Dietary protein is one of our most essential nutrients. Proteins contribute to key body functions like blood clotting, fluid balance, production of hormones and enzymes, vision, and cell repair. The word protein comes from the Greek word meaning "of prime importance." Proteins are similar to carbohydrates and lipids in that each molecule contains atoms of carbon, oxygen, and hydrogen. The major difference is that proteins also contain nitrogen, which comprises approximately 16% of the molecule, along with sulfur, phosphorus, and iron.
The four elements carbon, oxygen, hydrogen, and nitrogen are combined into a number of different structures called amino acids. Each amino acid possesses an amino group (NH2) and an acid group (COOH), with the remainder being different combinations of carbon, hydrogen, oxygen, and in some cases sulfur.
The basic building blocks of all proteins are amino acids. Proteins are created when two amino acids join and form what is called a peptide bond. These peptide bonds are formed when two amino acid groups join together by splitting off a water molecule. Thus, a dipeptide is formed. A tripeptide is formed when three or more aminos are bonded together. An oligopeptide has at least three amino bonds, but less than 50. When more amino acids are added polypeptides are formed. A polypeptide may be made up of anywhere from 50 to 100 amino acids. Most of the foods we eat contain polypetides. All peptide bonds are broken when water is added back to the molecule.
There are 20 amino acids. Nine of these amino acids are considered to be essential. Meaning that the human body can not synthesize them so they must be supplied in the diet. The essential amino acids are:
The other 11 amino acids are know as nonessential because the human body can synthesize them. The nonessential amino acids are:
It should be noted that all 20 amino acids are necessary and present in the body simultaneously for protein synthesis and other body functions to occur.
Proteins (amino acids) are found in both animal and plant foods. While all plant and animal products contain all 20 amino acids, the amount varies. For a food to be able to support growth and life it must contain all nine essential amino acids. Foods that contain all nine essential amino acids are called complete proteins. Those foods that have a deficiency of one or more of the nine essential amino acids are called incomplete proteins. The essential amino acid that is in short supply is called the limiting amino acid.
Animal products are generally thought to be higher quality proteins than plant products only because animal proteins are complete proteins. Not only do animals proteins contain all of the essential amino acids, but they are in much larger amounts and in the proper proportion.
Plant proteins can still provide the human body with all the amino acids needed for optimal growth and development, but the proteins are usually in much smaller amounts. Further more most plant proteins have one or more limiting amino acid making them incomplete proteins. While most plant proteins are considered to be incomplete amino acid sources, they may be combined and eaten in proper combinations to provide complete proteins. The only protein that is an exception to this rule is the soybean. The soybean is considered a complete protein and is comparable to animal protein.
As mentioned earlier, the human body has the capability of synthesizing the 11 nonessential amino acids. When these amino acids are in short supply transamination or the transfer of an amine group (nitrogen group) from an amino acid to a carbon skeleton to form a new amino acid takes place.
In order for an amino acid to be used for energy or used to synthesize other compounds it must lose its nitrogen group without transferring it to another carbon skeleton. This process is called deamination. Once the nitrogen is removed from the amino acid carbon skeleton it is turned to urea in the liver, transferred through the blood to the kidneys, and eventually eliminated in the urine. The carbon skeleton is then used either as energy or converted to other compounds.
As mentioned earlier, protein is very critical in the regulation of human metabolism. It is used to form muscle, connective tissue, blood clotting factors, blood transport proteins, lipoproteins, visual pigments, and the protein matrix inside the bones.
Protein is also used to maintain the body fluid balance by producing albumin and globulin. Without sufficient protein in the blood stream, edema would quickly develop.
Dietary protein also contributes to the acid-base balance by producing buffers that help regulate the amount of free hydrogen ions in the blood. This accepting or donating of hydrogen ions helps to keep the blood pH slightly alkaline (pH 7.35 to 7.45).
The immune system is also composed of proteins. Antibodies are proteins. Without enough dietary protein, the immune system will lack the cells needed to function properly, thus anergy or the lack of an immune response can appear.
Dietary protein can also be used as an energy source . If a diet does not contain enough carbohydrate to supply needed glucose, protein can be used to synthesize glucose. This process is called gluconeogenesis. While protein is not ordinarily considered a major energy source when the diet is balanced, it can be when carbohydrates are not available in a fasted state. The costly process of gluconeogenesis causes much of the muscle wasting that occurs in starvation.
