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From The January 2001 Issue of Nutrition Science News
The Protein Connection
by Alan Christiansen, N.D.
In the last few years, protein probably has been the focus of more confusion and faddism than any other nutrient. Best-selling authors Andrew Weil, M.D., and Dean Ornish, M.D., extol the health virtues of a low-protein, predominately vegetarian diet, while Robert Atkins, M.D., as well as Rachael Heller, M.D., and her husband Richard Heller, M.D., insist that humans need diets high in animal protein.
Unfortunately, the confusion about protein needs escalates even further when you enter the realm of sports nutrition. Bodybuilders and strength-training athletes are often advised to consume in excess of 300 g protein daily in order to build and maintain muscle. Yet the American Dietetic Association (ADA) adamantly insists that these active individuals require no more protein than their sedentary counterparts.1 The question is, how much protein is enough?
The word protein, coined in the 1800s, comes from the Greek proteios, meaning "of first importance." Proteins were the first substances found to be vital for the formation of living tissue.
Protein is ultimately derived from nitrogen, an organic chemical found in the soil. Nitrogen-fixing bacteria extract it from the soil and make the nitrogen available to growing plants. Herbivores then consume nitrogen from these plants, while carnivores absorb it from the herbivores themselves. Eventually, wastes from animals and decaying plants and animals release nitrogen back to the soil.
The Structure of Protein
In the human body, proteins operate in either structural or functional categories. Structural proteins serve primarily as building blocks. In fact, about 75 percent of our body mass is created by the main structural proteins keratin, collagen and elastin, which make up tissues, hair, nails and skin. On the other hand, some functional uses of protein include creating energy, forming enzymes and immune antibodies, and helping the liver excrete toxins.
Proteins are created from collections of subunit amino acids, which can vary in group size from a few to thousands. If the myriad of proteins found in nature are thought of as words, the letters making up the words are the 20 main amino acids. An amino acid is the combination of an amino group with a nitrogen (-NH2) and an acid group (-COOH). The presence of nitrogen makes protein distinct from all other nutrients.
The breakdown of proteins requires more internal metabolic effort than does the breakdown of fats or carbohydrates. After consuming protein-containing foods, the digestive system gradually breaks up proteins into predominately single amino acids and infrequent subunits of a few amino acids called dipeptides and tripeptides. Because of the labor-intensive process, adults cannot efficiently assimilate more than 35 to 45 g of protein per meal.2 The absorbed amino acids are then actively drawn into the bloodstream from the small intestine. This transport consumes calories and creates some of our body's warmth.3
Although all foods contain protein, only some contain all of the essential amino acids in usable proportions. These foods are said to contain complete protein (see table 1).4
Many vegetarian foods can also form complete proteins when combined properly. For example, combining corn—high in all other amino acids but low in tyrosine, methionine and cysteine—with pinto beans, which are higher in these three amino acids, forms a complete protein. Combined plant protein makes up 65 percent of dietary protein worldwide, yet only about 32 percent in the United States.5 Well-planned vegetarian diets can definitely supply adequate protein; however, unbalanced diets can leave a person protein deficient. Soy is the most complete protein found in plant foods. Although it contains many essential amino acids, some may not be well absorbed from soy. Vegetarians would be well advised to eat protein from dairy and egg sources in addition to soy foods.6
The usability of a protein source is expressed through various measurements, which are widely cited by companies promoting protein supplements. The three commonly used measurements are protein efficiency ratio (PER), net protein utilization (NPU), and biological value (BV).
PER is a measurement of growth in body mass relative to the amount of a protein ingested. NPU and BV are similar. These are percentages of nitrogen (NPU) or protein (BV) excreted by an animal in relation to the amount of nitrogen or protein consumed. Thus, they are measurements of retained protein. For example, beans have a low NPU since much of their nitrogen is bound up with indigestible fibers, and therefore some of its protein is excreted from the body.
Maintaining Protein Balance
Americans' protein intake changed little from the 1900s to the 1970s, at which time it began to rise. During this same time, the American diet also saw proportionate increases in total calories. A pro-protein author such as Atkins correlates this rise in calories to a rise in carbohydrate consumption and to higher rates of obesity. In reality, carbohydrate intake did not change relative to total calories. Americans are not getting heavier from too many carbohydrates or fat, we are getting heavier from simply too much food.7
Today, the average American consumes 1.1 grams of protein for every kilogram of body weight. This equates to 15 percent of dietary calories from proteins, which exceeds the RDA's recommended minimum of 0.8 g/kg body weight.7 Most nutritional researchers agree on a range of 0.81.1 g/kg of body weight as a safe and adequate amount for healthy, sedentary adults.8
When an individual consumes too little protein, her body dips into protein storage and thereby begins to break down lean tissue. First, the body uses structural proteins such as muscle, and, if the deficit persists, it also uses functional proteins.
