Samenvatting

Goal: The goal of this thesis is to study the association between protein intake and the change in lean mass during a cutting and a bulking phase. The term cutting means being in a caloric deficit for a period of time to reduce the amount of body fat and maintain as much muscle mass as possible. The term bulking is used to describe a period of time with a focus on gaining muscle mass. Method: The MUCMUS study is an observational prospective study with strength athletes (aged 18+ and training at least 4 times a week). These athletes were either bulking or cutting for a period of ±10 weeks. Participants recorded their nutritional intake in a 3-day food diary and their training regimen in a 7-day training diary. Both diet and training were programmed by the athletes themselves. Both groups were measured twice; once before their period of bulking or cutting and once after ±10 weeks. Protein and energy intake was estimated by a 3-day food diary. Body composition was based on whole body lean mass (bone-free and fat-free mass), body fat mass and body fat percentage measured with a DXA-scan. Total body weight was measured with the calibrated weighing scale. The data was analysed using linear regression, with change in lean mass (kg) as the dependent variable, and protein intake (g/kg body weight) as the main determinant, with adjustments for the potential confounder caloric intake. Results: The dataset, which consisted of the two visits and a complete food diary, was used for analysis. Out of the 26 participants, 13 were in the bulking phase and 13 in the cutting phase. Protein intake ranged from 1.46 to 3.57 g/kg body weight with a mean of 2.76 ± 0.70 kg. When corrected for caloric intake during the bulking phase, a significant association between protein intake and change in muscle mass (b=0.58, p=0.041) was found. This beta showed that an increase of 1 g/kg of protein leads to an increase of 0.6 kg of muscle mass. There were no significant associations in crude models without correction. Conclusion: Within the small population for this study, one significant association was found between protein intake and changes in lean body mass during the bulking phase. Based on this study, no concrete recommendation can be given for strength athletes or dietitians. Keywords: Protein, muscle mass, strength athletes, body composition, body fat percentage.