Muscle-building research is useful for athletes and bodybuilders as well as for elderly patients and those with muscle-wasting conditions. Research shows that a healthy diet, resistance exercise, and nutritional supplements are beneficial for improving body composition and the body of research on nutritional supplements for athletes is growing. However, supplementation will not improve muscle size and strength without resistance training, such as weight lifting or weight-bearing aerobics. The sports-nutrition and weight-loss industries in the United States comprise a growing market, and there were reports of $14.3 billion in sales of such products in 2004.1 In addition, according to the Nutrition Business Journal, this product market increased sales by 14% to reach $15.6 billion in 2005 and is estimated to grow at a rate of approximately 5%-7% per year for the next eight years.2
BODY COMPOSITION AND ITS EFFECTS ON MUSCLES Insulin Resistance
Insulin resistance is a state in which the pancreas secretes increasingly higher levels of insulin to facilitate glucose uptake into skeletal, hepatic, and adipose tissue cells. Obesity, the most common cause of insulin resistance, is associated with a decreased number of receptors and with postreceptor failure to activate the tyrosine kinase. The beta subunitofthe insulin receptor is a tyrosine kinase, which is activated when insulin binds to the alpha subunit; the kinase activity autophosphorylates and mediates multiple actions of insulin. Specific glucose receptors in muscle and adipose tissue are poorly responsive to high levels of insulin in the blood. Moderate weight loss has been shown to reduce insulin resistance. Hyperinsulinemia increases intracellular lipid accumulation, which, in turn, may increase insulin resistance. Insulin-resistant skeletal muscle has lower oxidative capacity and has decreased fatty-acid oxidation favoring lipid accumulation.3 In addition, high lipid levels in skeletal muscle also result in a lower oxidative capacity.4
Hormones have also been shown to have a great impact on insulin resistance. Stress and its resulting increase in epinephrine and cortisol affects insulin resistance. These adrenal hormones have been shown to increase glycogen breakdown from the liver and affect glucose utilization unfavorably.5,6 Estrogen supplementation may also increase insulin resistance, particularly in postmenopausal women.7 Increased testosterone in females and increased estrogens in males decrease peripheral glucose utilization.8 Studies have shown that dehy-droepiandrosterone (DHEA) supplementation increases peripheral glucose utilization.9 Insulin-like growth factor-1 (IGF-1) is a polypeptide stimulated by growth hormone, which affects growth and glucose metabolism. Studies have shown that IGF-1 increases peripheral glucose utilization as well as decreases protein catabolism.10
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