The Role of Protein and Amino Acids in Sustaining and Enhancing Performance (1999) / Chapter Skim
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Pages 19-76
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From page 19... ...
National Academy Press 1 CommiDee Overview INTRODUCTION Proteins form the major constituents of muscle, catalyze virtually all chemical reactions in the body, regulate gene expression, and comprise the major structural elements of all cells. Individual amino acids, the components of proteins, also serve as neurotransmitters, hormones, and modulators of various physiological processes.
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From page 20... ...
, the significance of which is unclear. Because the administration of these hormones is known to stimulate protein synthesis under some conditions, the Army maintains considerable interest in exploring their potential both to ameliorate the losses in lean body mass sustained by troops under conditions of extreme negative energy balance and to stimulate an increase in muscle mass and physical performance.
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From page 21... ...
on physical and cognitive performance and response to stress (IOM, 1994~. Data were presented on the effect of protein-to-carbohydrate ratio on mental alertness, the effect of physical activity on protein requirements, and the influence of branched-chain amino acids, tyrosine, and hyptophan in pharmacological amounts on cognitive function.
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From page 22... ...
In mature humans, a homeostatic mechanism maintains the balance between tissue protein synthesis and breakdown by drawing on the free amino acid pool. Methods for Assessment of Protein Requirements Because the majority of nitrogen in the body is associated with protein and amino acids, nitrogen has been used as a marker for assessing whole-body and tissue protein flux and status.
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With the improvement of this technology and the widespread availability of stable isotope-labeled metabolites, amino acid kinetic studies have come to augment nitrogen balance in examining the effects of dietary protein, energy, and physical activity on overall protein metabolism. Amino acids labeled with stable isotopes of hydrogen (2H)
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From page 24... ...
Also based on nitrogen balance data, the recommendation for total essential or indispensable amino acids (IAAs) as a percentage of protein intake is 43 percent for infants and 11 percent for adults (FAO/WHO/UNU, 1985~.
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Importance of the Debate over Indispensable Amino Acid Requirements Although consensus exists at present for the adult protein requirement this is not the case for the adult requirement of indispensable amino acids. Since the 1985 FAOMlIO/UNU report, Young and coworkers have presented data that contradict the findings of the report; based on these data, Young suggests that the adult requirement for total IAAs is 31 percent of the protein requirement, or about three times the FAO/WHO/UNU estimate (McLarney et al., 1996; Young, 1987, 1994; Young and El-Khoury, l 995a; Young and Marchini, 1990; Young et al., 1989; see also Chapter 10~.
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From page 26... ...
However, if the FAO estimates are incorrect and indispensable amino acids are required in the higher amounts proposed by Young, individual protein sources may duplicate the scoring patterns poorly, and protein quality may then become a significant determinant of protein requirements. Argumentfor Higher Indispensable Amino Acid Requirements Young has based his argument for higher indispensable amino acid requirements on two related measures: the obligatory oxidative losses of these amino acids and the calculated obligatory losses based on daily nitrogen loss.
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, nitrogen balance did not differ significantly between the MIT and the FAO patterns; this finding suggests that both patterns support overall body protein economy. The Rebuttal Young agrees with Millward that there are inherent difficulties in defining requirements for indispensable amino acids.
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Resolution of the Debate The practical implications of the debate between Young and Millward revolve primarily around lysine: the lysine content of cereal proteins is limiting for growth. If Millward is correct, then all dietary proteins, whether plant or animal, contain enough lysine and other amino acids to support adequate protein nutriture of adults if consumed in amounts that meet the protein requirement (although some military personnel in the 1 8-22-year age group are still growing, a factor that might influence the requirement for some amino acids)
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From page 29... ...
The implications of this debate for the current state of knowledge of protein and amino acid requirements for the military depend in part on the current intake of dietary protein and amino acids by military personnel and in part on other factors influencing protein requirements in these individuals, as discussed below. STRESSORS THAT INFLUENCE PROTEIN REQUIREMENTS As discussed by Friedl in Chapter 3, the stressors encountered most frequently by military personnel are high levels of physical activity with or without energy restriction; illness, injury, and infection; and environmental extremes.
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Contractile Activity and Muscle Protein Turnover Muscle protein synthesis appears to decrease during exercise and to rebound after exercise; thus, any net change in nitrogen balance or muscle protein is observed only over a period of several days. Muscle protein
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From page 31... ...
A potentially significant implication of this adaptation is that individuals who habitually consume high-protein diets may face the risk of significant losses of protein stores if suddenly forced to curtail protein intake. Energy Balance and Protein Requirements The direct influence of energy intake on nitrogen balance has been recognized for many years (Cuthbertson and Munro, 1937~.
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From page 32... ...
