Nutritional status assessment

Plasma glutamine concentrations can be evaluated to determine an individual's glutamine status. Among high-performance athletes (speed skating, swimming, skiing) under a variety of training conditions, Smith and Norris35 observed plasma glutamine concentrations to range from 402 to 741 ^mol/l. Under conditions of rest, the average plasma glutamine concentration in these athletes was 585 ^mol/l, while under heavy training conditions, a significant reduction to 522 ^mol/l was observed. Additionally, under conditions of overtraining, plasma glutamine concentrations decreased to 488 ^mol/l. Therefore, it has been proposed that glutamine concentrations decrease when the work volume exceeds the athlete's tolerance level; however, low glutamine levels are not necessarily indicative of the training status of an athlete. Hiscock and Mackinnon22 also observed differences in resting plasma glutamine concentrations based on the athlete's sport, with cyclists having the highest concentration (1395 ^M/l), runners and swimmers having intermediate levels (691 and 632 ^M/l, respectively), and power lifters having the lowest levels (556 ^M/l), suggesting that the physical and metabolic demands of a sport may influence an athlete's glutamine status. However, it is unclear if this is due to the effect of the sport or a combination of the sport and dietary practices of these athletes. Furthermore, dietary protein intake, when expressed as gram per kilogram of body weight, was observed to be negatively associated with plasma glutamine concentrations, which may be attributed to the role of glutamine in maintaining acid-base balance, with high protein diets increasing the acid load and thereby increasing glutamine needs by the kidneys.

In addition to exercise, an individual's dietary intake can also have an influence on his plasma glutamine concentrations. Blanchard et al.28 observed high-carbohydrate (70%) diets to increase plasma glutamine concentrations compared to low-carbohydrate (45%) (i.e., high-protein) diets in endurance-trained men completing exercise trials. However, muscle glutamine concentrations did not differ between the two groups, and no association was observed between plasma glutamine concentrations and changes in muscle glycogen concentrations. This suggests that the effect of carbohydrate intake on plasma glutamine is not influenced by the muscle glycogen stores. Likewise, Gleeson et al.29 observed low-carbohydrate diet (7%) to be associated with a reduction in plasma glutamine concentrations during recovery compared to a high-carbohydrate diet (75%). Low-carbohydrate and high-protein intakes have been suggested to result in lowering plasma glutamine levels due to a disruption in the acid-base balance, stimulating the kidneys to increase the uptake of glutamine to buffer the hydrogen ion concentration and restore normal pH.

Additionally, glutamine may serve as a precursor for gluconeogenesis, with low carbohydrate intakes, further reducing plasma glutamine concentrations. Branched-chain amino acid supplementation was also observed to maintain plasma glutamine concentrations among triathletes compared to placebo, which resulted in a 23% reduction after the triathlon.20 This ability of branched-chain amino acids to maintain glutamine concentrations is attributed to their influence of glutamate metabolism in the skeletal muscle with the subsequent release of NH3, which is used in the production of glutamine. Therefore, when assessing an individual's glutamine status, it is important to assess both his dietary intake and exercise patterns.

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