Anaerobic metabolism occurs in the cytosol of the muscle cells. As seen in Figure 7.11.1, a small amount of A.T.P. is produced in the cytosol without the presence of oxygen. Anaerobic metabolism uses glucose as its only source of fuel and produces pyruvate and lactic acid. Pyruvate can then be used as fuel for aerobic metabolism. Aerobic metabolism takes place in the mitochondria of the cell and is able to use carbohydrates, protein or fat as its fuel source. Aerobic metabolism is a much slower process than anaerobic metabolism but produces the majority of the A.T.P..

Muscle strength is developed and maintained by weight or resistance training that often is called anaerobic exercise. Anaerobic exercise consists of short duration, high-intensity movements that rely on immediately available energy sources and require little or no oxygen during the activity.

During exercise, especially when it is performed for longer than two to three hours, muscle tissue is broken down and some of the amino acids are catabolized to fuel muscle contraction. To avert excessive borrowing of amino acids from muscle tissue to synthesize energy during prolonged exercise, protein needs to be obtained from the diet.

However, when following a vegetarian diet, the amino acid lysine can be challenging to acquire. Grains, nuts, and seeds are lysine-poor foods, but tofu, soy, quinoa, and pistachios are all good sources of lysine

The P.D.C.A.A.S. is a method adopted by the US Food and Drug Administration (F.D.A.) to determine a food’s protein quality. It is calculated using a formula that incorporates the total amount of amino acids in the food and the amount of protein in the food that is actually digested by humans.

The second component of protein quality is digestibility, as not all protein sources are equally digested.

The amino acid profile of different foods is, therefore, one component of protein quality.

Foods that contain some of the essential amino acids are called incomplete protein sources, while those that contain all nine essential amino acids in ratios that support growth are called complete protein sources, or high-quality protein sources. Foods that are complete protein sources include animal foods such as milk, cheese, eggs, fish, poultry, and meat, and a few plant foods, such as soy and quinoa (Figure 7.7.2). The only animal-based protein that is not complete is gelatin, which consists of the protein, collagen.

The USDA recommends lean meats, such as round steaks, top sirloin, extra-lean ground beef, pork loin, and skinless chicken. Additionally, a person should consume 8 ounces of cooked seafood every week (typically as two 4-ounce servings) to assure they are getting the healthy omega-3 fatty acids that have been linked to a lower risk for heart disease.

Protein-rich animal-based foods commonly have high amounts of B vitamins, vitamin E, iron, magnesium, and zinc. Seafood often contains healthy fats, and plant sources of protein contain a high amount of fiber. Some animal-based protein-rich foods have an unhealthy amount of saturated fat and cholesterol.

The protein food group consists of foods made from meat, seafood, poultry, eggs, soy, beans, peas, and seeds.

The I.O.M. used data from multiple studies that determined nitrogen balance in people of different age groups to calculate the R.D.A. for protein. A person is said to be in nitrogen balance when the nitrogen input equals the amount of nitrogen used and excreted. A person is in negative nitrogen balance when the amount of excreted nitrogen is greater than that consumed, meaning that the body is breaking down more protein to meet its demands. This state of imbalance can occur in people who have certain diseases, such as cancer or muscular dystrophy.

The healing process begins with proteins which dilate blood vessels at the site of injury. An additional protein called fibrin helps to secure platelets that form a clot to stop the bleeding. Next, cells move in and mend the injured tissue by installing newly made collagen fibers. The collagen fibers help pull the wound edges together. In the remodeling phase, more collagen is deposited, forming a scar. Scar tissue is only about 80 percent as functional as normal uninjured tissue. If a diet is insufficient in protein, the process of wound healing is markedly slowed.

The most abundant protein in blood is the butterfly-shaped protein known as albumin. Albumin’s presence in the blood makes the protein concentration in the blood similar to that in cells. Thus, fluid exchange between the blood and cells is minimized to preserve the status quo.

As with other macro-nutrients, the liver is the checkpoint for amino acid distribution and any further breakdown of amino acids, which is very minimal. Recall that amino acids contain nitrogen, so further catabolism (breakdown) of amino acids releases nitrogen-containing ammonia. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine.

In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood. This movement of individual amino acids requires special transport proteins and the cellular energy molecule, adenosine triphosphate (A.T.P.).

The stomach empties the chyme containing the broken down egg pieces into the small intestine, where the majority of protein digestion occurs. The pancreas secretes digestive juice that contains more enzymes that further break down the protein fragments. The two major pancreatic enzymes that digest proteins are chymotrypsin and trypsin

The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin, which initiate the breakdown of the protein. The acidity of the stomach facilitates the unfolding of the proteins that still retain part of their three-dimensional structure after cooking and helps break down the protein aggregates formed during cooking. Pepsin, which is secreted by the cells that line the stomach, dismantles the protein chains into smaller and smaller fragments.

Unless you are eating it raw, the first step in egg digestion (or any other protein food) involves chewing.

A protein’s structure also influences its nutritional quality. Large fibrous protein structures are more difficult to digest than smaller proteins and some, such as keratin, are indigestible. Because digestion of some fibrous proteins is incomplete, not all of the amino acids are absorbed and available for the body to utilize, thereby decreasing their nutritional value.

To compare how so many different proteins can be designed from only twenty amino acids, think about music. All of the music that exists in the world has been derived from a basic set of seven notes C, D, E, F, G, A, B and variations thereof. As a result, there is a vast array of music and songs all composed of specific sequences from these basic musical notes. Similarly, the twenty amino acids can be linked together in an extraordinary number of sequences, much more than are possible for the seven musical notes to create songs

Titin is the largest known protein. It accounts for the elasticity of muscles, and consists of more than twenty-five thousand amino acids

Eleven of these are called nonessential amino acids because the body can synthesize or make them. However, nine of the amino acids are called essential amino acids because we cannot synthesize them either at all or in insufficient amounts. These must be obtained from the diet

Proteins contain the elements carbon, hydrogen, and oxygen just as carbohydrates and lipids do, but proteins are the only macronutrient that contains nitrogen

Protein is necessary for proper immune system function, digestion, and hair and nail growth, and is involved in numerous other body functions

Four common forms of semi-vegetarianism are: Lacto-Ovo vegetarian. This is the most common form. This type of semi-vegetarian eats eggs and dairy. Lacto-vegetarian. This type of semi-vegetarian eats dairy products but not eggs. Ovo-vegetarian. This type of semi-vegetarian eats eggs but not dairy products. Pescatarian-This type of semi-vegetarian eats fish, but no meat or eggs.