that protein is necessary to keep you healthy. So let’s talk a little bit about what it is, why you need it and where to get it…
What is protein?
Proteins are large biological molecules that contain nitrogen (this is the primary thing that differentiates protein from carbohydrates and fat). These nitrogen-containing molecules are sometimes called the “building blocks of life”, as they provide the structure for many parts of living organisms (including humans). However, the proteins themselves are not the very basic structure. Proteins are made of chains of smaller particles called amino acids.
There are 20 amino acids that are required in the human body. Of those 20, nine are essential amino acids. An additional five are considered “conditionally essential”, meaning that the body can create them, but only if the organ that creates them is functioning properly. Premature infants, people with liver disease, and those missing the enzymes necessary to convert certain amino acids will be required to ingest one or more of the five conditionally essential amino acids.
How does protein in food become protein in the body?
When you consume protein, digestion breaks the protein down to the amino acids. These amino acids become part of your body’s amino acid “pool” which is used to create new proteins for the body.
When your body needs to make a new protein, the pattern for the protein is copied from your DNA and this pattern is used by the endoplasmic reticulum (big fancy name for a tiny little organelle inside your cells) to build the protein. The process starts when individual amino acids are pulled from the pool. They are then hooked together in a specific order to create a sort of amino acid “string”. Finally, the string is folded in a very special way (think origami) to become a protein. That protein then gets combined with other proteins (sometimes the same type of proteins, sometimes proteins with different amino acids and/or different shapes) and building “materials” to become your muscles, skin, organs, hair, nails, etc.
What foods contain protein?
Protein is readily available in our food supply. However, not all protein foods contain all of the amino acids that the body needs to make muscles and other tissues. Animal sources of protein, such as beef, poultry, fish, eggs and some dairy products (except for things like butter that are basically all fat), contain all of the essential amino acids and, therefore, are considered “complete” proteins.
This is by no means to say that you must be a meat eater to get all of the amino acids that you need! Although the protein in plant foods is wrapped in carbohydrate, and therefore, not as easy to digest, eating a variety of whole plant foods allows the vegetarian or vegan to easily get all the required amino acids. After all, almost all plant foods contain at least some protein, with fruits containing the smallest amount.
For vegetarians, eggs and dairy are great sources of all the essential amino acids. For vegans, quinoa, buckwheat, hempseed, chia seeds, soybeans and Ezekial bread contain all of the essential amino acids. For more information on how to ensure you are getting enough protein as a vegetarian or vegan, check out this post.
Although your body needs all of the essential amino acids in order to properly build all the proteins in your body, you don’t have to get all of them in each bite… you just have to get enough each day to keep your amino acid pool populated with the ones needed to maintain your body.
What happens if we eat more protein than we need?
Unfortunately, this is not an easy question to answer as the real answer is “it depends.” The bottom line is that the body does not store excess protein. If there is more protein in the diet than the body needs to create bodily proteins, the body has to turn the amino acids into something else.
The body can create at least some glucose from 18 of the 20 amino acids. Once the amino acids have been turned into glucose, that glucose can then be used for energy or stored for later use. The other 2 amino acids can be turned into ketones and used for energy. If you are eating too much protein and have enough carbohydrate intake so that the body doesn’t need more glucose, the extra amino acids will be turned into fatty acids and stored as fat. (Ugh! Why does extra get turned into fat?!?)
As an additional note, whether the excess amino acids becomes glucose, ketones, or fatty acids, the nitrogen has to be stripped off (remember it is the part of the protein molecule that differentiates protein from the other macronutrients). That nitrogen then goes through a process to become urea or ammonia and is eliminated from the body, primarily through the kidneys as urine. If you’ve ever heard someone say that eating too much protein is hard on the kidneys, this is why… because the kidneys are responsible for taking the nitrogen from the excess protein and getting rid of it.
When does the body break down its own protein (ie, muscles)?
If you like to workout, then you’ve probably heard that you have to be careful about how much you eat to be sure your body doesn’t use your muscles for energy. So, let’s talk a little about what actually happens, keeping in mind that I’m a nutritionist and not an exercise scientist so this is a very basic explanation…
The body is constantly in the process of breaking down old, worn out protein and creating new protein. For most people, the amount that is degraded is equal to that which is created. Of course, there are exceptions to this, such as for the body builder who is constantly building new proteins faster than he or she breaks down old proteins so his/her muscles get bigger.
If a person is starving or has not consumed food for a prolonged period of time, the body doesn’t actually break down protein at a faster rate than normal. Obviously, this is different than what we’ve heard for years that the body will use the muscle for energy if we don’t eat enough food. The truth is that the body doesn’t break down the muscles any faster, it just doesn’t create new proteins at the same speed as usual (the creation rate is decreased by 30 – 40%). I’m not sure that the detail matters as the result is a net loss of bodily protein, but I think it’s worth understanding what’s really happening. (Yes, that’s the nutrition nerd in me coming out again. 🙂 )
Just for completeness of the story, there are a few situations in which the body will break down less critical body proteins to scavenge the amino acids. This normally occurs when the body is under “metabolic stress”, which is caused by serious conditions like sepsis (a life-threatening situation caused by an extreme infection) and severe injury or trauma (ex: severe burns). Please goodness may none of us have to deal with or worry about these situations for ourselves or our loved ones!
Wow! This turned into a much longer post than I had planned. But as I started writing I wanted to be sure I didn’t leave any common questions about protein unanswered. Hopefully, I did that. But just in case, tell me in the comments if you have any questions about protein that I didn’t answer.
— Compound Interest. A Guide to the 20 Common Amino Acids. Accessed on March 10, 2015. http://www.compoundchem.com/wp-content/uploads/2014/09/20-Common-Amino-Acids-v3.png
— English N. 12 Complete Proteins Vegetarians Need to Know About. Accessed on March 10, 2015. http://greatist.com/health/complete-vegetarian-proteins
— Gropper SS, Smith JL. Advanced Nutrition and Human Metabolism. 6th ed. Belmont, CA: Wadsworth Cengage Learning; 2013.
— Mahan LK, Escott-Stump S, Raymond JL. Krause’s Food and the Nutrition Care Process. 13th ed. St. Louis, MO: Elsevier Saunders; 2012.