Obesity is claimed by many to be caused by greed on the part of the individual, such that a positive energy balance results in incremental weight gain. Evidence for this is claimed to come from overfeeding studies whereby normal weight subjects are deliberately over fed to cause weight gain. For example, one study1 fed adult female volunteers a diet that was considerably higher in energy than their daily requirement. Total excess energy fed to the subjects ranged from 18900 to 35400 kcal over the course of a 3 week period. The subjects gained an average of 4.3 kg, of which 46 % was lean body mass and 54 % which was fat mass. Hormonal changes included increases in testosterone, insulin-like growth factor and insulin, which may have been the cause of the increase in lean body mass. In addition, there were also increases in urinary 17-ketosteroids, 17-hydroxysteroids, adrenaline and creatinine, suggesting an anabolic environment within the subjects.
Because obese individuals have increased lean body mass and adipose tissue compared to normal weight individuals, it could be suggested that such a study confirms that obesity is caused by overeating. However, while it is true that overeating is associated with obesity, the cause and effect are not as might be assumed. Obesity is a metabolic disorder that causes leptin resistance, and this in turn disrupts the signal to the hypothalamus regarding the amount of energy stored in adipose tissue. In fact, the leptin resistance causes the signal to the hypothalamus to indicate low bodyfat stores, and as a result appetite is stimulated in order to refill energy reserves. The overeating is therefore caused by obesity, or more correctly the overeating is caused by a metabolic disorder that is associated with excess adipose tissue. Therefore forced calorie restriction does not cause weight loss because it is not the cause of obesity.
Another interesting thing regarding overfeeding studies is that they rely on calorie dense foods in order to allow the subjects to consume enough excess energy. Such foods increase the sugar intakes of the subjects considerably, thus providing an increase in fructose intake (sucrose consists of one fructose moiety and one glucose moiety). Fructose is thought to be the cause of insulin resistance, and insulin resistance is thought to be the cause of leptin resistance. Therefore overfeeding studies are in essence, often fructose feeding studies. It is very doubtful that overeating of bland high fibre and high protein foods in the absence of carbohydrate would cause any weight gain because the subjects appetite would simply not allow enough energy intake to cause increases in adiposity. Without access to sugar, it is very difficult to overeat, as illustrated by the weight loss seen in low or no carbohydrate diets without the need for calorie restriction.
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