The role of this literature review was to investigate the metabolic and endocrine effects of resistance training and the implications this has for health.

The majority of the literature on this topic that was found was in relation to the metabolic effect of resistance training on insulin sensitivity. This was mainly due to the fact that insulin resistance is a growing problem and growing at an increasingly rapid rate. The literature on the topic shows some very positive results in regards to insulin resistance and resistance training. Further studies showed positive results with conditions such as sarcopeania and general muscle wastage in the elderly.

Testosterone
Firstly we will look at Resistance trainings effect on testosterone. It has been shown through many studies that the resting levels of testosterone remain the same after a progressive resistance training program. However subjects in most studies who had an initially higher level of testosterone seemed to experience greater muscular hypertrophy gains than those who began the studies with a lower initial testosterone concentration. A study by Kamel (2003), and found that free testosterone index, IGF-1, and physical activity were strong predictors of muscle mass and strength. The mechanisms by which testosterone increases muscle strength are not yet clear. One likely theory is that testosterone exhibits some of its effect on muscle through IGF-1.

Resistance training has been shown to increase the expression of androgen receptors in certain muscles, of which the trapezius seems to be the most responsive, Kadi, Bonnerud, Eriksson et al (2000). However little effect of this was shown in other muscles such as the vastus lateralis. As muscle wastage through inactivity is noted as individuals age it too may be related to a decline in there testosterone levels. A decline in testosterone levels with aging has been observed by both cross-sectional and longitudinal studies. Although there is great inter-individual variability in testosterone and bioavailable testosterone levels with advancing age, half of healthy men between the ages of 50 and 70 years will have bioavailable testosterone levels below the lowest levels seen in healthy men between the ages of 20 and 40. This age-related decline in testosterone is due to a combination of hypothalamic-pituitary and testicular failure Kamel (2003). This will in turn exacerbate an increased muscle tissue loss and lead to many secondary conditions such as sarcopeania Kamel (2003).

Growth Hormone
Resistance training has been shown to cause an acute rise in Growth hormone levels, however resting levels remain unchanged. Many stresses on the body, both mental and physical cause an acute increased growth hormone level. However these levels tend to return to normal usually within a half hour of ceasing resistance training Erhnborg (2003). Growth hormone is another potent anabolic hormone which exhibits its major effects also through its action on resting IGF-1 levels.

Secretion of growth hormone is controlled by growth hormone-releasing hormone (GnRH) and growth hormone-inhibiting hormone (somatostatin) released from the hypothalamus. Growth hormone has skeletal growth effects and additional effects such as increasing cartilage growth. GH achieves its indirect growth-promoting actions by acting principally on the liver and other tissues which in turn release somatomedins (insulin-like growth factor 1-IGF-I and 2 IGF-2). The somatomedins act synergistically with thyroid hormones promoting indirect growth Jenkins (2001). In its skeletal growth effects GH acts through IGF's on both in bone growth and in cartilage growth. In its extra skeletal effects GH also acts through IGF's on tissue growth and cellular differentiation of hormone actions. The somatomedins in turn provide negative feedback to both the hypothalamic and pituitary in controlling growth hormone releasing and inhibiting hormone, and growth hormone release from the anterior pituitary gland. Growth hormone has direct growth promoting actions. Its direct growth promoting actions are achieved in conjunction with cortisol. Exercise as well as other factors has been shown to increase levels of GH in turn stimulating IGF-I levels. However these secretion levels are influenced by type, intensity and duration of exercise, together with the training status of the individual (Ehrnborg, Bengtsson, Rosn, 2000). And usually return to normal resting levels within approximately 10 - 30 minutes of ceasing exercise Pyka, Wiswell & Marcus (1992).

Inconsistent findings for resting Te and IGF-1
IGF-1 and physical activity are strong predictors of muscle mass and strength. There is evidence from studies suggests that testosterone increases both IGF-1 and its binding protein.

Insulin
Some of the most positive studies with regards to the endocrine system and resistance training have been with regards to insulin. Resistance training or exercise in general has been shown to have the acute effect of lowering secretion of insulin due to the bodies need for increased levels of glucose. Resistance training has been shown in numerous studies to decrease peripheral insulin resistance. This finding has huge implications for sufferers of conditions of syndrome X and type II diabetes. It has been found that older individuals who vigorously train on a regular basis exhibit a greater glucose tolerance and a lower insulin response to a glucose challenge than sedentary individuals of similar age and weight Ivy (1996).

Ivy (1996) showed that improvements in insulin resistance and glucose tolerance with exercise training are highly related to an increased skeletal muscle insulin action. The increased insulin action is associated with an increase in the insulin-regulatable glucose transporters, GLUT4, and enzymes that are responsible for the phosphorylation, storage and oxidation of glucose.

Syndrome X is the metabolic syndrome is characterized by a group of metabolic risk factors in one person. They include: Central obesity (excessive fat tissue in and around the abdomen) Atherogenic dyslipidemia (blood fat disorders -- mainly high triglycerides and low HDL cholesterol -- that foster plaque buildups in artery walls) Raised blood pressure (130/85 mmHg or higher) Insulin resistance or glucose intolerance (the body can't properly use insulin or blood sugar) Prothrombotic state (e.g., high fibrinogen or plasminogen activator inhibitor [-1] in the blood) Proinflammatory state (e.g., elevated high-sensitivity C-reactive protein in the blood) The underlying causes of this syndrome are overweight/obesity, physical inactivity and genetic factors. People with the metabolic syndrome are at increased risk of coronary heart disease, other diseases related to plaque build-ups in artery walls (e.g., stroke and peripheral vascular disease) and type II diabetes. Subjects undergoing resistance training improved many of the abnormalities associated with this metabolic syndrome: namely glucose intolerance, hyperinsulinemia, abdominal adiposity, hypertension, and hypertriglyceridemia, Castaneda (2002). These results show positive signs for the inclusion of resistance training in the treatment of many other medical conditions. These results with regard to syndrome X show that resistance training maybe a lot more beneficial than it has been regarded in the past.

