The assessment of habitual dietary intake of British Army recruits and the influence of protein supplementation on training adaptations

Basic Training (BT) is an arduous course with new recruits experiencing energy expenditures of 3000-4000kcal⸱day⁻1 whilst completing 11-13km⸱day-1 in addition to physical training and military specific activities. New recruits are required to pass physical performance tests including a mid-thigh pull, medicine ball throw and 2km maximal run test. Historically, strength improvements have been small and strategies to increase strength improvements in this population are desired. An adequate nutritional intake is needed to support recovery and performance, including strength adaptations, however habitual dietary intake data in British Army recruits is limited. Dietary protein intake is crucial to modulating the effects of strength training. Protein supplementation has been shown to be an effective strategy at increasing a sub-optimal (<1.6g⸱kg-1⸱day-1) habitual protein intake and supporting strength adaptations. To inform the experimental studies of this thesis, the first part of the literature review (chapter 2) was a scoping review to identify which methods were typically used to assess dietary intake in military-research settings. The only method to date that had been validated was weighed food records and therefore this method should be used where feasible. The second part of the literature review (chapter 3) was a systematic review with meta-analyses to establish if protein supplementation could be an effective strategy to support training adaptations in military recruits. The aim was to evaluate the literature on the effects of protein supplementation on adaptations to arduous concurrent exercise training with potential applications to military recruits. Protein supplementation improved strength (SMD= +4.92kg, 95% CI = -2.70-12.54kg) and fat-free mass (FFM) (SMD = +0.75kg, 95% CI = 0.44-1.06kg). The first experimental study (chapter 5) aimed to compare habitual dietary intakes of British Army recruits to the Military Dietary Reference Values to determine changes in energy and macronutrient intake during 2 BT. This study also aimed to measure urinary nitrogen balance during BT. Nitrogen balance declined throughout BT (week 2=4.6 ± 4.1g, week 6=1.6 ± 4.5g, week 11=-0.2 ± 5.5g, p > 0.05) with most participants in negative nitrogen balance during BT (week 2=64%, week 6=55%, week 11=82%). The key findings from this study were that nitrogen balance was attained with protein intakes of 1.5g⸱kg⁻1⸱day⁻1 but higher protein intakes may be individually needed later in BT due to a decline in nitrogen balance during BT and high variability between participants. The final experimental studies (chapters 6 & 7) were two randomised controlled trials which aimed to establish the influence of different doses (20g and 60g) of protein prior to sleep on chronic training adaptations (chapter 6) and acute muscle recovery (chapter 7) during BT. These studies were conducted in the same cohort with 122 (99 men and 23 women) British Army recruits randomised into either a dietary control (CON), carbohydrate placebo (PLA), moderate (20g) protein (MOD) or high (60g) protein (HIGH) group. Muscle strength improved to a similar extent in the CON (+7 ± 19%), PLA (+7 ± 13%), MOD (0 ± 16%) and HIGH (+7 ± 13%) groups. Similar FFM changes (CON= +4 ± 3%, PLA= +4 ± 4%, MOD= +3 ± 3% and HIGH= +5 ± 4%) were also observed. Protein supplementation also did not accelerate muscle recovery from a load carriage test. Urinary 3-methyhistidine increased following the load carriage test, peaking 40-hours-post (CON=263.4 ± 91.6nmol⸱ml-1, PLA=294.1 ± 47.4nmol⸱ml-1, MOD=264.8 ± 57.5nmol⸱ml-1, HIGH=265.3 ± 76.8nmol⸱ml-1). Urinary myoglobin also increased following load carriage, peaking immediately-post (CON=128.9 ± 60.5ng⸱ml-1, PLA=151.7 ± 129.0ng⸱ml-1, MOD=163.5 ± 162.2ng⸱ml-1, HIGH=167.5 ± 104.8ng⸱ml-1). It is possible the training stimulus was not great enough to induce strength and FFM adaptations, and exercise induced muscle damage, limiting the impact of protein supplementation on chronic adaptations and acute muscle recovery. Currently, general protein supplementation does not appear to be warranted during BT but if the programme is revised to increase the stimulus of strength training, protein supplementation may be needed. Similarly, protein supplementation does not appear to be warranted to accelerate muscle recovery from load carriage over comparable distances (5km) and carrying similar loads (15kg). However, it is not known if protein supplementation would be beneficial over longer distances or when carrying heavier loads. Despite general protein supplementation not appearing to be beneficial, it is recommended that recruits who may be at risk of sub-optimal protein intakes (<1.6g⸱kg⁻1⸱day⁻1) should have their diet assessed to determine if nutritional inadequacies need to be addressed.