Results of studies depend in part on the energy intake of the participants, the kind and intensity of the exercise studied, and the length of time given for adaptation to varying protein intakes (Butterfield, 1987~. It has long been held that individuals experiencing an energy deficit whether it is the result of a decrease in energy intake, an increase in energy expenditure, or both exhibit an increase in protein breakdown and protein requirements (Calloway and Spector, 1954; Calloway, 1975~.
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From page 33... ...
should be sufficient to preserve muscle mass and nitrogen balance if energy intake is sufficient. Although the protein requirements of women have been studied inadequately, the small amount of available data suggests that the needs of nonpregnant women may be similar to those of men.
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Changes in dietary protein and energy requirements to sustain muscle protein synthesis during recovery from infections and severe injury are of considerable importance to the military situation (Wolfe, Chapter 13~. A full discussion of the impact of illness and injury on protein and energy requirements is beyond the scope of this report; the reader is referred to a comprehensive review such as that by Souba and Wilmore (1994~.
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From page 35... ...
As discussed later, hormonal therapies have been used in combination with nutritional support in an attempt to improve this situation. In badly burned children, growth hormone has demonstrated positive effects on muscle protein synthesis (Gore et al., 1991~.
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From page 36... ...
, if subjects consumed sufficient energy to maintain energy balance, weight losses were corrected and nitrogen balance was maintained. These findings suggest that the loss of lean tissue mass and the negative nitrogen balance often experienced at high altitude are due entirely to the negative energy balance caused by altitude-induced anorexia and increased metabolic rate, rather than to an increase in protein requirements.
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From page 37... ...
Thus, the MRDA for protein expressed on a g/kg BW basis is 1.3 for both men and women. Based on observations suggesting that protein requirements may be increased for individuals engaged in specific types of exercise, the question arises whether recommendations established for protein intake by soldiers during World War II are still appropriate for military personnel today.
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In the small number of studies in which energy expenditure was estimated, soldiers in the field tended to be in negative energy balance (as evidenced by weight loss) , although their protein intake met or exceeded the MRDA (none of these studies included women)
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The MRDA of 80 g/d would therefore be sufficient to meet the apparent protein requirements of most pregnant or lactating women. Summary In summary, the protein intake of soldiers in both garrison and field situations appears adequate relative to the current MRDA.
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From page 42... ...
Protein and Amino Acid Supplements and Cognitive Performance Although it is well known that several amino acids are precursors to neurotransmitters or neurotransmitters themselves, the brain's precise need for these amino acids is not well known. The concentrations of these precursor amino acids in blood influence their availability to brain neurons and, as a result, the ability of neurons to synthesize neurotransmitter products (because the rate of production of some transmitters is directly influenced by local concentrations of their precursors)
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From page 43... ...
Chronic, substantial reductions in protein intake can reduce brain wyptophan levels and serotonin production in laboratory animals (Fernstrom and Wurtman, 1971) , with the ingestion of proteins naturally low in tryptophan
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From page 44... ...
These data are insufficient to allow an estimate to be made of a brain tryptophan "requirement" in humans in relation to habitual dietary protein intake. Some human populations (for example, those that subsist on a corn-based diet)
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In addition, data on tyrosine supplementation are insufficient to demonstrate conclusive effects on cognitive performance. Protein, Amino Acids, Muscle Mass, and Physical Performance Measurement of Muscle Mass Although skeletal muscle (SM)
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From page 46... ...
Creatinine excretion in humans varies by gender, due to gender differences in muscle mass, and is affected by total dietary protein and the amount of muscle-containing foods (red meat) consumed.
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From page 47... ...
. The coefficient of variation associated with muscle mass determination by CT or MRI is 2-3 percent when the instruments are calibrated by comparing the results obtained with excised cadaver tissue or filled balloons to the actual weights of these standards (Heymsfield et al., 1997~.
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This approach has been validated by comparing total body muscle mass determined by CT to appendicular skeletal muscle estimated by DXA. The correlation coefficient between the two is 0.95, which indicates a strong association and the feasibility of using DXA for the quantitation of muscle mass in humans in viva.
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From page 49... ...
Muscle is metabolically sensitive to insulin action; insulin administration decreases protein breakdown but does not appear to increase muscle protein synthesis in humans except when preceded by a large infusion of amino acids. In insulin-dependent diabetes, muscle protein breakdown is increased, with little effect on muscle protein synthesis.
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1995~. Injection of IGF-I twice daily for 1 month in postmenopausal women resulted in increased nitrogen balance, whole-body protein synthesis, skeletal muscle protein synthesis, and net protein synthesis and in decreased protein breakdown (Butterfield et al., 1997~.
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found no effect of GH on skeletal muscle protein synthesis in catabolic cancer patients. Sandstrom and coworkers reported no effect of IGF-I administration on nitrogen balance or protein breakdown in postoperative patients receiving dextrose with no added amino acid source.
