Immune and Hormonal Changes following Intense Military Training

Military Medicine, Dec 2003 by Gomez-Merino, Danielle, Chennaoui, Mounir, Burnat, Pascal, Drogou, Catherine, Guezennec, Charles Yannick

This study was designed to determine whether the immune and hormonal systems were affected by a 5-day military course following 3 weeks of combat training in a population of 26 male soldiers (mean age, 21 � 2 years). The combination of continuous heavy physical activity and sleep deprivation led to energy deficiency. At the beginning of the training program and immediately after the combat course, saliva samples were assayed for secretory immunoglobulin A and plasma samples were assayed for interleukin-6, dehydroepiandrosterone sulfate, prolactin, catecholamines, glucocorticoids, and testosterone. Secretory immunoglobulin A was lower and circulating interleukin-6 was increased by the end of the course, which was attributed to sympathoadrenergic stimulation. Dehydroepiandrosterone sulfate, prolactin, and testosterone levels fell significantly. These results suggest that prolonged and repeated exercise such as that encountered in a military training program induces immune impairment via a decrease in mucosal immunity and a release of interleukin-6 into the circulation. The impaired secretion of dehydroepiandrosterone sulfate and prolactin, two immunomodulatory hormones, was thought to be a response to the chronic stressors. Lowered testosterone reflects a general decrease in steroid synthesis as a consequence of the physical and psychological strain.

Introduction

The body is often exposed to combinations of stressors, especially in military operations where the Stressors are often prolonged, hard, continuous physical exercise combined with sleep, energy, and water deficiency, cold, heat, time pressure, or periods of waiting and inactivity.1-3 The effects of these various Stressors on the soldier's health are complex but could be deleterious, as it has been shown that chronic stress experienced by soldiers in wartime leads to immunosuppression.4

Recently, Brenner5 reported that 18 weeks of basic infantry training did not have an adverse impact on either immune function or the incidence of upper respiratory tract infections (URTIs). In contrast, Kramer et al.6 in a study of the effects of food restriction during a military training on T lymphocyte proliferative responses in vitro found that (i) the military training program reduced the T lymphocyte proliferative response and (ii) that this was enhanced if the soldiers had a restricted diet. Impaired cellular immune responses have also been evidenced in soldiers after 8 weeks of stressful training.7

In athletes, epidemiological studies have noted an increase in infections, especially URTIs, after severe physical stress such as marathon8 or ultramarathon runs.9 Resistance to respiratory infections is provided by the mucosal system with the major immunoglobulin being secretory immunoglobulin A (IgA). A decrease in salivary IgA concentration has been proposed as a marker of excessive training, which could help to identify athletes more prone to or at risk from respiratory illness.10

Previous studies on mucosal immunity have focused on the acute changes in IgA levels after individual sessions of intense exercise or training. They have described consistent decreases or a chronic suppression of salivary IgA along with an increased incidence of respiratory illness.10-12

On the other hand, muscle cell injury and high, sustained metabolic work rates caused by prolonged exercise (e.g., marathon) have been demonstrated to involve a sequential release of proinflammatory cytokines and more particularly interleukin (IL)-6.13 The proinflanimatory cytokines help regulate the inflammatory cascade and are considered to be primary mediators for repair processes in muscle cells and metabolically active tissues.14

The present study was designed to find out whether a military training in the French Army comprising 3 weeks of conditioning followed by a 5-day combat course induced specific alterations in the immune and hormonal systems. The 5-day course included heavy and continuous physical activities inducing energy deficiency associated with psychological strain and sleep deprivation. In a study of hormonal adaptation, we recently evidenced lowered leptin levels, an index of energy availability, after this training schedule.3 Other metabolic and hormonal changes during the combat course have been documented by our laboratory.2

The immune mucosal system was studied by determination of salivary IgA concentrations and IgA relative to total protein before the training and immediately after the 5-day course. In addition, the immune system was studied by assay of circulating IL-6. In view of the putative immunomodulatory action of dehydroepiandrosterone sulfate (DHEAs) and prolactin during chronic stress,15 we measured plasma DHEAs and prolactin during the military training program. In an attempt to discern mechanisms responsible for alteration in immune status, we also assayed circulating glucocorticoids and catecholamines. We also focused on the consequences of psychological and physical restraints on steroid synthesis via changes in plasma testosterone levels.