Evaluating the dietary intake, energy expenditure and balance of young professional male rugby league players across the season

The primary aim of this thesis was to evaluate the dietary intake, energy expenditure and energy balance of young professional male rugby league players across the season. Twenty participants from one European Super League U19 academy were recruited. Prior to assessing ‘free-living’ dietary intake, study 1 investigated the validity of a traditional and contemporary dietary assessment tool to measure the total energy intake of young professional rugby league players (n =12). Findings highlighted a small and moderate mean bias for under-reporting by Snap- N-Send a very likely higher (4.96 (0.97) MJ; ES =0.30 (0.07); p =0.0021) total energy expenditure across a five-day microcycle matched for physical activity demands. Findings highlighted the large resting metabolic rates (11.63 (2.46) MJ.day-1), total energy expenditures (18.93 (3.18) MJ.day-1) and moderate physical activity levels (1.6 (0.2)) of players. Players were in a self-reported negative energy balance (-1.63 (1.73) MJ.day-1; p =0.233) and lost body mass (-0.65 (0.78) kg; p =0.076). To support a desired increase in player body mass, case study 1 established the large energy requirements (resting metabolic rate: 14.7 MJ.day-1; total energy expenditure: 22.4 MJ.day-1; physical activity level: 1.5) of one young professional rugby league player across a two-week pre-season microcycle, using the Behaviour Change Wheel to design and implement a behavioural intervention over a twelve week period. Study 4 Findings highlighted the large resting metabolic rates (10.26 (1.49) MJ.day-1), total energy expenditures (16.15 (0.77) MJ.day-1) and moderate physical activity levels (1.6 (0.2)) of players. Players were in a self-reported negative energy balance (-0.70 (1.11) MJ.day-1; p =0.145) and lost body mass (-0.3 (0.6) kg; p =0.222). Sub-analysis within study 4 (n = 5), demonstrated an almost certainly higher total energy expenditure across a pre-season (study 3) vs. in-season microcycle (0.02 MJ.kg.BM- 1; ES =1.14 (0.41); p =0.004). Study 5 and study 6 utilised data collected within study 3 and 4 to investigate the validity of wearable technology or Snap-N-Send to measure the total energy expenditure or total energy intake of young professional rugby league players across ‘true’ free-living conditions (n = 8), respectively. To support appropriate energy intakes following match-play, case study 2 investigated three-day changes in resting metabolic rate following a competitive young professional RL match (n = 5). Findings demonstrated almost certainly, most likely and likely increases in resting metabolic rate at 24 (2.38 (1.02) MJ.day-1; ES =1.06 (0.43); p =0.006), 48 (1.44 (0.93) MJ.day-1; ES =0.62 (0.38); p =0.025) and 72 hours (0.94 (0.78) MJ.day-1;ES =0.40 (0.32); p =0.055) after baseline. In conclusion, studies presented within this thesis establish the large resting and total energy requirements of young professional male rugby league players across the season, which are increased in the days following training-based collisions and competitive match-play. Players require support to consistently achieve desired manipulations of energy balance required for optimal physical development across pivotal youth development periods.