Obesity is associated with an increased risk of disease. In particular, obesity increases the risk of developing type 2 diabetes and cardiovascular disease. Obesity is characterised by excessive adipose tissue accumulation, and it is the metabolic changes that occur with this accumulation that are thought to cause disease. However, not all adipose tissue is the same, and broadly can be divided into subcutaneous peripheral fat and abdominal visceral fat. This distinction is important because only the latter is associated with an increased risk of disease. Triglycerides are stored in different locations either in the abdominal tissue or subcutaneous tissue, because these two loci have different functions. Fat accumulation around the organs in the abdomen is readily used as an energy source, whereas subcutaneous fat is a more permanent store of energy. As a result, in the short-term, dieting tends to reduce abdominal fat, but does little to remove subcutaneous fat.
In a review of the literature, researchers1 analysed the factors that affect the levels of subcutaneous and abdominal fat tissue before and after weight loss in subjects who were overweight or obese at baseline. In particular, the researchers were interested in the percentage change in abdominal and subcutaneous fat caused by weight loss. The only variable that influenced the ratio of abdominal to subcutaneous fat was the amount of weight loss. Small amounts of weight loss caused preferential depletion of abdominal fat. However, much larger weight loss was required to affect the subcutaneous fat stores. A very low calorie diet for less than 4 weeks was particularly effective at removing abdominal fat, but the effect was lost at 12 to 14 weeks. Therefore, the health benefits of modest weight loss are likely due to the removal of tissue from the abdominal fat stores, which appear to be readily used as energy in times of low calorie intakes.
However, the long-term success of low calorie diets is not good. The main problem with the forced calorie restriction theory of weight loss is that negative feedback causes weight loss to cease after a few weeks of success. This is largely because the hypothalamus is leptin resistant in obese individuals, and as such the signal alerting the brain to excessive body fat stores is disrupted. Fat from abdominal tissue is readily used as energy in the short-term, but once reduced, the hypothalamus considered the body in a state of starvation and adjusts physical activity and resting metabolic rate downwards accordingly. In addition, energy intake is stimulated through hunger and the individual finds it impossible not to eat increased food as compensation. The result of this is weight re-gain and re-distribution of triglycerides to the abdominal store of fat. This explains the failure of the low calorie diet to cause weight loss after 12 to 14 weeks.
RdB