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From page 52... ...
Resting total plasma amino acid concentrations did not change during the race, although a large number of amino acids decreased or increased over the course of the race. This finding suggests that the daily recovery periods were too short to restore amino acid balance, but it was not possible to interpret the significance of individual changes or to draw conclusions about the effect of strenuous exercise on amino acid metabolism.
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From page 53... ...
Similarly, research in healthy individuals infused with a-keto acids of the BCAAs, who showed an improvement in performance that was theoretically due to augmentation of the TCA intermediate pool, actually experienced a depletion in the TCA cycle intermediate pool (Katz et al., 1986; Sahlin et al., 19904. Thus, Wagenmakers cautions against the use of BCAA supplements as ergogenic aids for military troops, suggesting that these supplements may deplete TCA cycle intermediates when exercise is intense and of long duration and could thus hasten the time to fatigue.
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From page 54... ...
There may, however, be situations in which such supplementation would theoretically be beneficial. For example, with exercise during moderate protein restriction, exogenous glutamine may enhance skeletal muscle protein synthesis and aid general maintenance of acid-base homeostasis.
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From page 55... ...
Although, as noted above, the structure and function of immunologically important proteins is dependent upon the balanced availability of amino acids, there is growing evidence that the supplemental administration of certain amino acids can produce immunological benefits. Glutamine is the best example of this, as noted in an earlier CMNR report (IOM, 1999~.
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From page 56... ...
In considering this issue with respect to operational rations, it is desirable that the protein supplied be of high quality. During recovery from infection or trauma, including but not limited to blood loss, lost body protein must be replaced and this protein requirement is high for indispensable amino acids, especially lysine.
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From page 57... ...
In addition, some plant protein sources contain other substances not found to be associated with animal proteins, such as soluble fiber, which also decrease the levels of serum cholesterol and saturated fatty acids. Plant proteins, such as the cereal proteins, are low in one or more of the indispensable amino acids.
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From page 58... ...
have shown an improvement in protein synthesis when amino acids are provided immediately after exercise, compared to no feeding. Borchers and Butterfield (1992)
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From page 59... ...
Similarly, the potentially deleterious effects of high protein intakes in aged individuals (who are losing renal function in association with senescence) are not applicable to individuals of military age.
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Although phosphorus is known to decrease urinary calcium losses, negative calcium retention in response to high protein intakes is not necessarily prevented by increasing the phosphorus content of the diet (Hegsted et al., 1981~. In contrast to the results of Spencer and coworkers, the addition of meat to the diets of young men, which resulted in an increase in protein intake from 55 to 146 g and phosphorus from 890 to 1660 ma, led to increased urinary calcium losses and negative calcium balance (Schuette and Linkswiler, 1982~.
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From page 61... ...
would be expected to be associated with higher urinary calcium excretion than lower-protein diets. However, these protein intakes would not necessarily have a negative impact on calcium retention depending on other dietary factors such as the intake of phosphorus and calcium.
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Maher further points out that the purity of individual amino acids is a very important consideration in their use as supplements. Experience with the consumption of ~-tryptophan, which contained a suspected low-level contaminant, has shown that extreme harm can result (Hertzman et al., 1990~.
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From page 63... ...
High doses of single amino acids given to rat dams elevated their plasma amino acid concentrations and resulted in offspring with lower bir~weight, decreased brain weight, and altered behavior. Significant effects have been reported for such amino acids as leucine, isoleucine, valine, histidine, threonine, hyptophan, and tyrosine (Burns and Kacser, 1987; Frieder and Grimm, 1984; Funk et al., 1991; Huether, et al., 1992; Matsueda and Niiyama, 1982~.
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From page 64... ...
1991. Administration of branched-chain amino acids during sustained exercise Effects on performance and on plasma concentration of some amino acids.
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From page 65... ...
1954. Nitrogen balance as related to caloric and protein intake in active young men.
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From page 66... ...
1996. Lysine requirement of adult males is not affected by decreasing dietary protein intake.
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From page 67... ...
1993. Growth hormone acutely stimulates skeletal muscle but not whole-body protein synthesis in humans.
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1990. Dietary protein increases urinary calcium.
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From page 69... ...
1992. Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders.
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1989. Dietary protein requirements and body protein metabolism in endurance-trained men.
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From page 71... ...
1989. Dietary protein requirements of physically active individuals.
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1995. Stimulation of muscle protein synthesis by long-term insulin infusion in severely burned patients.
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1988. Influence of protein intake and training status on nitrogen balance and lean body mass.
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From page 74... ...
1996. Growth hormone increases muscle mass and strength but does not rejuvenate myof~brillar protein synthesis in healthy subjects over 60 years old.
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C~I~E O~ 73 Jacobs, RJ. Smith, J.~.
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