Female Endocrinology in Arduous Training

THE APPROACH

We undertook a detailed characterisation of reproductive function, stress (and the hypothalamic-pituitary-adrenal (HPA) axis), bone health and energy metabolism during 11-month officers training at the Royal Military Academy, Sandhurst. We conducted dynamic function tests tracking subtle changes in responsiveness of the adrenal and pituitary before and after the first 28 weeks of training. At four regular intervals over the 12 months we performed dual-energy x-ray absorptiometry and high resolution peripheral quantitative CT scans, blood tests, hair sampling (for cortisol) and heartrate variability, as well as questionnaires about stress and diet. Fitness was measured by the military fitness tests (the centrepiece of which is a 1.5 mile (2.4km) best effort run, which correlates well with maximal oxygen uptake (VO2max).

Due to the highly compressed nature of the training programme, it wasn’t possible to carry out indirect calorimetry or VO2max testing, but we did conduct three 10-day assessments of energy balance and energy availability during representative periods of the course. We measured energy intake, total energy expenditure from doubly labelled water, and exercise energy expenditure from accelerometry. We also tracked diurnal salivary cortisol and menstrual function using diaries and ovulation using regular urine sampling for progesterone: creatinine ratio for the entire year.

THE IMPACT

The key findings were:

• The course was highly stressful, related to externalised locus of control, lack of sleep and personal difficulties. The HPA axis was upregulated, with increased basal cortisol measured from hair and increased cortisol response to ACTH. However there was a reduction in early morning cortisol across the course, suggesting habituation to the stress of training
• Participants expended 3,400 kcal per day, driven by an exercise energy expenditure of 2,300 kcal per day. Overall body composition did not change (although fat mass and lean mass fluctuated during training), however. Blood tests also suggested energy sufficiency, with total triiodothyronine and insulin-like growth factor 1 unchanged, but significant increases in leptin and insulin resistance.
• Although there was evidence of menstrual disturbance, commensurate with reports from studies of basic military training elsewhere, we noted pronounced anovulation – up to 93% of cycles in non-contraceptive users in the most arduous term of training. The pituitary LH and FSH responses were reduced during the first 28 weeks of training, regardless of contraceptive use. There was an increase in some markers of follicular size, suggesting ovarian dysregulation with larger follicles developing unruptured
• Stress fracture rates were very low during training. Bone health was not impaired during training, with no change in total bone mineral content, and an increase in strength of the lower limb. There was blood bone marker evidence of increased formation of bone and reduced resorption, however we identified a potential hotspot for fractures between 14 and 28 weeks of training, where secondary mineralisation lagged deposition of new collagen

Potential impact:

• We have highlighted to policymakers that nutrition was adequate, although there was evidence of metabolic maladaptation during training
• Whether the metabolic and reproductive maladaptive effects we saw are consistent with long-term poor outcomes is unknown: this is an important question for future studies
• In order to mitigate metabolic and reproductive maladaptation during training, which were likely driven by HPA axis upregulation, policy makers should consider addressing sleep deprivation, resilience training and the ethnographic ‘shock’ of entry into the army