Nutrition related health of female recruits in the New Zealand Army : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Nutritional Science at Massey University, Albany, New Zealand
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2023
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Massey University
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Background: Basic training for military recruits is a physically demanding course that is generally a sudden change from an individual’s habitual lifestyle. Female recruits have physiological disadvantages in comparison to males, which contribute to lower aerobic fitness and higher risk of musculoskeletal injuries. Nutrition related health, including iron and vitamin D status, dietary intake and body composition is essential to support the health and physical fitness of female recruits. However, little is known about how these nutrition related health factors change during completion of basic training, their determinants or associations with measures of physical fitness in female recruits in the New Zealand Army.
Objectives: The objectives of this research in females undertaking 16-weeks of basic training in the New Zealand Army were to 1) characterise iron status in female recruits during basic training and investigate associations with physical fitness; 2) investigate associations between iron stores, dietary patterns and non-dietary determinants of iron stores in female recruits at the commencement of basic training; 3) characterise vitamin D status in female recruits during basic training and investigate potential determinants at the commencement of basic training; and 4) characterise body composition, physical fitness and dietary intake during basic training and investigate associations between these three factors in female recruits.
Methods: Data were collected at weeks 1 and 16 of basic training from female recruits who enlisted in the New Zealand Army between February 2014 and March 2016. Demographic, lifestyle and medical history information were self-reported via an online questionnaire at week 1. Dietary intake data from a food frequency questionnaire (FFQ) administered online and body composition measurements were collected at weeks 1 and 16. Body composition was determined by the InBody230 bioelectrical impedance analyser and included body mass (BM), skeletal muscle mass, fat mass (FM), fat free mass (FFM), total body water and percent body fat (%BF). Height and body mass index (BMI) were also determined. Biochemical data were assessed at weeks 1 and 16 and included serum ferritin (SF), transferrin saturation (TS), soluble transferrin receptor (sTfR), erythrocyte distribution width (RDW), haemoglobin (Hb) and serum 25-hydroxyvitamin D (25(OH)D). A 2.4km run, push-ups and curl-ups were performed at weeks 1 and 8 to assess physical fitness. Changes during basic training in iron status, 25(OH)D, body composition, physical fitness and dietary intake were investigated using paired t-tests. To explore dietary determinants of iron stores at the commencement of basic training, dietary patterns (DPs) from the FFQ were identified using factor analysis. The DPs were then examined alongside potential non-dietary determinants of iron stores. Following univariate analysis, age, %BF, previous blood donation, ≥6-hours of exercise per week and a vegetarian DP were analysed using a multiple linear regression model. To explore vitamin D status, changes in 25(OH)D were characterised by ethnicity and season. Following univariate analysis, age, BMI, ethnicity, season, exercise and SF were analysed as potential determinants of 25(OH)D at the commencement of basic training using a hierarchical linear regression model. Associations between physical fitness and iron status indicators and %BF were investigated using Pearson’s correlation coefficients. Associations between frequency intake of food categories and %BF were explored using the rho-Spearman’s correlation.
Results: Of the 108 female recruits invited to take part in this research, 106 volunteered to participate. During basic training, the mean ± standard deviation (SD) changes for iron status indicators were that SF decreased (56.6 ± 33.7 to 38.4 ± 23.8μg/L, P<0.001), TS decreased (38.8 ± 13.9 to 34.4 ± 11.5%, P=0.014), sTfR increased (1.21 ± 0.27 to 1.39 ± 0.35mg/L, P<0.001), RDW increased (12.8 ± 0.6 to 13.2 ± 0.7%, P<0.001) and Hb increased (140.6 ± 7.5 to 142.9 ± 7.9g/L, P=0·009). At week 16, sTfR was positively associated (r=0.29, P=0.012) and TS was negatively associated (r=–0.32, P=0.005) with the week 8 run time. There were no significant associations between iron status and push-ups or curl-ups. Serum ferritin was positively associated with %BF (P<0.009) and negatively associated with blood donation in the past year (P<0.011), explaining 17.5% of the variance in SF. There was no association between SF and DPs in the multiple linear regression model. From week 1 to week 16, the mean ± SD for 25(OH)D was 102.5 ± 33.6 to 67.4 ± 22.6nmol/L (P<0.001) for basic training commenced in summer and 67.4 ± 21.5 to 73.8 ± 18.9nmol/L (P=0.033) for basic training commenced in winter. Overall, more than two-thirds of participants had suboptimal vitamin D status (<75nmol/L) at the end of basic training, regardless of the season training commenced. Increasing age and BMI, being of Pacific or Māori ethnicity and commencing basic training in winter were negatively associated with 25(OH)D. Collectively these determinants explained 45.0% of the variance in 25(OH)D at the commencement of basic training. From week 1 to week 16, the mean ± SD change for FM was -3.8 ± 3.6kg, %BF was -5.5 ± 3.7% and FFM was 3.8 ± 1.8kg (all P<0.001). There was no change in BM or BMI. All measures of physical fitness improved during basic training (P<0.001). There was a significant increase in frequency intake of protein, grains, fats, discretionary items and beverages (all P<0.001). A higher %BF at week 1 was positively associated with the 2.4km run time and negatively associated with push-ups at both weeks 1 and 8 (all P<0.05).
Conclusions: Overall, 16-weeks of basic training in the New Zealand Army provides adequate nutrition to support training-induced adaptations in physical fitness and body composition of most female recruits. Optimal %BF and FFM were associated with improved physical fitness, while BMI is an unreliable measure of body composition in this physically active cohort. However, storage and functional iron parameters indicated a decline in iron status and 25(OH)D indicated a decline in vitamin D status in female recruits during basic training. Diminished tissue iron status was associated with impaired aerobic fitness. A lower %BF and blood donation in the past year were the strongest determinants of reduced iron stores while wintertime and being of Pacific or Māori ethnicity were the strongest determinants of reduced 25(OH)D at the commencement of basic training. Therefore, while positive changes in nutrition related health are occurring during basic training, several factors are negatively impacting the iron and vitamin D status of female recruits, both before and during basic training. These factors are limiting the potential of female recruits to achieve optimal health and physical fitness. Delivering education, clinical screening and early supplementation of iron and vitamin D are recommended strategies to counter suboptimal iron and vitamin D status and enhance the success of female recruits during basic training in the New Zealand Army.
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Figures 2.1 & 2.3 are re-used with permission. Chapter 4 is reproduced under a Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/by-nc/4.0/).
Keywords
New Zealand. Army, Women, Basic training (Military education), Women soldiers, Nutrition, Requirements, Health and hygiene, New Zealand, Iron in the body, Vitamin D in the body, Human body, Composition, Physical fitness