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As a direct target gene of the glucocorticoid receptor with a diurnal response pattern, KLF15 signaling may explain the complex role of glucocorticoids in metabolism and protein balance and mechanistically favor the intermittent value of glucocorticoids via exercise or pharmaceuticals. Intracellular adaptation of glucocorticoid regulators to exercise is tissue specific, resulting in decreases in glucocorticoid action in skeletal muscle and increases in glucocorticoid action in the liver and visceral fat (227). Expression of GRβ selectively increases in cells exposed to inflammatory signals; this increased expression leads to glucocorticoid resistance (196, 200) and may reduce the therapeutic potential of glucocorticoids (201). Mechanical stress also stimulates IGF-R signaling cascades via focal adhesion kinase (FAK), an attachment complex protein necessary for mechanical IGF-I-mediated hypertrophy in skeletal muscle cells (163).
Morning enclomiphene aligns with the natural circadian testosterone rhythm and avoids any potential sleep interference from the hormonal signaling cascade it initiates. Correcting zinc and vitamin D deficiency, treating hypothyroidism, and reducing visceral fat and insulin resistance also meaningfully raise IGF-1. Adequate dietary protein (0.7 to 1.0g per pound of body weight daily) is the most critical nutritional factor; protein deficiency rapidly suppresses IGF-1 regardless of exercise. IGF-1 acts on virtually every organ system through its own receptor (IGF-1R), which is closely related to the insulin receptor. Glucose tolerance remains stable in most participants, though those with pre-existing insulin resistance should monitor fasting glucose and HbA1c at 4-week intervals during dose escalation. Visceral adipocytes have significantly higher HSL receptor density than subcutaneous fat cells, making them disproportionately responsive to GH-mediated lipolysis. Most cases resolve as the body adapts to new hormone levels.
Following a rapid post-natal growth phase, skeletal muscle mass is typically maintained at a steady state in adulthood through a controlled balance between muscle protein synthesis (MPS) and breakdown (MPB)—unless in the presence of physiological (exercise) or pathological (age or disease) stimuli. Despite the importance of these hormones for the regulation of skeletal muscle mass in response to different types of exercise, their interaction with the processes controlling muscle mass remain unclear. However, there is growing evidence for selected anabolic hormones to influence the form and function of the motoric system, and, as such, there is a need for increased research in this area. While only a limited number of human studies have examined the effects of steroids on the motor system, there is growing evidence, from animal studies in particular, that certain anabolic hormones, such as testosterone and IGF-1, exert effects on regenerative ability and anti-apoptotic effects on the central and peripheral tissues of the motoric system. Furthermore, muscle fiber specific force is increased where IGF-1 injected into skeletal muscles is specifically targeted to motor neurons and retrogradely transported by the motor axons back to the motor neuron soma as visualized by immunocytochemistry (Payne et al., 2006). In this perspective article, we review the effects of selected anabolic hormones on the motoric system and speculate on the role these hormones may have on influencing muscle and physical function via their impact on the nervous system.
Single-bout, short-term exercise, such as the Wingate test or 30-s run, causes a rise in IGF-1 level. In physically active people, IGF-1 activity may be connected with inflammatory response intensity, mainly with changes to the level of interleukin-1β IL-1β, and IL-6, which reduce the concentration of IGFBP-3 and modulate bioavailability of IGF-1. — Changes in blood IGF-1 level are related to exercise frequency single bout vs repeated, exercise type eccentric vs concentric, and exercise duration short vs long. Ehrnborg et al. observed high blood hGH concentration after a maximal exercise test in athletes practicing such sports as cycling, athletics, rowing, swimming, triathlon, tennis, weight lifting, football, and alpine and cross-country ski-ing. In the study, hGH concentration was significantly higher in the wrestlers than in non-athletes, which indicate an enhancement of anabolic processes by regular physical activity.
Following GH release, which induces the hepatic generation of IGF-1, circulating levels of IGF-1 and GH, feedback to the hypothalamus to inhibit further GH secretion (Daughaday, 2000). However, this has not been confirmed (Miller et al., 2006; Sakamaki-Sunaga et al., 2016) as no differences between follicular phase and luteal phase RET responses have also been observed, at least with regard to strength gains and hypertrophy; and as such, the role of estrogen in mediating responses to RE, remains unclear. However, the proposed effects of estrogen may be defined by the stage of the menstrual cycle. Indeed estrogen replacement has been shown to attenuate the age-related decline in muscle mass observed in postmenopausal women (Enns and Tiidus, 2010). This reduction can eventually lead to very low resting concentrations of circulating testosterone particularly in men, creating the so-called andropause (Vingren et al., 2010). Because testosterone is bound to SHBG with high affinity, it is not available to most tissues for action. With aging, there is a linear decline in bioavailable circulating testosterone in both men and women (Kraemer et al., 1998; Hakkinen et al., 2000), with these reductions leading to osteoporosis in both sexes (Mohamad et al., 2016).
Subsequent decreased sensitivity of monocytes to glucocorticoids 24 h following exercise may act to protect the body from prolonged, exercise-induced cortisol secretion (172). Diurnal pattern of anabolic/catabolic regulators may facilitate anabolic benefit of intermittent exposure. In the periphery, the cellular response to glucocorticoids differs by cell type (167–169), cell cycle stage (167), and exposure to stress (170). Endogenous levels of cortisol are systemically controlled by the hypothalamic-pituitary-adrenal (HPA) axis and locally by the action of 11β-hydroxysteroid dehydrogenase (11β-HSD) enzymes. Cortisol levels are regulated both at the systemic and tissue level to maintain glucocorticoid homeostasis.

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