Obesity is a complex issue because the reasons for weight gain and weight loss are not fully understood. The mainstream concept of weight gain is that energy balance becomes positive on account of too much energy consumed in the form of food, or too little energy expended in the form of exercise. Often laziness or greed are inferred as the prime causes of weight gain. However, this paradigm of weight gain is vastly oversimplified in many cases and a number of additional factors such as mineral deficiencies, stress, genetic traits and infant adiposity are known to be involved in the development of obesity. While exercise has been shown to help shift energy balance to negative creating weight loss in the short term, the complex interaction between appetite, satiety and exercise makes predicting successful long term weight loss difficult.
It is becoming clear that metabolic abnormalities are present in many of the individuals that gain excessive weight. Research is starting to accumulate that food choices play an important role in this process. Fructose has been identified as a contributing factor to the development of insulin resistance, which can lead to metabolic syndrome, type 2 diabetes and cardiovascular disease. Fructose causes insulin resistance because it can only be metabolised by the liver, and the liver can only convert a small amount of fructose into glycogen. Once the glycogen stores of the liver are full, fructose is diverted to unrestrained de novo lipogenesis which results in the production of saturated fatty acids. These are packaged in very low density lipoproteins (VLDL) and it is thought that these contribute to insensitivity of muscle cells to insulin.
Interestingly exercise may reduce obesity, not only because it can shift the energy balance of the body, but also because it increases oxidation of saturated fatty acids as a source of energy. This would decrease circulating levels of VLDL and thus decrease one of the causes of insulin resistance. Research has shown that exercise increases the sensitivity of muscle to insulin and that uptake of glucose into muscle is greatest immediately following exercise and highest in the muscle groups worked during exercise. It would make sense that training the largest muscles in the body should lead to larger improvements in insulin sensitivity. Because liver glycogen is depleted during exercise, ingested fructose is more likely to enter glycogenolysis than de novo lipogenesis, further reducing circulating fatty acids and removing a cause of insulin resistance.
Weight loss is also a misleading term, because the goal of weight loss is really body fat loss. However, weight loss can also be accounted for by losses in water through dehydration, loss of bone though demineralisation, loss of body protein from muscle, or loss of glycogen from muscle and liver. Generally initial weight loss seen with exercise or calorie restrictive diets can often be accounted for by loss of water and glycogen. Some weight loss supplements that contain diuretics such as dandelion remove water and produce a misleading weight loss effect. Water and glycogen loss can be compensated for by addition of adequate carbohydrates following exercise and increasing intake of water. Weekly fluctuations in weight are most likely caused by changes in the glycogen and water content of the skeletal muscles and liver following exercise.
Loss of protein in the form of skeletal muscle is not desirable because as muscle is lost the metabolic rate is decreases, making further body fat loss more difficult. Resistance training followed by aerobic or anaerobic exercise has been shown to reduce the loss of protein associated with calorie restricted diets. In addition, supplementation of the branched chain amino acids can prevent the loss of protein associated with exercise and increase protein synthesis in skeletal muscle. Circulating levels of leucine rise during exercise as the muscle derives more of its energy needs by oxidation of leucine. At the same time pyruvate oxidation is inhibited and pyruvate is converted to lactate and exported from the muscle to the liver, where it can be synthesised into glucose in order to maintain blood sugar levels. Whey protein is a good source of branched chain amino acids.
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