While carbs and fat get alternately praised and punished, protein is basically the golden child among the macronutrients. That’s totally unfair to carbs and fat, first of all, but protein certainly does enough to earn its reliably good reputation. We know protein is a great thing to have, but why exactly do we need it, and what does our body even do with it? Here’s a rundown of what actually happens when you eat protein.
What protein actually is
Like we mentioned, protein is one of the three macronutrients (i.e. nutrients the body needs in sizable amounts). Unlike carbs and fat, protein is not usually a major energy source, although we definitely get some of that from it—protein provides 4 calories per gram. But protein is often referred to as a building block in the body because of its central role in growth and development.
Almost all animal-derived products—meat, poultry, eggs, dairy, fish—contain a significant amount of protein, so they get labeled as “proteins” when we’re talking about our diets and nutrition. But protein is also present in a lot of plant-based foods. There’s a good amount in beans, peas, nuts, and seeds, for instance, while vegetables and grains generally contain smaller amounts, according to the FDA. (Whole grains will have more protein than refined grains, though, which are missing the part of the grain that often supplies a lot of the protein content, as SELF previously reported.)
The different kinds of proteins
Proteins are made of small units called amino acids. Amino acids are organic compounds containing structures made of elements including nitrogen, hydrogen, carbon, and oxygen. Hundreds or thousands of amino acids link up to form super long chains, and the sequence of that chain determines the protein’s unique function, the U.S. National Library of Medicine explains.
There are 20 different amino acids in total, which can be broken down into two main groups, per the FDA. Nine of the 20 are what are referred to as essential amino acids, meaning that the body is unable to produce them itself and so we must get them from food. The other 11 are nonessential because the body is able to synthesize them out of the essential amino acids or the normal process of breaking down proteins, according to the U.S. National Library of Medicine. Many of these nonessential amino acids are also considered conditional amino acids, because they can become essential in rare, severe instances when the body is unable to synthesize amino acids properly, per the U.S. National Library of Medicine.
Now, when a protein is a good source of all nine of the essential amino acids, we call it a complete protein, according to the FDA. All animal products are complete proteins, and so is soy. When a protein is missing or pretty low in any of those essential amino acids, it’s considered incomplete. Most plant foods are considered incomplete proteins.
The good news for vegetarians, vegans, and lovers of plant foods in general is that you can still easily get all the essential amino acids from eating a wide variety of incomplete proteins. As the FDA explains, incomplete proteins are often just lacking in one or two amino acids, so they can often make up for whatever the other one is lacking. (Pretty romantic, right?) For instance, grains are low in an amino acid called lysine, while beans and nuts are low in methionine. But when you eat, say, beans and rice or wheat toast with nut butter, you’re getting all the amino acids that you do when you eat, say, chicken. While people used to be encouraged to eat foods in combinations at meals, we now know this is not necessary, according to the U.S. National Library of Medicine, as long as you’re eating a variety of complementary incomplete proteins throughout the day.
Why we even need protein
That building block nomer is no exaggeration. The stuff is an integral component of every cell in the body, including, yes, your muscles. “If we don’t get enough protein, our bodies actually won’t be able to rebuild properly and we’ll start to lose muscle mass,” Colleen Tewksbury, Ph.D., M.P.H., R.D., senior research investigator and bariatric program manager at Penn Medicine and president-elect of the Pennsylvania Academy of Nutrition and Dietetics, tells SELF.
In addition to muscle growth, protein is essential to the growth and repair of virtually all cells and body tissues—from your skin, hair, and nails to your bones, organs, and bodily fluids, according to the FDA. That’s why it’s especially important to get enough of it during developmental periods like childhood and adolescence.
Protein also plays a role in crucial bodily functions like blood clotting, immune system response, vision, fluid balance, and the production of various enzymes and hormones, per the FDA. And because it contains calories, it can provide the body energy for storage or use. (But this definitely isn’t its main gig, which we’ll get into in a bit.)
What happens in your body when you eat protein
It’s not like we eat a piece of chicken and that protein goes directly to our biceps. Dietary protein gets broken down and reassembled into the various kinds of proteins that exist in the body. No matter what kind of protein you’re eating—plant or animal, complete or incomplete—the body’s first objective is to break it back down into all the different amino acid units it was assembled from, Tewksbury explains.
Breaking down protein requires more time and effort than carbs, but not as much as fat. It begins in the mouth, as proteins and especially animal proteins typically take more chewing than other kinds of foods, Tewksbury says. That mechanical process is the very first step of digestion.
Then, those pieces of protein move to the stomach to get mixed up with gastric juices containing acids and enzymes that help break down food. Next, that mixture gets passed on in steady increments to the small intestine, where more specialized enzymes and acids get injected (mainly by the pancreas) to help break that protein all the way down. Once you’ve got those little singular amino acids, they’re ready to get to work.
How the body uses protein
These amino acids get sent to the liver, where they’re shuffled around and reconfigured into any type of protein your body needs, Tewksbury explains. Your body is constantly regenerating and replacing cells and tissues, so there’s always a variety of proteins needed. For instance, some proteins in the body make up antibodies that help the immune system kick out bacteria and viruses. Others help with DNA synthesis, chemical reactions, or transporting other molecules, the National Institute of General Medical Sciences explains.
How much protein your body actually requires for the purpose of tissue growth and repair is determined by factors like sex, age, body composition, health, and activity level, according to the U.S. National Library of Medicine, but most of us are getting more than enough protein to fulfill these needs. The bummer is that once your tissues get all the amino acids that they need, they have no use for any extra.
So what happens to the rest, once our dietary protein intake exceeds what our tissues need? The body doesn’t have a protein holding tank like it does for carbs, where it can siphon away extras for quick access when we need it. “We have little to no way of being able to store protein [for future use] in our body,” Tewksbury explains. This is why you need to eat protein throughout the day, every day.
Since we can’t use excess protein for its intended purpose later on, the body breaks it down and stows it away in fat tissue, according to Merck Manuals. To do this, the liver removes the nitrogen from the amino acids and disposes of it through the urine, in the form of a waste product called urea, Linsenmeyer explains. What’s left behind is something called alpha keto-acids, which will most often then go through a chemical process that turns them into triglycerides to be stored in our fatty tissues, Linsenmeyer says. (This can technically be accessed at a later date when the body needs to tap into fat stores for energy.)
Alpha-keto acids can be converted into glucose and used for immediate fuel if necessary, when the body is in a fasting state or not getting enough calories coming in from other macronutrients, Linsenmeyer says. But this is not typical because the body prefers carbs as its primary source of energy, followed by dietary fat, which the body can adapt to use as fuel if it’s not getting enough carbs. “We can adapt to use protein for energy as well, but it’s not ideal,” Linsenmeyer says. “Ideally, [our bodies] want to leave it alone to build and maintain body tissues.”
Now, what we just walked you through is still oversimplifying the reality of what happens when we eat protein (or any food). Digestion and metabolism are complex processes happening constantly on a cellular level. But even just grasping the broad strokes can make you really appreciate what your body actually does with the protein you eat.