Chapter 6
Endocrine System Responses
and Adaptations
Copyright © 2012 American College of Sports Medicine
Endocrine System: Overview
• General Functions
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Helps body maintain normal function
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Prepares body for exercise
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Mediates several adaptations
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Is involved in every system
• Mechanism
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Releases chemical messenger (hormone)
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Transports hormone to target tissue
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Elicits chain of events leading to desired function
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Hormone Actions
• Autocrine Actions
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Hormone acts on cells that produced it
• Paracrine Actions
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Hormone acts on cells adjacent to the ones that produced it
• Endocrine Actions
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Hormone enters general circulation & travels systematically to
specific target tissues
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Role of Releasing Hormone
• Releasing Hormones
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Hormones that cause the release of other hormones
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Hypothalamus
• Segment of brain that acts as an endocrine gland
• Provides link between nervous & endocrine systems
• Synthesizes & releases neurohormones to the pituitary gland
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Role of Releasing Hormone (cont’d)
• Hormones Released by the Hypothalamus
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Corticotropin-releasing hormone (CRH)
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Gonadotropin-releasing hormone (GnRH)
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Growth hormone-releasing hormone
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Growth hormone-inhibiting hormone
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Thyrotropin-releasing hormone (TRH)
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Prolactin-inhibiting hormone (PIH)
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Role of Releasing Hormone (cont’d)
• Hormones Released by the Pituitary in Response to the
Release of Hormones from the Hypothalamus
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Growth hormone (GH)
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Follicle-stimulating hormone (FSH)
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Luteinizing hormone (LH)
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Adrenocorticotropic hormone (ACTH)
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Thyroid-stimulating hormone (TSH)
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Prolactin
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Beta-endorphins
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Types of Hormones
• Steroid Hormones
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Made of three 6-carbon rings & one 5-carbon ring
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Synthesized from cholesterol via synthetic pathways
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Actions:
• Released into circulation
• Arrive at target tissue
• Diffuse through cell membrane
• Bind to specific receptor within cell
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Types of Hormones (cont’d)
• Common Steroid Hormones
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Androgens: testosterone (TE)
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Glucocorticoids: cortisone
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Progesterone
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Prostaglandins
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Anabolic steroids: commonly used/abused by athletes
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Types of Hormones (cont’d)
• Peptide Hormones
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Proteins of various sizes
• Peptides: small chains
• Polypeptides: large chains
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Direct product of:
• mRNA translation
• Cleavage from larger parent molecules
• Other postsynthesis modifications
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Circulate & bind to specific receptors on cell membranes of target
tissues
Copyright © 2012 American College of Sports Medicine
Types of Hormones (cont’d)
• Common Peptide Hormones
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Insulin
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Insulin-like growth factor-I (IGF-I)
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Superfamily of GH molecules
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Types of Hormones (cont’d)
• Amine Hormones
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Have amine (NH2) group at end of molecule
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Derived from amino acids
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Classified as protein hormones
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Synthesized from:
• Tyrosine
• Phenylalanine
• Tryptophan
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Must bind to surface-bound receptor on target tissue
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Can act as neurotransmitters in autonomic nervous system
Copyright © 2012 American College of Sports Medicine
Types of Hormones (cont’d)
• Common Amine Hormones (catecholamines)
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Epinephrine
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Norepinephrine
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Dopamine
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Major Endocrine Glands in the Human
Body
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Hormone Activity: Production, Release,
and Transportation
• Production & Release
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Peptides & amines are synthesized in advance & stored in
vesicles until needed
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Steroids are synthesized from cholesterol & released (not stored)
• Transportation
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Released into circulation & transported to target tissues
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Half-life: time it takes for half of hormone to be degraded
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Transport (binding) proteins
• Protect hormone from metabolism
• Deliver hormone to its receptor
Copyright © 2012 American College of Sports Medicine
Hormone Activity: Production, Release,
and Transportation (cont’d)
• What Determines Hormonal Concentrations in the Blood?
