Protein and Performance (PROPER) in Endurance Athletes (PROPER)

The present study is a parallel group design with randomised allocation to either a 1) reduced protein diet (RP) or a 2) normal protein diet (NP). The two groups will be pair-matched based on habitual dietary protein intake, endurance training, endurance performance, and sex. The aim of the study is to investigate the effects of reducing dietary protein (~1g protein/kg body mass) compared to an eucaloric normal protein diet (~2g protein/kg body mass) for 6 weeks in well-trained endurance athletes on endurance performance, body composition, skeletal muscle protein synthesis, and health-related outcomes. The hypothesis is that endurance performance will maintain or even be improved in well-trained endurance athletes after a 6-week dietary intervention of an eucaloric, protein reduced diet compared to a normal protein rich diet.

Design: The present study will include two phases: a 3-week run-in period, and a 6-week intervention of controlled diets containing either a reduced dietary protein (RP) or normal (habitual) protein (NP). The run-in period serves as a period for obtaining general activity data and habitual dietary and training habits in order to match the groups before entering the dietary intervention period. Subjects and dietary goals: The study is aiming at including 20 male endurance athletes exercising ~15hrs+/week. The 20 males will be pair-matched based on habitual endurance training, habitual dietary protein intake, and endurance performance, and allocated in a randomised order to: 1) a reduced-protein diet (RP) or 2) a normal-protein diet (NP). 3-weeks run-in period: After a minimum of 7 days after written consent has been obtained, the run-in period will begin. Habitual endurance training volumes will be registered in an online platform (TrainingPeaks) and daily activity level will be monitored using an accelerometer (SENS) throughout the run-in and diet intervention period. "Heavy water" (D2O) will be ingested in a large bolus (3.5ml/kg LBM) upon beginning of the run-in period and plasma enrichments will be maintained throughout the study period by ingesting small daily boluses. This serves to determine skeletal muscle protein synthesis throughout the study. 6-weeks dietary intervention: The diets will be eucaloric and the estimated energy percentages for macronutrients will be as follows: RP - Protein ~7E%, Carbohydrate ~63E%, Fat ~29E%. NP - Protein ~16E%, Carbohydrate ~53E%, Fat ~30E%. Weekly urine and faeces samples will be collected during the run-in period and dietary intervention and the controlled diets will be supplied weekly for the participants. Testing: Extensive endurance performance and metabolic testing will be performed prior to-, and during the intervention on weeks -1, 0 (Pre), 3 (Mid), and 6 (Post). The tests include: Body composition by DXA-scanning, resting metabolic rate by online respirometry using a Vyntus (Jaeger, PCX), venous blood-, and skeletal muscle biopsy sampling from m. Vastus Lateralis, a standardised breakfasts, endurance cycling on an electro-magnetically braked bike (Lode Excalibur) or running on a motorised treadmill (Woodway), and haemoglobin mass measuring using a modified version of the CO-rebreathing method. The body mass will be kept unchanged during the 6-weeks dietary intervention.

Diet intervention, Endurance athletes, Endurance performance, Skeletal muscle protein synthesis, Protein intake, Health markers, Microbiome

A 6-week eucaloric dietary intervention of reduced protein intake (~1g protein/kg body mass). The diet will, hence, contain ~7E% protein, ~63E% carbohydrate, and ~29E% fat. Participants are free-living and receive all food pre-packed from the study kitchen. The energy provision will be set to match energy balance.

A 6-week eucaloric dietary intervention of normal protein intake (~2g protein/kg body mass). The diet will, hence, contain ~16E% protein, ~53E% carbohydrate, and ~30E% fat. Participants are free-living and receive all food pre-packed from the study kitchen. The energy provision will be set to match energy balance.

Endurance performance defined as maximal aerobic power (Wmax) during incremental test to exhaustion on a bike measured as 1-minute mean power output (W)

Alterations in Fractional utilisation of maximal oxygen uptake at 4mmol/L blood lactate in response to diet

Alterations in Fractional utilisation of maximal oxygen uptake at 4mmol/L blood lactate in response to diet

Energy expenditure is measured by combining resting metabolic rate and daily activity recording from accelerometer data and heart rate monitor

Haemoglobin mass measured via inhalation of carbon monoxide (CO) using the modified CO-rebreathing method

Haemoglobin mass measured via inhalation of carbon monoxide (CO) using the modified CO-rebreathing method

Blood volume measured via inhalation of carbon monoxide (CO) using the modified CO-rebreathing method

Blood volume measured via inhalation of carbon monoxide (CO) using the modified CO-rebreathing method

Inclusion Criteria: Endurance athlete Exercising at least 10hrs per week Habitual dietary protein intake ~2g/kg body mass BMI<25 Exclusion Criteria: Food allergies that exclude the controlled diet Inconsistent endurance training load