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function between non-vegetarians and vegetarians (a group demonstrated to have lower dietary protein intakes) [23,24]. Both the non-vegetarian and vegetarian groups possessed similar kidney function, and displayed the same rate of progressive deterioration in renal physi- ology with age [24]. Preliminary clinical and epidemio- logical studies have suggested a benefit of relatively high protein diets on major risk factors for chronic kidney dis- ease, such as hypertension, diabetes, obesity and meta- bolic syndrome. Future studies are necessary to further examine the role of relatively high protein weight loss diets, dietary protein source (quality) and quantity on the prevalence and development of kidney disease in at risk patient populations [25,26]. While it appears that dietary protein intakes above the RDA are not deleterious for healthy, exercising individuals, those individuals with mild renal insufficiency need to closely monitor their pro- tein intake as observational data from epidemiological studies provide evidence that dietary protein intake may be related to the progression of renal disease [21,26]. dietary protein intake and bone metabolism has also served as the cause for some controversy. Specifically, there is concern that a high intake of dietary protein results in the leaching of calcium from bones, which may lead to osteopenia and predispose some individuals to osteoporosis. This supposition stems from early studies reporting an increase in urine acidity from increased die- tary protein that appeared to be linked to drawing calcium from the bones to buffer the acid load. However, studies reporting this effect were limited by small sample sizes, methodological errors, and the use of high doses of puri- fied forms of protein [27]. It is now known that the phos- phate content of protein foods (and supplements fortified with calcium and phosphorous) negates this effect. In fact, some data suggest that elderly men and women (the segment of the population most susceptible to osteoporo- sis) should consume dietary protein above current recom- mendations (0.8 g/kg/day) to optimize bone mass [28]. In addition, data from stable calcium isotope studies is emerging, which suggests the main source of the increase in urinary calcium from a high-protein diet is intestinal (dietary) and not from bone resorption [29]. Also, given that exercise training supplies the stimulus for increasing skeletal muscle protein, levels in the range of 1.4 to 2.0 g/ kg/d are recommended to transform this stimulus into additional contractile tissue, which is an important pre- dictor in bone mass accrual during pre-pubertal growth [30,31]. More research needs to be conducted in adults and the elderly relative to exercise, skeletal muscle hyper- trophy and protein intake and their cumulative effects on bone mass. Overall, there is a lack of scientific evidence linking higher dietary protein intakes to adverse outcomes body of scientific literature which has documented a ben- efit of protein supplementation to the health of multiple organ systems. It is therefore the position of the Interna- tional Society of Sport Nutrition that active elderly indi- viduals require protein intakes ranging from 1.4 to 2.0 g/ kg/day, and that this level of intake is safe. supplements viduals often ingest protein powders. Powdered protein is convenient and, depending on the product, can be cost- efficient as well [32]. Common sources of protein include milk, whey, casein, egg, and soy-based powders. Different protein sources and purification methods may affect the bioavailability of amino acids. The amino acid bioavaila- bility of a protein source is best conceptualized as the amount and variety of amino acids that are digested and absorbed into the bloodstream after a protein is ingested. Furthermore, amino acid bioavailability may also be reflected by the difference between the nitrogen content from a protein source that is ingested versus the nitrogen content that is subsequently present in the feces. Consid- eration of the bioavailability of amino acids into the blood, as well as their delivery to the target tissue(s), is of greatest importance when planning a regimen of pre- and post-exercise protein ingestion. A protein that provides an adequate circulating pool of amino acids before and after exercise is readily taken up by skeletal muscle to optimize nitrogen balance and muscle protein kinetics [33]. mined by the somewhat outdated protein efficiency ratio (PER), and the more precise protein digestibility corrected amino acid score (PDCAAS). The former method was used to evaluate the quality of a protein source by quanti- fying the amount of body mass maturing rats accrue when fed a test protein. The latter method was established by the Food and Agriculture Organization (FAO 1991) as a more appropriate scoring method which utilized the amino acid composition of a test protein relative to a ref- erence amino acid pattern, which was then corrected for differences in protein digestibility [34]. The U.S. Dairy Export Council's Reference Manual for U.S. Whey and Lactose Products (2003) indicates that milk-derived whey protein isolate presents the highest PDCAAS out of all of the common protein sources due to its high content of essential and branched chain amino acids. Milk-derived casein, egg white powder, and soy protein isolate are also classified as high quality protein sources with all of them scoring a value of unity (1.00) on the PDCAAS scale. In contrast, lentils score a value of 0.52 while wheat gluten scores a meager 0.25. |
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