Protein breaks down into amino acids, which can be reassembled into new proteins. The balance between protein breakdown and buildup is known as protein turnover. Learn about protein turnover, the amino acid pool and nitrogen balance in the body in this lesson.
Your eyes and your body see the foods you eat in very different terms. For example, if you have a cheeseburger sitting on your dinner plate, your eyes see a bun, a hamburger patty and a slice of cheese. Your body, on the other hand, sees a supply of nutrients that it can break down and reassemble to carry out its many critical functions.
To your body, that cheeseburger on your plate means it will now have a new source of protein, fat and carbohydrates to process.In this way, you can think of your body as a factory that is constantly breaking down dietary nutrients or existing body tissues in order to build new components. In this lesson, we will focus on how your body maintains a balance between protein breakdown and buildup, which is a process referred to as protein turnover.
Amino Acid Pool
We know that protein, whether that protein is found in your food or in the tissues of your body, is simply a chain of amino acids bonded together. There are 20 different amino acids that your body uses, and each one contains the same basic structure, which includes atoms of nitrogen along with atoms of carbon, oxygen and hydrogen atoms.
Because amino acids contain nitrogen in their chemical makeup, they can be used to make and maintain a number of nitrogen-containing molecules found in various tissues, such as your muscles, skin, blood and even your DNA.When your body breaks down proteins, the now-available amino acids are said to be part of the amino acid pool. Now, don’t let that term fool you; there’s no actual pool filled with amino acids sitting in some trench of your body.
This term is simply used to describe the reserve of available amino acids stored in your body tissues for future use, much like your kitchen pantry acts as a reserve for ingredients that you will use in future recipes.
Because protein molecules are constantly turning over, meaning they are always being broken down into their amino acid parts and then reassembled into different molecules, it’s not hard to see that supplying your body with enough protein is pretty important. The amount of protein you need from your diet is based on nitrogen balance, which is the balance between the amount of nitrogen consumed in your diet and the amount excreted. Now, remember that most of the nitrogen we consume comes from those nitrogen-containing amino acids in the proteins that we eat.
The nitrogen we excrete exits the body through urine and other body wastes.When you are in nitrogen balance, you’re eating the right amount of protein to match the losses. Your body always likes periods of homeostasis, so when your nitrogen intake matches your nitrogen loss, your body’s pretty happy. This nitrogen balance is what we see in healthy individuals that eat sufficient amounts of protein.
Having said that, there are stages in life when this balance needs to shift. For example, a positive nitrogen balance occurs when nitrogen intake exceeds nitrogen loss. Your body orchestrates this shift during periods of growth such as childhood and pregnancy, and if you are into bodybuilding, it’s what helps you gain mass when you pump iron.
During positive nitrogen balance, tissues are able to grow because proteins are built up or synthesized at a greater rate than they are broken down.As you might imagine, this process can work in the opposite direction as well; a negative nitrogen balance occurs when nitrogen loss exceeds nitrogen intake. When your body is in a state of negative nitrogen balance, your intake of protein is not keeping pace with the needs of the body or your body is breaking down protein faster than it can be synthesized.
This can occur in individuals that do not have adequate amounts of protein-containing foods available to them, as we might see in people living in undeveloped countries or in dieters who reduce their nutrient or calorie intake too severely.It can also happen to athletes who over-train. For example, if a marathon runner trains too hard, his body will break down protein at too high of a rate to keep up with. Likewise, a person recovering from injuries or burns could enter a negative nitrogen balance due to the higher need for tissue repair.
Let’s review. Protein consists of a chain of amino acids bonded together.
Your body is constantly breaking down protein molecules into their individual amino acids, which are then used to build or synthesize new proteins. The balance between protein breakdown and buildup is referred to as protein turnover. When proteins are broken down, their nitrogen-containing amino acids become part of the amino acid pool, which is a reserve of available amino acids stored in your body tissues for future use.Nitrogen balance is the balance between the amount of nitrogen consumed in your diet and the amount excreted through body wastes, such as urine. When nitrogen intake exceeds nitrogen loss, a positive nitrogen balance occurs and tissues are able to grow because protein synthesis is greater than breakdown. This is seen during childhood, pregnancy and bodybuilding.
When nitrogen loss exceeds nitrogen intake, a negative nitrogen balance occurs. This happens when protein intake is too low or protein breakdown is greater than synthesis. This can happen in a person on a severe or starvation diet, an athlete who over-trains, or a person recovering from injuries or burns.
Upon completing this lesson, you will be able to:
- Define protein and amino acid pool
- Describe the processes of protein turnover
- Explain what happens during positive and negative nitrogen balance and how each of these occurs