While exercise is considered a relatively small source of energy during exercise when a balanced diet is consumed, research has shown that the greater the intensity of exercise the greater the loss of protein in the urine. Athletes, especially weightlifters believe that they need a higher protein consumption than the recommended RDA of 0.8 to 1.0 g/kg/d to perform optimally. Since these claims are anecdotal, many question whether high protein intakes actually provide additional energy for exercise or whether the additional protein actually enhances muscle synthesis, thereby improving the strength and power of an athlete.
During long term, low intensity exercise protein catabolism can account 5 to 10% of the energy production. Research indicats that the protein needs of a long distance runner may actually increase from 50 to 100% over the RDA recommendations. Lemon et al. (1991) and Dohm (1985) recommend that athletes engaged in endurance type activities consume approximately 1.5 g/kg/d during the first few months of training. Once the athlete reaches a trained state the protein may be reduced to 1.2 to 1.4 g/kg/d and still provide a positive nitrogen balance. In any case, Lemon and Friedman et al. (1989) both noted in their studies that dietary protein needs may vary with individual endurance athletes depending greatly on the athlete’s total energy intake (particularly carbohydrate), and protein quality. For example, female endurance athletes may need more protein since their energy intakes are usually lower.
The use of the branched chain amino acids leucine, valine, and isoleucine may be helpful to endurance runners by helping to maintaining the anabolic hormone responses and slowing the rate of protein catabolism. Carli, et al. (1992) found that human growth hormone (HGH), prolactin, adrenocorticotropic hormone and cortisol increased, while testosterone levels were maintained. Normally the testosterone levels would have been lowered during endurance exercise. It was therefore suggested that BCAA administration before exercise affects the response of some anabolic hormones, mainly human growth hormone and testosterone. Blomstrand et al. (1992) also reported that when BCAAs (7.5-12 g) were taken during exercise, the net rate of protein degradation caused by exercise could be prevented or decreased.
Individuals that are involved in strength and power type sports like bodybuilding, powerlifting, football or sprinting may have even higher dietary protein needs than the endurance athlete to maintain a positive nitrogen balance. These athletes have felt for many years that increased protein consumption would promote an accelerated rate of muscle synthesis and decrease the rate of protein catabolism, resulting in greater muscle mass accumulation. There are many conflicting views over how much protein is actually needed to optimally increase muscle mass and/or strength. However, Williams (1985) feels there is sufficient data available to make some general conclusions. It is generally agreed that a pound of muscle contains about 100 g of actual protein. So in order to gain one pound of muscle mass per week we would need to consume at least 14.29 g of extra protein per day along with the additional calories (100 / 7 = 14.29). While it is not know exactly how many extra calories are necessary to synthesize a pound of muscle mass, the National Research Council notes that 5 calories are needed to support one gram of lean tissue growth (Williams, 1992). So simple math would tell us that 500 extra calories (5 x 100 = 500) may be also necessary every day to gain one pound of lean tissue per week.
Tarnopolsky et al. (1992) using both nitrogen balance and metabolic tracers methodology recommended between that 1.4 and 2.4 g/kg/d for athletes involved in strength and power exercise. Later 1.76 g/kg/d was recommended as the accepted RDA for strength and power athletes by Lemon et al (1992) and Tarnopolsky. These studies showed that whole body protein synthesis was elevated at these intakes without an increase in protein oxidation.
Fern et al. (1991) found that 2.4 g/kg/d was considered protein overload, thus providing no further increase in protein synthesis for strength and power athletes. When strength athletes increased their protein consumption to 2.4 g/kg/d amino acid oxidation increased, but there was no further protein synthesis. Researchers considered this to clearly indicate a protein overload.
It is interesting to note that Consolazio et al. (1975) Marabel et al. (1979), and Dragan et al. (1985) all reported larger increases in strength, lean body mass (LBM) and nitrogen with much higher protein intakes (3.3, 2.8, and 3.5 g/kg/d respectively). These reports tend to corroborate the more anecdotal beliefs of weight lifters that extremely high dietary protein intakes are essential for optimal muscular development.
