Protein is an essential nutrient whose name comes from the Greek word protos, which means “first.” To visualize a molecule of protein, close your eyes and see a very long chain, rather like a chain of sausage links. The links in the chains are amino acids, commonly known as the building blocks of protein.

In addition to carbon, hydrogen, and oxygen atoms, amino acids contain a nitrogen (amino) group. The amino group is essential for synthesizing (assembling) specialized proteins in your body. The human body is chock-full of proteins. Proteins are present in the outer and inner membranes of every living cell.

Here’s where else protein makes an appearance:

• Your hair, your nails, and the outer layers of your skin are made of the protein keratin. Keratin is a scleroprotein, or a protein resistant to digestive enzymes. So if you bite your nails, you can’t digest them.

• Muscle tissue contains myosin, actin, myoglobin, and a number of other proteins.

• Bone has plenty of protein. The outer part of bone is hardened with minerals such as calcium, but the basic, rubbery inner structure is protein; and bone marrow, the soft material inside the bone, also contains protein.

• Red blood cells contain hemoglobin, a protein compound that carries oxygen throughout the body. Plasma, the clear fluid in blood, contains fat and protein particles known as lipoproteins, which ferry cholesterol around and out of the body.

Your body uses proteins to build new cells, maintain tissues, and synthesize new proteins that make it possible for you to perform basic bodily functions. About half the dietary protein that you consume each day goes into making enzymes, the specialized worker proteins that do specific jobs such as digesting food and assembling or dividing molecules to make new cells and chemical substances.

To perform these functions, enzymes often need specific vitamins and minerals. Your ability to see, think, hear, and move — in fact, to do just about everything that you consider part of a healthy life — requires your nerve cells to send messages back and forth to each other and to other specialized kinds of cells, such as muscle cells.

Sending these messages requires chemicals called neurotransmitters. Making neurotransmitters requires — guess what — proteins. Finally, proteins play an important part in the creation of every new cell and every new individual. Your chromosomes consist of nucleoproteins, which are substances made of amino acids and nucleic acids.

What Happens to the Proteins You Eat

The cells in your digestive tract can absorb only single amino acids or very small chains of two or three amino acids called peptides. So proteins from food are broken into their component amino acids by digestive enzymes — which are, of course, specialized proteins.

Then other enzymes in your body cells build new proteins by reassembling the amino acids into specific compounds that your body needs to function. This process is called protein synthesis. During protein synthesis

• Amino acids hook up with fats to form lipoproteins, the molecules that ferry cholesterol around and out of the body. Or amino acids may join up with carbohydrates to form the glycoproteins found in the mucus secreted by the digestive tract.

• Proteins combine with phosphoric acid to produce phosphoproteins, such as casein, a protein in milk.

• Nucleic acids combine with proteins to create nucleoproteins, which are essential components of the cell nucleus and of cytoplasm, the living material inside each cell.

The carbon, hydrogen, and oxygen that are left over after protein synthesis is complete are converted to glucose and used for energy. The nitrogen residue (ammonia) isn’t used for energy. It’s processed by the liver, which converts the ammonia to urea.

Most of the urea produced in the liver is excreted through the kidneys in urine; very small amounts are sloughed off in skin, hair, and nails. Every day, you turn over (reuse) more proteins than you get from the food you eat, so you need a continuous supply to maintain your protein status.

If your diet does not contain sufficient amounts of proteins, you start digesting the proteins in your body, including the proteins in your muscles and — in extreme cases — your heart muscle.

All proteins are made of building blocks called amino acids, but not all proteins contain all the amino acids you require. To make all the proteins that your body needs, you require 22 different amino acids. Ten are considered essential, which means you can’t synthesize them in your body and must obtain them from food (two of these, arginine and histidine, are essential only for children).

Several more are nonessential: If you don’t get them in food, you can manufacture them yourself from fats, carbohydrates, and other amino acids. Three — glutamine, ornithine, and taurine — are somewhere in between essential and nonessential for human beings: They’re essential only under certain conditions, such as with injury or disease.

Essential Amino Acids:

• Arginine*
• Histidine*
• Isoleucine
• Leucine
• Lysine
• Methionine
• Phenlyalanine
• Threonine
• Tryptophan
• Valine

* Essential for children; nonessential for adults

Nonessential Amino Acids:

• Alanine
• Asparagine
• Aspartic acid
• Citrulline
• Cysteine
• Glutamic acid
• Glycine
• Hydroxyglutamic acid
• Norleucine
• Proline
• Serine
• Tyrosine

Because an animal’s body is similar to yours, its proteins contain similar combinations of amino acids. That’s why nutritionists call proteins from foods of animal origin — meat, fish, poultry, eggs, and dairy products — highquality proteins.

Your body absorbs these proteins more efficiently; they can be used without much waste to synthesize other proteins. The proteins from plants — grains, fruit, vegetables, legumes (beans), nuts, and seeds — often have limited amounts of some amino acids, which means their nutritional content is not as high as animal proteins.

The basic standard against which you measure the value of proteins in food is the egg. Nutrition scientists have arbitrarily given the egg a biological value of 100 percent, meaning that, gram for gram, it’s the food with the best supply of complete proteins.

Other foods that have proportionately more protein may not be as valuable as the egg because they lack sufficient amounts of one or more essential amino acids. For example, eggs are 11 percent protein, and dry beans are 22 percent protein.

However, the proteins in beans don’t provide sufficient amounts of all the essential amino acids, so they (the beans) are not as nutritionally complete as proteins from animal foods. The prime exception is the soybean, a legume that’s packed with abundant amounts of all of the amino acids essential for adults.

