Dietary protein is important for optimal recovery from and adaptation to all types of exercise due to its ability to provide substrates for the repair and remodeling of muscle and body proteins (1, 2). The importance of this macronutrient is exemplified by the consensus statements from the American College of Sports Medicine (ACSM) identifying a target intake range of 1.2–2.0 g⋅kg⁻¹⋅day⁻¹ for active and athletic populations (3). However, these broad recommendations are primarily based on studies investigating the protein requirements of dichotomous strength (e.g., bodybuilding, resistance training) and endurance athletes and may not be directly translatable to athletes who fall within this continuum, such as those engaged in variable-intensity exercise.

Intermittent variable-intensity sports (e.g., basketball, soccer, ice hockey) have a unique activity pattern that incorporate characteristics of both aerobic and resistance exercise. For example, despite featuring periods of high intensity sprinting, these sports, similar to traditional more steady-state aerobic exercise, have a high reliance on aerobic metabolism (4). While there is a predominant reliance on carbohydrates for fuel during high-intensity exercise, aerobic exercise is also associated with an increase in the oxidation of amino acids (5), which may be accentuated during the latter stages of variable-intensity training or competition when glycogen availability becomes limited (6, 7). Alternatively, weight-bearing variable-intensity sports also incorporate rapid changes in velocity and direction (e.g., stop-and-go movements) that require high force production (during deceleration and acceleration), which may be more broadly aligned with that of dynamic strength or power exercises (8). Similar to high-force resistance exercise, variable speed, high-intensity “stop-and-go” type sports can also increase markers of muscle damage and induce decrements in performance (9). Nevertheless, despite the importance of dietary protein for replenishing exercise-induced oxidative losses and supporting the repair and remodeling of body (and especially muscle) proteins following exercise (1, 10), the impact of variable speed, high-intensity exercise on protein requirements, to our knowledge, have yet to be systematically investigated.

The majority of research identifying protein needs in active and athletic populations utilize the nitrogen balance technique, which has practical and analytical limitations (11, 12). Our group previously used the minimally invasive indicator amino acid oxidation (IAAO) method to demonstrate that protein requirements in endurance athletes are ~50% greater than sedentary individuals (13), which is consistent with the generally greater recommendations for active individuals estimated using nitrogen balance methodology (5, 14). Importantly, the IAAO method identifies the protein requirement that maximizes whole body protein synthesis (15), which would be important for an athlete aiming to enhance their recovery from and potentially adaptation to an exercise stimulus beyond merely maintaining nitrogen equilibrium (16, 17). Therefore, the objective of the current study was to prospectively investigate, for the first time, dietary protein needs of male athletes engaging in variable intensity exercise using IAAO methodology. We hypothesized that the safe protein intake would be greater than the current recommendations for non-active individuals determined by NBAL (0.83 g⋅kg⁻¹⋅day⁻¹) (18) and IAAO (1.2 g⋅kg⁻¹⋅day⁻¹) (12) but within ACSM commendations (1.2–2.0 g⋅kg⁻¹⋅day⁻¹) (3).

The aim of this study was to utilize the minimally invasive IAAO technique to determine the safe protein intake in active individuals following variable-intensity exercise that would model a stop-and-go, team sport-type (e.g., soccer) stimulus. We observed an EAR and safe protein intake (as determined by the upper 95% CI) to be 1.20 and 1.40 g⋅kg⁻¹⋅day⁻¹, respectively. Importantly, our 95% CI as a percentage of the EAR was similar to our previous observations in active females (i.e., ~17 vs 21%, respectively) (30) but less than studies in resistance-trained males at rest (~29%) (31) and non-exercising adults (~29%) (12), indicating our data displayed a robust and relatively homogenous bi-phase response. Although the Institute of Medicine does not consider that active individuals require a greater daily protein intake (32), our values exceed the current EAR (0.66 g⋅kg⁻¹⋅day⁻¹) and safe intake (0.83 g⋅kg⁻¹⋅day⁻¹) determined by the NBAL technique (18) and were numerically greater than those requirements established by the IAAO technique in non-exercised individuals (0.93 and 1.2 g⋅kg⁻¹⋅day⁻¹, respectively) (12). These results are consistent with the current broad (i.e., 1.2–2.0 g⋅kg⁻¹⋅day⁻¹) sports nutrition consensus recommendations based on primarily NBAL studies (3) and suggest that active individuals engaged in weight-bearing, variable-intensity exercise require a greater protein intake than their non-active peers.

