Introduction

Athletes who compete in endurance-based events are swimming, running, cycling, and skating faster than ever before, and, thus, world records in many events are being broken on a nearly annual basis (1). There are many factors that contribute to this improvement in human exercise performance. First, the coaches working with these athletes have improved scientific knowledge because of advances in the fields of sports medicine and exercise physiology. Second, sporting equipment changes have allowed some improvements in events (1,2). Third, and perhaps most important, is the greater level of exercise training that athletes are performing in our modern era (1-4). It is not uncommon, for example, for marathon runners to complete 150 to 250 km of intensive running per week or for tri athletes to spend 3 to 4 h per day in swimming, running, and cycling training. This large volume of exercise training results in physiological changes and adaptations that are highly beneficial to the human organism, such as enhanced cardiac output, enhanced arterial-venous oxygen difference, increased erythrocyte number, decreased body adiposity, and increased mitochondrial density (3). However, this great volume of exercise training can also place a tremendous amount of stress on the human body and can result in unwanted

From: Male Hypogonadism: Basic, Clinical, and Therapeutic Principles Edited by: S. J. Winters © Humana Press Inc., Totowa, NJ

physiological responses and medical problems, which can potentially compromise the ability of an athlete to perform.

A physiological system that is extremely sensitive to the stress of exercise training is the endocrine reproductive system. A growing body of research during the last 25 yr reveals how chronic exposure to endurance exercise training results in the development of human endocrine reproductive dysfunction (5-9). The majority of the research on exercise and endocrine reproductive dysfunction has focused on sporting women (10-12). A growing number of studies, however, have begun to address the question of how exercise training affects the endocrine reproductive system in men. Comparatively, however, research reports in this area are relatively few. Nevertheless, many researchers hypothesize that the effect of endurance exercise training on the male endocrine reproductive system may be comparable to that found in sporting women, i.e., endurance exercise-trained athletes of both sexes can develop hypogonadism-like characteristics. Specifically, some men who are chronically exposed to endurance exercise training develop low basal resting levels of total and free testosterone. Most of these men display clinically normal levels of testosterone, but the levels are at the low end of normal (13).

To date, no name or label has been applied to these men, other than "endurance-trained men with low resting testosterone." Although this is an accurate descriptive phrase for their condition, it is cumbersome to use. According to Taber's Medical Dictionary, hypogonadism can be defined as "defective internal secretion of the gonads" (14). This is a simplistic definition, not giving an indication of the level of "defect" (clinical vs subclinical) or the root of the "defect" (e.g., hypergonadotropic vs hypogo-nadotrophic). Nonetheless, we chose to use this definition and refer to "endurance-trained men with low resting testosterone" as "exercise-hypogonadal men." This term serves as an operational definition for this special case of exercise men discussed in this chapter. These men have several common characteristics:

(1) Their low testosterone levels are not a transient phenomenon related to the immediate stress-strain of an acute exercise bout.

(2) In many cases, an adjustment in the regulatory axis (to allow a new lower set point for circulating testosterone) has occurred.

(3) They typically have a history of early involvement in organized sport and exercise training, resulting in many years of almost daily exposure to varying intensities of physical activity.

(4) The type of exercise training history most prevalent in these men is prolonged endurance-based activities, such as distance running (10 km or marathons), cycling, race walking, or triathlon.

A second endocrine reproductive disturbance also occurs in male athletic populations. This problem is a pseudohypergonadism brought about by the use of anabolic-androgenic steroid pharmaceutical agents. The term "pseudohypergonadism" is used because in reality, the usage of exogenous anabolic-androgenic steroids results in atrophy of the gonads and reduced endogenous sex-steroid hormone production because of hypothalamic-pituitary suppression. The extremely high circulating levels of anabolic-androgenic phamaceutical agents mask the endogenous hypogonadism.

Evidence points to large numbers of athletes taking anabolic-androgenic steroid agents in an attempt to stimulate muscular growth, development, and strength. The use of these agents has been a persistent occurrence in sports for approx 50 yr. Unfortunately, athletes and coaches continue to ignore the reports of serious medical side effects associated with such use.

This review presents an overview of select endocrine reproductive problems that occur in men involved in exercise training. Specifically, this review discusses (1) how endurance exercise training affects the male reproductive endocrine system to induce hypogonadal-like conditions, resulting in suppressed circulating testosterone levels (denoted here as the "exercise-hypogonadal" state), and (2) anabolic-androgenic steroid abuse and the reproductive dysfunction that is associated with the use of these pharmaceutical agents.

The following discussion is delimited to the relationship between exercise endocrinology and reproduction in men. Research in this area on women is the subject of several authoritative reviews available in the literature (12,15).

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