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Background: To examine the effects of higher-protein diets on endogenous glucose metabolism in healthy, physically active adults, glucose turnover was assessed in five endurance-trained men (age 21.3 ± 0.3 y, VO2peak 70.6 ± 0.1 mL kg-1 min-1) who consumed dietary protein intakes spanning the current dietary reference intakes., Findings: Using a randomized, crossover design, volunteers consumed 4 week eucaloric diets providing either a low (0.8 g kg-1 d-1; LP), moderate (1.8 g kg-1 d-1; MP), or high (3.6 g kg-1 d-1; HP) level of dietary protein. Glucose turnover (Ra, glucose rate of appearance; and Rd glucose rate of disappearance) was assessed under fasted, resting conditions using primed, constant infusions of [6,6-2H2] glucose. Glucose Ra and Rd (mg kg-1 min-1) were higher for MP (2.8 ± 0.1 and 2.7 ± 0.1) compared to HP (2.4 ± 0.1 and 2.3 ± 0.2, P < 0.05) and LP (2.3 ± 0.1 and 2.2 ± 0.1, P < 0.01) diets. Glucose levels (mmol/L) were not different (P > 0.05) between LP (4.6 ± 0.1), MP (4.8 ± 0.1), and HP (4.7 ± 0.1) diets., Conclusions: Level of protein consumption influenced resting glucose turnover in endurance athletes in a state of energy balance with a higher rate of turnover noted for a protein intake of 1.8 g kg-1 d-1. Findings suggest that consumption of protein in excess of the recommended dietary allowance but within the current acceptable macronutrient distribution range may contribute to the regulation of blood glucose when carbohydrate intake is reduced by serving as a gluconeogenic substrate in endurance-trained men.

The current investigation examined the effect of variations in protein intake on Whole body protein turnover (WBPTO) at rest in endurance-trained males. Whole body protein turnover is influenced by both diet and exercise. Whether endurance athletes require more protein than the non-exerciser remains equivocal. Five male runners (21.3 +/- 0.3 years, 179 +/- 2 cm, 70.6 +/- 0.1 kg, 8.7% +/- 0.4% body fat, 70.6 +/- 0.1 VO(2)max) participated in a randomized, crossover design diet intervention where they consumed either a low-protein (LP; 0.8 g/kg), moderate-protein (MP; 1.8 g/kg), or high-protein (HP; 3.6 g/kg) diet for 3 weeks. Whole body protein turnover (Ra, leucine rate of appearance; NOLD, nonoxidative leucine disposal; and Ox, leucine oxidation), nitrogen balance, and substrate oxidation were assessed at rest following each dietary intervention period. The HP diet increased leucine Ra (indicator of protein breakdown; 136.7 +/- 9.3, 129.1 +/- 7.4, and 107.8 +/- 3.1 micromol/[kg . h] for HP, MP, and LP diets, respectively) and leucine Ox (31.0 +/- 3.6, 26.2 +/- 4.3, and 18.3 +/- 0.6 micromol/[kg . h] for HP, MP, and LP diets, respectively) compared with LP diet (P < .05). No differences were noted in nonoxidative leucine disposal (an indicator of protein synthesis) across diets. Nitrogen balance was greater for HP diet than for MP and LP diets (10.2 +/- 0.7, 1.8 +/- 0.6, and -0.3 +/- 0.5 for HP, MP, and LP diets, respectively). Protein oxidation increased with increasing protein intake (54% +/- 6%, 25% +/- 1%, and 14% +/- 2% for HP, MP, and LP diets, respectively). Findings from this study show that variations in protein intake can modulate WBPTO and that protein intake approximating the current recommended dietary allowance was not sufficient to achieve nitrogen balance in the endurance-trained males in this investigation. Our results suggest that a protein intake of 1.2 g/kg or 10% of total energy intake is needed to achieve a positive nitrogen balance. This is not a concern for most endurance athletes who routinely consume protein at or above this level.