Phenylalanine flux in the present study (~76 μmol⋅kg⁻¹⋅h⁻¹) was generally greater than that of non-exercising adults (~59 μmol⋅kg⁻¹⋅h⁻¹) (12), which is consistent with previous athletic populations utilizing the IAAO (13, 31) and would suggest a greater whole body protein turnover. Muscle protein turnover is elevated after acute aerobic (33, 34) and resistance exercise (35). Moreover, whole body and muscle protein turnover have also been reported to be increased in trained strength (36, 37) and endurance athletes (38, 39) at rest, which could suggest the trained status of our participants contributed to a chronically elevated phenylalanine flux independent of any acute effect of exercise. Therefore, while it is unclear what potential site(s) may be responsible for this increased phenylalanine flux and/or whether it was an acute effect of exercise or a chronic adaptation to training, the lack of effect of protein intake on this variable is consistent with a robust estimation of the average and safe protein intake from F¹³CO₂ (21).

Current recommendations for dietary protein in athletic populations are primarily based on research from the dichotomous training modalities of aerobic and resistance exercise (3, 5, 40). Moreover, the broad recommendations (i.e., 1.2–2.0 g⋅kg⁻¹⋅day⁻¹) from consensus statements such as the ACSM (3) are generally nonspecific, which perhaps reflects the multitude of factors that may influence requirements (e.g., exercise modality, energy status, sex, etc.) and, in the case of team sport-type athletes, the limited research on athlete sub-groups. We demonstrate that athletes who engage in variable-intensity, stop-and-go exercise characteristic of many team sports (e.g., soccer, hockey, rugby, etc.) have an increased protein requirement relative to the EAR for non-exercised individuals based primarily on nitrogen balance data (i.e., 1.2 vs 0.66 g⋅kg⁻¹⋅day⁻¹) (18). In addition, the safe intake in the present study was numerically greater than an IAAO-derived requirement in non-exercising adults (i.e., 1.2 g⋅kg⁻¹⋅day⁻¹) (12). Our results contribute to the growing body of research demonstrating the amount and type of physical activity has a modifiable effect on daily protein requirements, which currently is not reflected in general nutrition guidelines (32). Nevertheless, our protein requirement is within the range of current sports nutrition consensus statements (3). With the assumption that dietary amino acids can be used to support protein synthesis or be irreversibly oxidized, development of the IAAO model has demonstrated that hepatic protein synthesis is the reciprocal of F¹³CO₂ (41). We have also demonstrated that the synthesis of plasma albumin (a hepatic export protein) mirrors that of muscle protein synthesis during recovery from exercise (42). Collectively, these data suggests that team sport athletes who adhere to the current safe dietary protein intake, as most generally do when consuming adequate energy (43, 44), would obtain a sufficient quantity of this important macronutrient to maximize whole body protein synthesis to support their recovery from and/or adaptation to their sport-specific training. However, it may be relevant to highlight that the present safe intakes were derived from the consumption of a high quality, reference egg protein and may not directly translate to diets based solely or predominantly on plant-based sources (45), which may be deficient in one or more essential amino acids (46). Thus, athletes, who if omnivorous may obtain greater than ~40% of the dietary protein from plant-based sources (43), should ensure they consume complementary plant-based proteins (e.g., grains and legumes) or a combination of high quality plant and animal-based sources to ensure the current safe intake is met with nutritionally complete dietary protein(s) (45). Although it remains to be determined whether the present requirement represents an “optimal” intake from the standpoint of maximizing athlete performance (16, 17), the protein intake determined herein could be viewed as minimum target for athletes engaged in variable-intensity, weight-bearing exercise.

