Given that amino acids can be oxidized as energy during exercise, it is theoretically possible that this may impact on the need for extra dietary protein. The determination of dietary protein requirements for endurance athletes is a function of the duration and intensity of exercise, gender, age, training status, and habitual energy and CHO intake. In a simplistic approach to determining protein requirements, it is possible to calculate the estimated need for dietary protein by an athlete from first principles. For example, if a 70-kg male was running for 1.5 h at 70% VO2peak and protein accounted for 5% of the total energy expenditure, he would oxidize about 15 g of protein. If his basal protein requirement was 0.86 g/kg/day (60 g), this would represent an additional 25% increase in his daily protein requirement (1.07 g/kg/day). Most male and female endurance athletes habitually consume more protein than this (Table 7.3). These calculations are only rough estimates and most studies have used NBAL to try to quantify dietary protein requirements for endurance athletes.
Two often-quoted studies from the mid-1970s determined NBAL following the initiation of an endurance exercise program on a constant protein intake,73 and while consuming two different protein intakes.161 In a group of males starting an endurance exercise program they found that a protein intake of 1.5 g/kg/day was adequate to maintain a positive NBAL, whereas 1.0 g/kg/day was inadequate.161 In addition, they also found that the subjects on a constant protein intake showed progressive adaptation to the moderate exercise program by improving NBAL over the course of about 1 week.73 These latter findings suggested that there were adaptive changes to the stress of exercise (e.g., an increase in amino acid reutilization efficiency), and therefore, an increased protein intake was needed only at the initiation of an endurance exercise program. This is similar to our findings in men and women following a modest training program.40 An improvement in NBAL with moderate-intensity endurance exercise training is due to a lower resting,144 and exercise-induced,40 amino acid oxidation.
With moderate-intensity endurance exercise (<50% VO2peak), there does not appear to be an increase in protein requirements.80162 At these modest exercise intensities, protein utilization is enhanced80 and energy deficits are better tolerated.162 In another study of endurance exercise at moderate intensity (46% VO2peak), El-Khoury and colleagues66 used a combined isotopic tracer and nitrogen excretion method and found that a protein intake of 1 g/kg/day was adequate for young males. The improvement in NBAL observed by Gaine and colleagues144 occurred in men and women with modest aerobic capacity (39 ml/kg/min) after 4 weeks of modest exercise training (4 to 5 times/week at 65 to 85% max HR). Likely the most comprehensive study of endurance exercise training and protein metabolism was completed by Forslund and colleagues.96 They studied leucine oxidation, protein, carbohydrate, fat, and energy balance over a 24-h period in men performing low-to moderate-intensity exercise (90 min at 45 to 50% VO2peak) while consuming a higher (2.5 g/kg/day) and lower (1.0 g/kg/day) protein intake.96 Whole-body protein balance was slightly negative on the 1.0 g/kg/day diet and positive on the 2.5 g/kg/day diet.96 These results suggest that people performing moderate-intensity exercise do not require an increase in dietary protein intake or, at most, it is only marginally above 1.0 g/kg/day.96 However, most athletes exercise at intensities of 65 to 85% of VO2peak, where there may be a negative impact on protein homeostasis and NBAL.
For well-trained and elite endurance athletes, there does appear to be an increase in protein requirements.6'41'65'163-165 One study used NBAL to determine the protein requirements in a group of endurance-trained males who were young (27 years; VO2peak = 65 ml/kg/min) or middle-aged (52 years; VO2peak = 55 ml/kg/min).159 They found that a protein intake of 0.94 g/kg/day was required for NBAL, and whole-body protein synthesis (glycine tracer) increased with increasing protein intakes (0.61 > 0.92 > 1.21 g/kg/day). When accounting for interindividual variability by adding two standard deviations to the zero NBAL intercept, the estimated protein requirement for these males was about 1.28 g/kg/day.165 We performed an NBAL experiment in six elite male endurance athletes (VO2peak = 76.2 ml/kg/min; training >12 h/week) to determine what we considered to be close to the upper limit of protein requirements for endurance athletes.65 We determined the safe protein intake for the elite athletes to be 1.6 g/kg/day, whereas the estimate for a sedentary control group (N = 6) was 0.86 g/kg/day, which was very close to Canadian and U.S. recommendations.65 In a simulated Tour de France cycling study, Brouns and colleagues163 found that well-trained cyclists (VO2peak = 65.1 ml/kg/min) required protein intakes of 1.5 to 1.8 g/kg/min to maintain NBAL. In a final study, Friedman and Lemon164 calculated that the protein requirement for five well-trained runners was about 1.49 g/kg/day, using NBAL. Another study performed in our laboratory found that both male (VO2peak = 59 ml/kg/min) and female (VO2peak = 55 ml/kg/min) endurance athletes had a negative NBAL while consuming a dietary protein intake that was close to the Canadian, U.S., and Australian recommended intake (males = 0.94 g/kg/day; females = 0.80 g/kg/day).41 Finally, a recent study by Gaine and colleagues97 found that the zero intercept for NBAL in well-trained athletes (VO2peak
= 70.6 ml/kg/min) was 1.2 g/kg/day, which would equate to a +2 SD value of 1.5 to 1.7 g/kg/day.
To summarize the available data, it appears that low- and moderate-intensity endurance exercise does not result in an increase in dietary protein requirements. At the initiation of an endurance exercise program there may be a transient increase in dietary protein need, yet the body rapidly adapts to the increase in need. For the well-trained athlete (training 4 to 5 days per week for >45 min at >60% VO2max), there appears to be an increase of about 20 to 25% in dietary protein requirements. In the elite athlete, the increase in dietary protein requirements may be as high as 1.6 g/kg/day (or nearly twice the recommended intake for sedentary persons). Given that the fitness of the Tour de France cyclists163 and that of the athletes in our study65 are respectively among the most demanding and highest reported, this protein intake is probably the top limit of requirement needed. Clearly, there may be some more demanding events; however, the day-to-day training is not likely to exceed that reported for the athletes in these studies.65,163 In spite of these elevated requirements, there is no need for supplementation with a mixed diet of adequate energy intake, providing 15% of the energy from protein. For example, with an energy intake of about 3500 kcal/day (which is still modest), this would amount to about 125 g protein per day or ~1.6 to 1.9 g/kg/day.
One final point about protein requirements for endurance athletes is the possibility of a gender difference. We first found a gender difference in protein metabolism in 1990,64 whereby males increased urinary urea excretion on an exercise compared to rest day, whereas females did not. We concluded that this was due to a glycogen-sparing effect seen in the women.64 In the study where we found that the recommended intake for protein was inadequate for well-trained endurance athletes, we also found that the females had a less negative NBAL and their basal leucine oxidation was lower than that of the males.41 In a subsequent study, we also found that females had a lower leucine oxidation than males both at rest and during exercise before and after a 38-day training program.40 These findings may indicate that the dietary protein recommendations for endurance athletes (Table 7.4) may be 10 to 20% lower for females than for males.
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