This study aims to characterize the relationship between increased protein intake and hydration indexes. Five men participated in a 12-week, randomized, crossover, controlled diet intervention study. Subjects consumed eucaloric diets containing 3.6 (high protein), 1.8 (moderate protein), and 0.8 (low protein) g/kg/day of protein for 4 weeks each. Energy intakes were based on requirements established relative to resting energy expenditure and activity at baseline. Assessments included blood urea nitrogen, plasma osmolality, urine-specific gravity, and estimates of fluid balance. Repeated-measures analyses of variance and paired t tests were used to determine effects of treatment and time. Fluid intake and fluid balance were unaffected. Blood urea nitrogen was higher for high protein vs low protein and vs moderate protein, and urine-specific gravity was higher for high protein vs moderate protein. Baseline plasma osmolality was greater for high protein vs low protein and vs moderate protein. The effect of increasing dietary protein on fluid status was minimal.

The effect of a 4-wk aerobic exercise training program (30-45 min, 3-5 d/wk, >or=65% maximal heart rate) on mixed skeletal muscle protein fractional synthetic rate (FSR), fractional breakdown rate (FBR), and net protein balance (FSR - FBR) (NET) was examined in 8 healthy, previously unfit men and women [21.0+/- 0.4 y, 163.7+/- 4.4 cm, 75.6+/- 5.7 kg, 33.5+/- 4.1% body fat, VO(2 peak) 38.6+/- 2.3 mL/(kg.min)] fed eucaloric diets providing 0.85 g protein/(kg.d) for the 6-wk study. Measurements were made at baseline after 2 wk of diet intervention only, and after 4 wk of aerobic exercise training and diet intervention. Primed continuous infusions of ring-[(2)H(5)]-phenylalanine (2 micromol/kg; 0.05 micromol/(kg.min) and [(15)N]-phenylalanine (2 micromol/kg; 0.05 micromol/(kg.min) were used to assess skeletal muscle protein turnover at rest via the precursor-product method. Endurance training improved cardiovascular fitness, with a significant increase in VO(2 peak) (P<0.01) and a significant decrease in running time on a standard course (P<0.01). There were o significant changes in body mass or composition. There was a significant increase in FSR (0.077+/- 0.007 vs. 0.089+/- 0.006%/h, P<0.05) and decrease in NET (FSR - FBR) (-0.023 +/-0.004 vs. -0.072 +/- 0.012%/h, P < 0.05); FBR tended to increase (0.105+/- 0.014 vs. 0.143+/- 0.018%/h; P=0.06) after training. Findings show that aerobic training for 4 wk increases skeletal muscle protein turnover in previously unfit subjects.

Both exercise and dietary protein intake affect whole-body protein turnover (WBPTO). Few studies have investigated the effect of aerobic exercise training on WBPTO [leucine rate of appearance (Ra), oxidation (Ox), and nonoxidative leucine disposal (NOLD)] in untrained individuals consuming a specified level of protein. This study examined the effect of aerobic exercise training on WBPTO in untrained men and women during a controlled diet intervention providing 0.88 g protein/(kg . d). After a 2-wk adaptation to the study diet, 7 subjects [3 men, 4 women; 76.1 +/- 5.8 kg, 164.7 +/- 4.4 cm, 30.7 +/- 4.5% body fat, 39.1 +/- 2.8 VO(2max) (maximal oxygen uptake) mL/(kg . min)] participated in 4 wk of aerobic exercise training (running and walking 4-5 times/wk at 65-85% maximal heart rate). WBPTO (determined via constant infusion of 1-[(13)C] leucine), nitrogen balance, and body composition were determined at baseline and after 4 wk of training. Nitrogen balance (-1.0 +/- 0.7 vs. 0.9 +/- 1.1 g N/24 h, P = 0.03) improved with exercise training, whereas body mass and composition did not change. Leucine Ra did not change, Ox decreased [18 +/- 2 to 15 +/- 2 micromol/(kg . h), P