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Amount of hormone released
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Pattern of release (pulsatility)
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Rate of metabolism
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Quantity of transport proteins
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Time of day (circadian patterns)
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Plasma volume shifts (during exercise)
Copyright © 2012 American College of Sports Medicine
Hormone Activity: Production, Release,
and Transportation (cont’d)
• Negative Feedback Control
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Elevates a hormone when it is low
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Reduces a hormone when it is elevated
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Example: athlete using anabolic steroids—
• Reductions in testosterone production lead to
• Testicular shrinkage due to
• Negative feedback inhibition of endogenous TE production
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Receptor Interaction
• Receptor Specificity
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Lock-and-key principle: only one hormone will unlock or
activate each receptor
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Cross-reactivity: allows more than one hormone or molecule to
activate a receptor
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Receptor Interaction (cont’d)
• Steroid and Thyroid Hormones
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Steroids, because they are lipophilic:
• Diffuse through target cell membranes
• Bind to receptors in cytoplasm or nucleus
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Receptors are bound to heat-shock proteins in cytoplasm prior to
hormone binding
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Hormones and Exercise
• Exercise Presents a Potent Stimulus for Hormonal
Adaptations
• Resistance Training (RT) Acute Program Variables
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Intensity
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Volume
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Rest intervals
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Exercise selection & sequence
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Repetition velocity
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Frequency
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Hormones and Exercise (cont’d)
• Aerobic, Plyometric, Sprint, & Agility Training Variables
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Intensity
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Volume & duration
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Modality
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Work:rest ratio
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Frequency
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Hormones and Exercise (cont’d)
• Other Variables Affecting Hormonal Responses
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Genetic predisposition
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Gender
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Fitness level
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Nutritional intake
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Potential for adaptation
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Hormones and Exercise (cont’d)
• Classifications of Hormonal Responses and Adaptations
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Acute responses during exercise
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Chronic changes in resting concentrations
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Chronic changes in acute response to exercise
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Receptor changes
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Testosterone (testes)
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Elevated during & immediately following exercise
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Acute TE response weaker in women
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Changes in resting TE during RT are inconsistent
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Regulated by luteinizing hormone (LH)
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Bound to sex hormone-binding globulin (SHBG)
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Precursors: dehydroepiandrosterone (DHEA), androstendione, &
androstenediol
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Hormones and Exercise (cont’d)
• Growth Hormone (GH) Superfamily (anterior pituitary-brain)
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Secreted by anterior pituitary
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Released in pulsatile manner
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Under control of GHRH & inhibited by somatostatin
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Exercise is a potent stimulus for GH secretion
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No apparent change in resting GH with consistent exercise
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50% of GH binds to GH-specific binding proteins (GHBPs) that
extend its half-life & enhance its effects
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Hormones and Exercise (cont’d)
• Insulin-Like Growth Factors (IGFs) (liver)
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Structurally related to insulin & mediate many actions of GH
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Small polypeptide hormones secreted by liver in response to GH
stimulation
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Increase proliferation & differentiation of satellite cells & protein
synthesis
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Enhance muscle hypertrophy
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IGF-1 response to exercise is unclear
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No consistent pattern of change in IGF-1 during exercise
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Insulin (pancreas)
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Secreted from Islets of Langerhans (B cells) in pancreas
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Secreted in response to glucose intake
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Increases muscle protein synthesis when amino acid is adequate
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Aerobic & anaerobic training:
• Improves insulin sensitivity
• Reduces insulin resistance
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Hormones and Exercise (cont’d)
• Glucagon (pancreas)
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Protein hormone consisting of 29 amino acids
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Synthesized in A cells in Islets of Langerhans of pancreas
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Inhibited by glucose levels
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Stimulates breakdown of glycogen
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Increases energy availability
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Elevated during exercise as energy demands increase
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Hormones and Exercise (cont’d)
• Cortisol (adrenal cortex; perimeter of adrenal gland top of kidney)
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Catabolic glucocorticoid released from adrenal cortex in response
to stress under the control of CRH & ACTH
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Stimulates lipolysis in adipose cells
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Increases protein degradation in muscle cells
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Decreases protein synthesis in muscle cells
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Elevated during aerobic exercise, with greater response
occurring with greater intensity
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Increased during RT
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Resting concentrations reflect a long-term stress & are transient
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Catecholamines (adrenal medulla; adrenal gland-top kidney)
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Secreted by adrenal medulla
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Secreted in response to stress (physical, heat, hypoxia,
hypoglycemia)
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Increased during both aerobic & anaerobic exercise
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Magnitude of increase depends on:
• Muscle mass involved
• Posture
• Intensity
• Duration
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Hormones and Exercise (cont’d)
• β-Endorphins (anterior pituitary-brain)
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31-amino acid peptide cleaved in anterior pituitary from parent
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Act as neurotransmitters in nervous system & as analgesics
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Increase relaxation
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Enhance immune function
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Increased with exercise, with response depending on intensity &
duration
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Thyroid Hormones (thyroid gland-neck)
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Thyroxine (T4) & triiodothyronine (T3) released into circulation
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Travel mostly bound to transport proteins
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Increase basal metabolic rate
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Increase protein synthesis
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Augment actions of catecholamines
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Response to exercise not clear
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Fluid-Regulatory Hormones
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Fluid homeostasis is critical to exercise performance
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Elevations in AVP, atrial peptide, renin, aldosterone, &
angiotensin II during exercise
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Magnitude of increase dependent on intensity, duration, fitness
level, & hydration status
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Leptin (adipose tissue)
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Product of ob gene in adipose tissue
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Relays satiety signal to hypothalamus to regulate energy balance
& appetite
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Higher levels in obese individuals (4 times greater) & women
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Crosses blood-brain barrier to act with receptors
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Concentrations influenced by insulin, glucocorticoids,
catecholamines, thyroid hormones, TE, GH, & stimulants
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Not affected by exercise, independent of % body fat
Copyright © 2012 American College of Sports Medicine
Hormones and Exercise (cont’d)
• Estrogens (Ovaries)
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Include estradiol, estriol, & estrone
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Have long half-lives
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Synthesized & secreted primarily by ovaries in women, under
control of LH & FSH
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Produced from conversion of androgens in men
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Response to exercise unclear
Copyright © 2012 American College of Sports Medicine