Merck Manual

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Overview of Proteins
Overview of Proteins
Overview of Proteins

    Protein is an essential part of the human diet. It’s found in a variety of foods like eggs, dairy, seafood, legumes, meats, nuts, and seeds. Regardless of the source, the protein that we eat gets broken down and reformed into new proteins in our bodies. These proteins do everything from fighting infections to helping cells divide. You name it, they’re doing it.

    At its simplest, a protein is a chain of amino acids, bound to one another by peptide bonds. Like a string of beads. These strings get twisted and folded into a final protein shape. When we eat protein, it gets broken down into its individual amino acids. Most amino acids have a central carbon atom bonded to one amino or nitrogen-containing group and one carboxylic acid group - that’s why it’s called an amino acid. The carbon also has one hydrogen atom and a side chain which is unique to each amino acid. The exception to this is proline which has a tiny little ring structure instead.

    Although there are hundreds of amino acids in nature, humans use only about 20 of them to make basically every type of protein. They include: alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. Phew, that’s 20. One way to divide them is by defining which ones our bodies can make ourselves, and which ones we cannot. There are 5 amino acids - alanine, asparagine, aspartic acid, glutamic acid, and serine - that we can get from foods, but we can also make ourselves. These are called nonessential amino acids. Then, there are 6 of them that we call conditionally essential because healthy bodies can make them under normal circumstances - arginine, cysteine, glutamine, glycine, proline, and tyrosine. But we can’t make them in cases like starvation or certain inborn errors of metabolism. Finally, there are 9 of them that we can only get from food - histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. We call these the essential amino acids.

    Dietary protein provides the essential amino acids that are needed to make our own proteins, hormones, and other important molecules. A circle of life, of sorts. But to do so, we need to break the dietary protein down first, through a process called proteolysis. When we eat a protein-containing food, proteolysis begins when the food reaches the stomach. First, hydrochloric acid denatures the protein, unfolding it and making the amino acid chain more accessible to enzymatic action. Then, pepsin, which is a protein itself made by gastric chief cells, enters the picture. Pepsin cleaves any available protein into smaller oligopeptide chains which move into the duodenum, where a second set of digestive enzymes, made by the pancreas, further chop the oligopeptides into tripeptides, dipeptides, and individual amino acids. These can all be taken up into the intestinal cells, where di- and tripeptides are then converted into amino acids. Some amino acids remain in these cells and are used to synthesize intestinal enzymes and new cells. But most enter the bloodstream and are transported to other parts of the body.

    In general, animal-based protein foods like eggs, dairy, seafood, and meat provide all nine essential amino acids in adequate amounts. Soy foods are unique in that they are plant-based and also provide all nine essential amino acids in adequate amounts. Most other plant foods, including whole grains, legumes, nuts, and seeds, have high amounts of some amino acids and low amounts of others. Hearing this, it might be easy to assume that animal-based foods provide more protein than plant-based sources, but as it turns out a cup of tofu has the same number of grams of protein as 3 ounces of steak, chicken, or fish, and half a cup of lentils has more grams of protein than an egg! And not all plant foods are low in the same amino acids, so eating a variety of plant-based foods can provide all nine of the essentials. For example, pairing protein sources like rice and beans or hummus and pita bread or oatmeal topped with almond butter. However, in terms of volume, it may be necessary to eat more plant-based foods to get a similar amount of protein and amino acid profile provided by animal-based proteins.

    Generally speaking, daily protein requirements are based on studies that estimate the minimum amount of protein needed to avoid a progressive nitrogen loss. The World Health Organization guidelines and the US Recommended Dietary Allowance each estimate that daily protein requirements for healthy adults are about 0.80 grams per kilogram of body weight. Protein recommendations per day vary by age, too. Children 1 to 3 years of age are recommended to get 13 grams. For ages 4 to 8, 19 grams are recommended. And between ages 9 and 13, 34 grams are recommended. Whether a person is male or female also impacts protein needs. Females ages 14 and above are recommended to get 46 grams of protein per day. Males ages 14 to 18 need slightly more, about 52 grams per day. And males 19 and older are recommended to get 56 grams per day. Some groups like pregnant and breastfeeding women as well as athletes have elevated needs, and older adults may also benefit from eating more protein.

    Getting the right amount of protein per day can be achieved in multiple ways. For example, one serving of Greek yogurt with breakfast, a salad topped with 3 ounces of chicken for lunch, and 3 to 4 ounces of fish at dinner provides about 64 grams of protein. Getting the same amount of protein could also be achieved by eating a cup of tofu scramble and a slice of peanut butter toast for breakfast, a cup of shelled edamame with lunch, and one cup each of lentils and brown rice at dinner, which also offers about 64 grams of protein in total.

    Now, the fact is that it’s still unclear what an “optimal” amount of protein is and the research is ongoing. In clinical settings, there are certain individuals who may be at risk for a protein deficiency, including patients with malnutrition, trauma and burn injuries, and various conditions impacting nutrient absorption, like inflammatory bowel disease. These individuals may have increased protein needs compared to the general population. Except for certain circumstances like kidney disease, there usually isn’t a health risk associated with eating a lot of protein, because our bodies are able to process it.

    Alright, as a quick recap. Protein is a cornerstone of the human diet, and a major component of our bodies. There are 5 nonessential, 6 conditionally essential, and 9 essential amino acids. We need to get essential amino acids from our diet. Protein needs vary depending on life cycle stage, level of physical activity and health status. Some health conditions may put people at risk for protein deficiency or increase protein needs. Everyone, whether omnivorous, vegetarian, or vegan, can get enough protein by eating a variety of foods.

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