Resistance trainings effects on the body's regulation of glucose seem to occur in more than one way. Skeletal muscle is the primary reservoir for glucose. As insulin is release it binds with the muscle receptor and glucose up taken by muscle, in turn this lowers blood glucose levels. Here, at the muscle is where the majority of the effect of resistance training is taking place. Firstly through resistance training an increased muscle mass is achievable, or alternatively with the elderly a decreases catabolic muscle break down. Due to this more or a constant capacity for storage of muscle glucose is available. Put simply, more muscle equals more potential for muscle glucose uptake.

However this effect would be pointless should the muscle be irresponsive to the body's insulin, which is the case with syndrome X and type 2 diabetes. Peripheral insulin resistance refers to the fact that insulin is continually released during times of elevated blood glucose, however due to underlying factors, it takes more and more of this insulin for the body to exhibit the same effect as in that of a health subject. Resistance training has been shown to increase the muscles responsiveness to insulin. A smaller does of insulin causes a greater response.

The effects of resistance training on lipid metabolism are unclear. However, there is some evidence that it increases HDL cholesterol levels in normal subjects and lowers triglyceride and LDL cholesterol levels in type 2 diabetic subjects Willey (2003).

Resistance training has also shown to have numerous other benefits such as promoting favourable energy balance and reduced visceral fat deposition through enhanced basal metabolism and activity levels while counteracting age and disease related muscle wasting. Resistance training improves insulin sensitivity and glycemic control, increases muscle mass, strength, and endurance and has positive effects on bone density, osteoarthritic symptoms, mobility impairment, self-efficacy, hypertension, and lipid profiles Willey (2003).

It is also considered a lot more feasibly for people with these specific symptoms due to other underlying factors. Any type of exercise improves insulin resistance and has beneficial effects in preventing and treating type 2 diabetes. However, aerobic exercise is hindered, and may in fact be dangerous, in many type 2 diabetic patients because of advancing age, obesity, and other co morbid conditions. Weight lifting or resistance training offers a safe and effective exercise alternative for these people, Willey (2003). Exercise increases systemic and retinal blood pressure, but there is no evidence that it accelerates diabetic retinopathy either acutely or with chronic training.

Overall resistance training seems to be the best form of exercise to aid in the treatment of both type II diabetes, and its closely related syndrome X. Resistance training and the prevention of muscle wastage in the elderly Sarcopeania, or age-related decline in muscle mass and function, is a recently recognized epidemic that constitutes a great threat to the functional independence and quality of life of older adults. This condition has been linked to functional disability, falls, decreased bone density, glucose intolerance, and decreased heat and cold tolerance in older adults. The occurrence of sarcopeania has been linked to decreased physical activity, malnutrition, increased cytokine activity, oxidative stress, and abnormalities in growth hormone and sex steroids. Options currently available for the management of sarcopeania are limited because the mechanisms involved in its development and progression are poorly understood. At the present time, resistance training is the best intervention to slow down or reverse the age-related decline in muscle mass or strength Kamel (2003). As has been shown earlier resistance training can positively alter some of these hormonal lackings and may be the most beneficial way in treating this disease.

There is evidence to support that age-related decline in growth hormone and serum insulin-like growth factor-1 may contribute to the development of sarcopeania. Studies of GH-deficient adults have revealed important similarities with the changes in body composition frequently observed in older adults. Individuals deficient in GH have more adipose tissue and less fat-free mass when compared with age-matched controls. In addition, GH-deficient adults have a more central distribution of adiposity as measured by circumferences. This decline in GH and IGF-1 levels with aging is attributed to changes in the effect of the hypothalamic factors, somatostatin (SRIF) and growth hormone-releasing hormone (GHRH), on the pituitary gland Evans (1996).

As well as a growth and other hormone levels declining myosin heavy-chain synthesis rate which is correlated with measures of muscle strength and circulating insulin-like growth factor I in men and women and free testosterone levels in men is shown to alter with age yet may change with resistance training. A decline in the rate of myosin heavy-chain synthesis implies decreased ability to remodel this important muscle contractile protein, which likely contributes to declining muscle mass and contractile function Evans (1996).

Through a review of the literature with regards to the topic Metabolic and endocrine adaptations to resistance training: implications for health, I it has been shown that resistance training can alter acutely many hormones in the body whether through a down regulation or an acute increase which is shortly returned to normal. The levels of IGF-1 can be raised even at rest through the application of a resistance training program. This is a positive result for those looking to increase muscle mass as IGF-1 is a very potent anabolic hormone.

Through this literature review it has been shown that health can be improved for those with both type II diabetes and the closely related syndrome X. Both of these conditions were shown to be positively affected by the implementation of a resistance training program. Sarcopenia another disease related to muscle wastage and androgen levels is also positively treated with resistance training. As has been stated resistance training can help elevate resting IGF-1 levels which can combine to become one of the growth stimulators to combat this catabolic condition. By increasing the muscle mass of these three populations we can positively affect their quality of life and decrease the incidence of the condition progressing further. With an increased muscle mass victims of sarcopenia can reduce the number of falls, through better limb control, which can lead to more serious complications. Through as greater muscle mass, diabetics can better self regulate there blood glucose levels and the secondary characteristics of syndrome X have been proven to be reduced. References available on request.

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