Protein isn't just part of what's on your plate or in your shaker bottle. It's a major part of who you are, and the same goes for every other living thing. And once you step away from the nutritional label and look at a microscope, you see that "protein" isn't even a thing, but rather, a category of things.

There are millions of different types of protein molecules, all containing different combinations of 20 standard amino acids, and all having different functions and effects in the body. These are by far the most complicated molecules in the human body, and can contain anywhere from 50-2,000 amino acids including the essential 20. To date, about 10,000 different proteins have been identified in the human genome. There are thought to be nearly one trillion different proteins on Earth.[1]

The body of a 154-pound man contains about 24 pounds of protein. Almost half is in the form of skeletal muscle, with another 3-4 pounds in the form of skin and blood. Connective tissue and organs make up the rest of the protein.[2]

In nutritional terms, protein is one of the three primary macronutrients our bodies need to survive. It is a "macro" nutrient because, like carbohydrates and fats, we consume a significant amount of it and use it for fuel. We need a comparatively small amount of vitamins and minerals every day, so they are referred to as "micronutrients."

Protein is contained in various combinations and quantities in every food we eat—even vegetables. Unlike all these other macro and micronutrients, your body can't store protein, so you have to get it consistently through the foods you eat, and/or the supplements you take.