Many problems associated with aging, such as poor circulation, fatigue, osteoporosis, and weak immunity occur in proportion to the loss of lean body mass.9
A deficit of protein on a daily basis may not develop into a recognized disease, but it may cause fatigue and premature aging. Early signs of such a deficit can include erratic blood sugar and dull hair. These symptoms arise because when the body lacks nutritive protein, it uses structural proteins from the organs, lean tissue as well as skin and hair.
Maintaining protein balance is even trickier for athletes, especially strength athletes. Weight lifting leads to muscle-fiber breakdown. However, the fibers repair and become denser. To allow for this muscle growth, weight lifters require more protein, especially during the first few years of training.
A strength athlete's greater protein need has been confirmed by measuring the amount of nitrogen lost in their sweat, stool and urine compared to the amount of nitrogen ingested in dietary protein. This ratio is called nitrogen balance. If an athlete loses more protein than he ingests, a state of negative nitrogen balance occurs, causing catabolizing of protein from other tissues. If he ingests more than he loses, he is in positive nitrogen balance, which allows for tissue growth.
Strength-training athletes generally require 1.4 to 1.7 g/kg of body weight of protein daily to stay in positive nitrogen balance. For the average 170-pound male, this equals 108 to 131 g daily, or 18 to 21 percent of a 3,000-kcal diet.10 Preliminary data suggest that novice (less than 1 year) strength athletes require more protein than do those with several years' experience because the novices experience more tissue growth.
Similarly, endurance athletes are wise to eat adequate amounts of protein. Prolonged bouts of aerobic exertion require protein, especially branched chain amino acids, in order to burn carbohydrates. When adequate protein does not come from the diet, it is taken from tissues less essential to the activity at hand. For instance, the gaunt upper bodies often seen in long-distance runners is a product of chronic protein catabolism that spares the legs.
Endurance athletes commonly consume high-carbohydrate diets before and after exercise to restore supplies of muscle glycogen, which are stored carbohydrates. Studies are beginning to show that consuming protein with carbohydrates helps fill glycogen stores better than carbohydrates alone, because glycogen formation is dependent on free amino acids.11
Protein Requirements
In general, younger people need more essential amino acids than adults because children have smaller body masses and therefore less structural proteins to fall back on.
In addition to age-related variance, protein needs vary as a result of pregnancy and lactation, chronic diseases, and many genetically determined differences.
Protein is not a panacea, and excessive amounts will not make anyone stronger or faster. Nonetheless, a little time spent comparing one's intake with activity level can provide guidance on protein intake to maximize performance as well as long-term health.
Sidebars:
Complete Protein Sources
Powdered Protein
Creating A Protein Profile
The Essential Amino Acid Reader
Alan Christiansen, N.D., has a private naturopathic practice in Scottsdale, Ariz.
References
1. American Dietetic Association, ADA's complete food and nutrition guide. New York (NY): Chronimed; 1996. p 533.
2. Mahan K, Escott-Stump S, Krause's food, nutrition and diet therapy. Saint Louis (MO): Saunders; 1996. p 66.
3. Guyton A. Textbook of medical physiology. Saint Louis (MO): Saunders; 1993. p 728.
4. Pennington J. Food values of portions commonly used 15th ed. Location: Harper and Row; 1989.
5. Young VR, Pellett PL. Plant proteins in relation to human protein and amino acid nutrition. Am J Clin Nutr. 1994; 59:1203-12.
6. Caso G, et al. Albumin synthesis is diminished in men consuming a predominantly vegetarian diet. J Nut 2000 Mar;130(3):528-33.
7. USDA. Excerpt of the report of the dietary guidelines advisory committee on dietary guidelines for Americans. 2000.
8. Dennis BH, et al. Nutrient intakes among selected North American populations in the lipid research clinics prevalence study: composition of energy intake. Am J Clin Nutr 1985;41:312-29.
9. Blain H, et al. The preventive effects of physical activity in the elderly. Presse Med 2000 24;1240-8.
10. Lemon PWR. Effects of exercise on dietary protein requirements. Int J Spors Nutr Exercise Metab 1998;12(8):426.
11. van Loon LJ, et al. Maximizing postexercise muscle glycogen synthesis: carbohydrate supplementation and the application of amino acid or protein hydrolysate mixtures. Am J Clin Nutr 2000 Jul;72(1):106-11.
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