here is still a widely held belief that obesity is caused by a simple positive energy balance that results from too much food and too little exercise. Proponents of the ‘eat-too-much, do-too-little’ theory of obesity therefore suggest that a cure for obesity is exercise. The assumption goes that physical activity will increase energy expenditure and this will result in a negative energy balance that will cause weight loss. Many of the people who make such a claim are normal weight and exercise themselves, suggesting that they have obtained their seemingly logical conclusion from first hand experience. At first glance the theory seems plausible, and is in fact it is so widely held to be the truth that many people think it absurd to question it. However, scientific evidence over 5 decades shows that subjecting obese people to exercise does not cause weight loss and in fact may even be detrimental to their health.
The inability of most people to understand that exercise does not cause weigh loss in obese individuals derives from a misunderstanding of the cause of obesity. Obesity is caused by poor quality diet, or overconsumption of the type of foods of which the food industry love to sell in large quantities. Sugar and other fructose containing molecules are likely the primary driver of insulin resistance, a condition that is thought to be required for the development of obesity. This is because fructose can only be metabolised by the liver, and high intakes cause increased de novo lipogenesis, or the endogenous production of fatty acids. These fatty acids then become deposited in liver and skeletal muscle where they interfere with the action of the insulin receptor cascade. Intense exercise can prevent the detrimental effect of sugar to some extent because physical activity depletes the liver of glycogen. Ingestion of sugar following exercise results in restoration of liver glycogen, and not increased de novo lipogenesis.
However, exercise being able to protect from obesity is not the same as being able to reverse the disease once fully developed. Obesity is a metabolic disorder characterised by insulin and leptin resistance, and the latter results in the hypothalamus receiving incorrect guidance as to the quantity of fat in body stores. In fact as leptin resistance develops, the hypothalamus receives a weaker signal regarding adiposity which is interpreted as there being very low levels of stored energy. As a result the hypothalamus decreases the thermic effect of food (TEF), decreases resting metabolic rate (RMR) and reduces the energy available for physical activity. This leads to the observation that obese individuals exercise less than lean counterparts. This is true, but the cause and effect have been misinterpreted with the assumption that it is the lack of exercise that causes the obesity. In fact the opposite is true.
The hypothalamus is able to regulate energy expenditure in this way such that any increase in physical activity results in even more extreme efficient use of energy in response. Some evidence in the nutritional literature suggests that in obese individuals, a situation of perceived starvation results from the metabolic changes caused by insulin resistance. The symptoms shown by obese individuals subjected to a negative energy balance through vigorous forced exercise have been shown to resemble those of starvation and can cause physical and metal deterioration of health. Long-term studies show that physical activity does not cause successful weight loss in obese individuals, probably because of an abnormal response to the stress of exercise. Forcing the issue with chaperoned physical activity results in a fall in RMR over time, something that has been reported in many studies investigating the effect of exercise on obesity.
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