The Relative Energy Deficiency in Sports (REDs) in Swedish Athletes Study

The syndrome of REDs describes impairment of physiological functions e.g., reproduction and bone health with negative impact on performance. The etiology behind is problematic low energy availability (LEA), defined as low energy intake relative to exercise energy expenditure relative to fat free mass. Eating disorders can either proceed or result from REDs. LEA induces metabolic and endocrine alterations, and energy balance at a lower level prevents further weight-loss. Therefore, most athletes with LEA have a body weight within the normal range, and low resting metabolic rate is a recognized LEA surrogate marker.

LEA suppresses anabolic hormones e.g. estradiol/testosterone, while cortisol levels increase, and bone formation markers are reduced. Low testosterone levels have been reported in male endurance athletes, with potential negative effects on fertility. Among Irish male athletes from different sports 23% were identified with low libido and associated with increased injury risk. Menstrual disturbances due to LEA is frequently reported, and is associated with low bone mineral density and injuries.

Glucose is an important substrate for the muscles and an essential substrate for brain neurons and immune cells. Low glucose availability is considered a major trigger to the alterations of the hypothalamic-pituitary axis hormones causing suppression of the sex and thyroid hormones and increase in cortisol. Blood glucose homeostasis is therefore protected with several mechanism (hepatic gluconeogenesis, increased lipolysis and ketogenesis) during prolonged LEA. Hence, hypoglycemia is a marker of a prolonged and severe catabolic state.

LEA has been associated with gastrointestinal problems in female athletes, derived from hormonal and dietary factors affecting motility and digestion. Long-term LEA in patients with anorexia nervosa is associated with profound alterations of the gut microbiome, with negative impact on gastrointestinal function and immunologic processes, and a reduced microbial diversity is therefore suggested as a potential useful biomarker of REDs in athletes. It is therefore relevant to investigate the associations between the gut microbiome, Gastrointestinal function and immunology status in athletes with and without clinical indication of LEA.

LEA affects both men and women, athletes as well as recreational active people however, the threshold and endocrine response seem to differ between gender. Although menstrual disturbances has been frequently reported in female athletes for decades, few studies have investigated the impact on performance, and no study has yet investigated the performance effects of low libido in male athletes. Furthermore, no study has investigated microbial diversity in male and female athletes with clinical symptoms of LEA. Therefore, it is highly relevant to investigate aspects of physiology, nutrition, performance, and sport environment regarding REDs and eating disorders in both male and females.