While these results are very interesting, they still did not prove that higher intakes of more than 2.4 g/kg/d actually were responsible for improving muscle mass during resistance training. Researchers are not exactly sure what role the extra calories might have provided by consuming that much extra protein, could have had on protein synthesis. It is suspected that the more calories you take in over energy balance, the less protein you may actually need for optimal protein synthesis (Bucci 1993). In any case a higher protein intake has not been shown to impede sports that involve strength and power.
Because the kidneys are involved in the removal of urea from the consumption of protein, many have come to believe that excessive protein intakes may be harmful. The only study that have ever shown high protein intakes to harm kidney function was done by Brenner et al. (1982). However, these studies were done on patients that already had compromised kidney function. So it can not be assumed that athletes with a normal kidney function will ever have kidney disease due to high intakes of dietary protein. Currently there is no evidence to show that protein intakes in the of 1.76 g/kg/d would contribute to any health problems (Lemon, 1995).
While high protein intakes have not been shown to adversely affect a normally functioning kidney, the consumption of protein is usually accompanied by the consumption of saturated fats and cholesterol. Excessive intakes may mean high levels of saturated fats and cholesterol too. However this problem can be completely avoided by the use of most common dietary protein.
Individuals with a personal history of liver or kidney problems should be aware of their susceptibility to adverse reactions from excess dietary protein. Because the liver is the major organ involved in protein metabolism, excessive protein consumption may cause stress to the inadequately functioning liver (NRC, 1989).
High-protein diets may also lead to excessive production of ketones and urea. An inadequately functioning kidney may have trouble removing the ketones and urea from the blood. Thus an increase in blood acidity will occur causing a host of health problems (NRC, 1989).
It should also be warned that athletes who exercise in hot, humid climates and consume high protein diets may be more susceptible to dehydration. Urinary output is the main path for water loss. Because high protein diets have a diuretic effect, due to the excessive production of urea and ketones, frequent urination may be necessary to remove these byproducts from the blood (William, 1992). Thus dehydration may occur much quicker.
A popular sports nutrition dietary fad revolves around the myth that a protein to carbohydrate ratio of the 40-30-30 diet allegedly maintains the proper balance between the hormones insulin and glucagon thus, improve athletic performance, reducing body fat by increasing the utilization of stored fat, and decreasing the likelihood that nutrients will ever be stored as fat.
In the popular book Enter the Zone (Regan Books, 1995), Dr. Barry Sears defines what he refers to as the three blocks, a protein block (7 grams), a carbohydrate block (9 grams) and a fat block (1.5 grams). According to Dr. Sears each 100 calories consumed during a day would contain 7 grams of protein, 9 grams of carbohydrate, and 1.5 grams of fat. But is 30% protein considered to be excessive? If you follow the recommendations of the Dr. Sears, no.
The Zone diet is based on dietary protein needs. Dr. Sears recommends that for each pound of lean body mass that you have, you need to consume between .5 and 1.0 gram of protein per day (depending on your activity level). These dietary protein recommendations however, do not exceed the current recommendations of the available literature. So for every 7 grams of protein, you must consume 9 grams of carbohydrates and 3 grams of fat. For example, if we have a very active athletic person who has 150 pounds of lean body mass, this person would need 150 grams of protein per day or 21 blocks, according to Dr. Sears. This same person would also need about 193 grams (21 blocks) of carbohydrate and 64 grams (43 blocks) of fat per day. The total daily caloric intake would be approximately 1948 calories per day. While the caloric intake appear to be too low for the activity level, the protein intake is right on the money. However it is very unlikely that this athlete will be able to maintain a positive energy balance with such few calories.
When the 40-30-30 diet is randomly applied to any caloric amount, the dietary protein intake could easily exceed recommendations. For instance, if we have a very active athlete at 250 pounds who needs 5000 calories per day to maintain body mass and apply the 40/30/30 ratio, 500 grams of carbohydrate, 375 grams of protein, and 167 grams of fat would be consumed per day. While the carbohydrate to protein to fat ration (blocks) is still within Dr. Sears’ recommendations, this amount of protein is much more than the recommended 1.76 g/gk/d for strength athletes or the 1.2 to 1.5 g/kg/d for endurance athletes. In fact, it is almost 3.3 g/kg/d. These extremely high proteins intakes have not been sufficiently shown to have any benefits on athletic performance and may have adverse affects on those with abnormal kidney or liver function.
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