Centre for Human Performance

Female Endocrinology in Arduous Training

Aiming to comprehensively characterise the metabolic and endocrine adaptations of women to 1-year arduous basic training in the military.

Female Endocrinology in Arduous Training

The Challenge

Women have recently been allowed to enter the ground close combat roles in the UK military for the first time, yet rates of adverse outcomes from arduous training are already higher among women than men, e.g. higher training-associated stress fracture rates and through-career musculoskeletal injury rates, higher rates of psychological distress and also reproductive dysfunction. These conditions are linked to hormonal and metabolic changes, but these have never been characterised in detail at the onset of arduous training, in women or men. It is hoped that understanding the physiological changes as women enter compressed training in the military will help mitigate against unnecessary physiological disadvantages for the future.

The FEAT Study won the Royal Society of Medicine Colt Prize for outstanding contribution to Defence Research, 2019


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.


Our findings apply for all women entering the military and form the beginnings of an evidence base to mitigate against unnecessary physiological disadvantage and facilitate more equitable training.


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

Outputs and recognition

  • Gifford RM, O'Leary TJ, Double RL, et al. Positive adaptation of HPA axis function in women during 44 weeks of infantry-based military training. Psychoneuroendocrinology 2019;110:104432 doi: 10.1016/j.psyneuen.2019.104432
  • Gifford RM, Reynolds RM, Greeves J, Anderson RA, Woods DR. Reproductive dysfunction and associated pathology in women undergoing military training. Journal of the Royal Army Medical Corps 2017;163(5):301-10 doi: 10.1136/jramc-2016-000727: 2017 Feb 17]
  • Gifford RM, Howie F, Wilson K, et al. Confirmation of ovulation from urinary progesterone analysis: assessment of two automated assay platforms. Sci Rep 2018;8(1):17621 doi: 10.1038/s41598-018-36051-6: 2018/12/06]
  • Gifford RM, Taylor N, Stacey M, Woods DR. Sex, gender or occupational psychology: what matters most to preventing heat-related illnesses and improving outcomes for women in ground close combat? BMJ Military Health Apr 2020 Online First 

  • Koivula F, Wardle SL, Double R, Woods DR et al. Sleep Patterns During Arduous Military Training in Men and Women: 1045 Board #279 May 29 2:00 PM - 3:30 PM. Medicine & Science in Sports & Exercise 2019;51(6):277-78 
  • Sophie L. Wardle, Rebecca L. Double, Robert M. Gifford, Woods DR et al. Sex Differences in Energy Balance during Arduous Military Training, American College of Sports Medicine, Mar 19, Florida Medicine & Science in Sports & Exercise 2019;51(6):277-78
  • Gifford R, O’Leary T, Woods DR Hypothalamic-pituitary-gonadal (HPG) axis suppression during basic military training in women despite increased adiposity and insulin resistance. Society for Endocrinology BES 2019. BioScientifica
  • R Gifford, DR Woods, RM Reynolds; Impact of military training on dynamic endocrine function, ENDO, Endocrine Society, New Orleans, Apr 19, endocrineabstracts.org
  • Gifford R, Woods DR, female resilience to extreme climactic exposures and arduous military training: mitigating risks to optimize performance Military Health Systems Research Symposium, Florida, USA, Aug 2019
  • R Gifford, DR Woods, RM Reynolds; Female military responses to heat, cold and basic military training. Association of Physicians of Great Britain, Apr 19 

Contact Professor David Woods

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