The moderately greater requirements from the present study relative to a non-exercised population is ultimately due to the incorporation of the exercise stimulus (12). The variable intensity, stop-and-go nature of the LIST test mimics the physiological demands of a soccer match and can increase markers of exercise-induced muscle damage (47), which would ultimately stimulate skeletal muscle remodeling to facilitate post-exercise recovery (10). It is possible that this adaptive remodeling contributed to the elevated rates of phenylalanine flux and the greater protein requirements in the present study compared to a non-exercised population (12). Alternatively, the metabolic demands of the exercise stimulus [i.e., ~70% VO_2max (4, 23)] would have also increased amino acid oxidation (48). Assuming a contribution of ~5% toward total energy expenditure, the LIST could have resulted in ~0.13 g⋅kg⁻¹ of protein oxidized over the 75-min exercise period. This increased oxidative loss could potentially explain ~65% of the difference between the EAR determined herein and previous non-exercised population estimates by IAAO (12). These oxidative amino acid losses have previously been suggested to contribute to the greater protein requirements of strength athletes (i.e., football and rugby) who included sprint training in their programs (37). Therefore, while we are unable to determine what specifically contributed to the moderately elevated protein requirement in our athletes, our results with variable-intensity exercise are consistent with well-established research highlighting the physiological demands of endurance and resistance exercise modalities increase the metabolic requirement for this important macronutrient (5, 40). Additional research on lower impact weight bearing sports (e.g., ice hockey) and/or training modalities that include both resistive and aerobic components (e.g., concurrent training, crossfit) could be conceivable future avenues of study to advance our understanding of the nutritional requirements that span the classic strength-endurance continuum of exercise demands. Moreover, the suggested benefit of protein supplementation to improve exercise performance and anaerobic adaptations during training programs (49) could be suggestive of a slightly elevated protein requirement during consecutive days of training or competition (e.g., tournament play) for team sport athletes. Inasmuch as the oxidative losses were the primary determinant of the increased demand, our results could subsequently suggest that protein requirements may be lower on a non-training day for similar team sport athletes. Alternatively, team sports that require lower aerobic metabolism and/or include additional breaks in play (e.g., American football) might have proportionally lower protein requirements on a single day of exercise. Ultimately, the array of variables that may encompass variable intensity team sport-type athletes and their exercise demands would require additional research to titrate the impact of each on protein requirements in this population. Thus, we posit that the present results provide the background to which the impact that additional exercise/training variables may have on protein requirements of team sport can be compared.

In summary, we report that male athletes engaged in weight-bearing team sport-like variable intensity exercise have an estimated average protein requirement (1.2 g⋅kg⁻¹⋅day⁻¹) and safe intake (1.40 g⋅kg⁻¹⋅day⁻¹) that are greater than the current safe intake (estimated by NBAL) (18) and by IAAO in non-exercised males (12). Provided athletes adhere to current sports nutrition guidelines and consume adequate energy (3), these minimum requirements would likely be satisfied with a complete diet containing high quality and/or complementary protein sources.

All participants were informed of the purpose of the study, the experimental procedures, and all the potential risks involved. This study was carried out in accordance with the Declaration of Helsinki with written informed consent from all subjects. The protocol was approved by the University of Toronto Health Sciences Research Ethics Board.

DM, GC-M, PP designed the study. JP, DW, and HK collected and analyzed the data. DM and JP drafted the manuscript and all authors were responsible for revising its intellectual content. All authors read and approved the final manuscript.

HK is an employee of the study funder (Ajinomoto Co., Inc.) but has no competing financial interests. All other authors have no competing interests to declare. The reviewer BW and the handling Editor declared their shared affiliation.