Protein in Nutrition
Proteins are the basic building blocks of the tissues of the body. They are broken down in the stomach during digestion by enzymes known as proteases to provide the body's necessary amino acids for repair and growth, including the essential amino acids that the body cannot synthesize by itself. Aside from their role in protein synthesis, amino acids are also important nutritional sources of nitrogen. For muscle building purposes, a positive nitrogen balance equals muscle growth and a negative nitrogen balance equals muscle loss. In conjunction with applying the proper stress in the form of progressive resistance training weightlifting, ingesting enough protein to keep nitrogen balance high will ensure continued muscle growth.
In addition to being used for growth and repair, proteins can also be used by the body as a source of energy, when glycogen (muscle sugar energy) stores are low. The body converts proteins into glucose (blood sugar energy) through a process called gluconeogenesis. Protein has the same caloric value as carbohydrates (4 kilocalories per gram) and can therefore provide the same amount of energy when the need arises.
Dietary sources of protein include meats, eggs, nuts, grains, beans, protein supplements and dairy products such as milk and cheese. Of the 20 amino acids used by humans in protein synthesis, 11 “nonessential” amino acids can be synthesized in sufficient quantities by the adult body, and are not typically required in the regular diet. The nine essential amino acids, plus arginine and histidine for kids, are not created by the body and must come from dietary sources.
Most animal sources, such as meat and dairy products, have the complete combination of all the essential amino acids. However, it is not necessary to consume a single food source that contains all the essential amino acids, as long as all the essential amino acids are eventually present in the diet: protein combining like beans and rice or nuts and grains will provide complete proteins.
Different proteins have different levels of biological availability to the human body. Many methods have been introduced to measure protein utilization and retention rates in humans. They include biological value, Net Protein Utilization or NPU, and PDCAAS (Protein Digestibility Corrected Amino Acids Score) which was developed by the FDA as an improvement over the Protein Efficiency Ratio (PER) method. These methods examine which proteins are most efficiently used by the body. In general they conclude that animal complete proteins that contain all the essential amino acids such as milk, eggs, and meat, and the complete vegetable protein soy are of most value to the body.
Egg whites have been determined to have the standard biological value of 100 (though some sources may have biological values higher), which means that most of the absorbed nitrogen from egg white protein can be retained and used by the body. The biological value of plant protein sources is considerably lower than animal sources. Vegetable sources of protein can be very low, such as in peanuts, which have a biological value of 40.
Different references cite different protein needs for the varied populations. For a long time, the American Dietetics Association fought hard to prove that dietary needs of humans—even heavily-muscled, hard-training competitive bodybuilders’ protein needs—were low, some suggesting as low as 0.5 grams per kg of lean bodyweight. With an overabundance of both anecdotal and clinical evidence suggesting otherwise, even the protein-needs-conservative ADA has felt the need to change its suggested requirements recently. According to the recently updated Dietary Reference Intake guidelines, women aged 19-70 need to consume 46 grams of protein per day, while men aged 19-70 need to consume 56 grams of protein per day as a bare minimum to avoid a deficiency. The difference is because men's bodies generally have more muscle mass than those of women, or this may be attributed to weight difference by taking 0.8 g (of protein)/kg of lean body weight.
Because the body is continually breaking down protein from tissues, even adults who do not fall into the above categories need to include adequate protein in their diet every day. If enough energy is not taken in through diet, then just like the process of starvation, the body will use protein from the muscle mass to meet its energy needs. This leads to muscle wasting over time. If the body does not consume adequate protein through the diet, then muscle will also waste as more vital cellular processes use protein to sustain other body functions.
Other recommendations suggest 0.8 grams of protein per kilogram of lean body weight per day while other sources suggest that higher intakes of 1-1.4 grams of protein per kilogram of body weight for enhanced athletes or those with a large muscle mass. Still other credible sources recommend much higher intake for strength and endurance athletes who use a high amount of both protein and other macro nutrient energy sources. It is not uncommon for high-level competitive strength, endurance and bodybuilding athletes to take in as much as 2 grams per pound of lean body mass—much higher than traditional dietetic and medical sources may suggest.
How much protein is needed in a person's daily diet is determined in large part by overall energy intake, as well as by the body's need for nitrogen and essential amino acids. Physical activity and exertion as well as enhanced muscular mass can dramatically increase the need for protein. If you are a hard training athlete or bodybuilder, always make sure to take in enough protein to feed your body and maintain or build new lean tissue. Also, remember to drink water liberally on a high protein to help flush and protect your kidneys.
— Reprinted With Permission