Soybeans are an excellent source of proteins for vegetarians, especially vegans, which are vegetarians who avoid all products of animal origin, including milk and eggs. The term used to describe the value of the proteins in any one food is amino acid score. Because the egg contains all the essential amino acids, it scores 100.

Another way to describe the quality of proteins is to say that they’re either complete or incomplete. A complete protein is one that contains ample amounts of all essential amino acids; an incomplete protein does not. A protein low in one specific amino acid is called a limiting protein because it can build only as much tissue as the smallest amount of the necessary amino acid.

You can improve the protein quality in a food containing incomplete/ limiting proteins by eating it along with one that contains sufficient amounts of the limited amino acids. Matching foods to create complete proteins is called complementarity.

For example, rice is low in the essential amino acid lysine, and beans are low in the essential amino acid methionine. By eating rice with beans, you improve (or complete) the proteins in both. Another example is pasta and cheese. Pasta is low in the essential amino acids lysine and isoleucine; milk products have abundant amounts of these two amino acids.

Shaking Parmesan cheese onto pasta creates a higher-quality protein dish. In each case, the foods have complementary amino acids. Other examples of complementary protein dishes are peanut butter with bread, and milk with cereal. Many such combinations are a natural and customary part of the diet in parts of the world where animal proteins are scarce or very expensive.

Here are some categories of foods with incomplete proteins:

• Grain foods: Barley, bread, bulgur wheat, cornmeal, kasha, and pancakes
• Legumes: Black beans, black-eyed peas, fava beans, kidney beans, lima beans, lentils, peanut butter, peanuts, peas, split peas, and white beans
• Nuts and seeds: Almonds, Brazil nuts, cashews, pecans, walnuts, pumpkin seeds, sesame seeds (tahini), and sunflower seeds

In order for the foods to complement each other, you must eat them together. In other words, rice and beans at one meal, not rice for lunch and beans for dinner.

The National Academy of Sciences Food and Nutrition Board, which sets the requirements (for example, RDAs) for vitamins and minerals, also sets goals for daily protein consumption. As with other nutrients, the board has different recommendations for different groups of people: young or older, men or women.

As a general rule, the National Academy of Sciences says healthy people need to get 10 to 35 percent of their daily calories from protein. More specifically, the Academy has set a Dietary Reference Intake (DRI) of 45 grams protein per day for a healthy woman and 52 grams per day for a healthy man.

These amounts are easily obtained from two to three 3-ounce servings of lean meat, fish, or poultry (21 grams each). Vegetarians can get their protein from 2 eggs (12–16 grams), 2 slices of prepacked fat-free cheese (10 grams), 4 slices of bread (3 grams each), and one cup of yogurt (10 grams).

As you grow older, you synthesize new proteins less efficiently, so your muscle mass (protein tissue) diminishes while your fat content stays the same or rises. This change is why some folks erroneously believe that muscle “turns to fat” in old age.

Of course, you still use protein to build new tissue, including hair, skin, and nails, which continue to grow until you cross over into The Great Beyond. By the way, the idea that nails continue to grow after death — a staple of shock movies and horror comics — arises from the fact that after death, tissue around the nails shrinks, making a corpse’s nails simply look longer. Who else would let you in on these secrets?

Protein Deficiency

The first sign of protein deficiency is likely to be weak muscles — the body tissue most reliant on protein. For example, children who do not get enough protein have shrunken, weak muscles. They may also have thin hair, their skin may be covered with sores, and blood tests may show that the level of albumin in their blood is below normal.

Albumin is a protein that helps maintain the body’s fluid balance, keeping a proper amount of liquid in and around body cells. A protein deficiency may also show up in your blood. Red blood cells live for only 120 days. Protein is needed to produce new ones.

People who do not get enough protein may become anemic, having fewer red blood cells than they need. Protein deficiency may also show up as fluid retention (the big belly on a starving child), hair loss, and muscle wasting caused by the body’s attempt to protect itself by digesting the proteins in its own muscle tissue.

That’s why victims of starvation are, literally, skin and bones. Given the high protein content of a normal American diet (which generally provides far more protein than you actually require), protein deficiency is rare in the United States except as a consequence of eating disorders such as anorexia nervosa (refusal to eat) and bulimia (regurgitation after meals).

Anyone who’s building new tissue quickly needs extra protein. For example, the Dietary Reference Intake (DRI) for protein for women who are pregnant or nursing is 71 grams per day. Injuries also raise your protein requirements. An injured body releases above-normal amounts of protein-destroying hormones from the pituitary and adrenal glands.

You need extra protein to protect existing tissues, and after severe blood loss, you need extra protein to make new hemoglobin for red blood cells. Cuts, burns, or surgical procedures mean that you need extra protein to make new skin and muscle cells. Fractures mean extra protein is needed to make new bone.

The need for protein is so important when you’ve been badly injured that if you can’t take protein by mouth, you’ll be given an intravenous solution of amino acids with glucose (sugar) or emulsified fat. Do athletes need more proteins than the rest of us? Recent research suggests that the answer may be yes, but athletes easily meet their requirements by increasing the amount of food in their normal diet.

Yes, you can get too much protein. Several medical conditions make it difficult for people to digest and process proteins properly. As a result, waste products build up in different parts of the body. People with liver disease or kidney disease either don’t process protein efficiently into urea or don’t excrete it efficiently through urine.

The result may be uric acid kidney stones or uremic poisoning (an excess amount of uric acid in the blood). The pain associated with gout (a form of arthritis that affects nine men for every one woman) is caused by uric acid crystals collecting in the spaces around joints. Doctors may recommend a low-protein diet as part of the treatment in these situations.