The Danish government has recently implemented a tax on all foods that contain over 2.3 % saturated fat. According to government ministers the aim is to persuade against buying such foods in an effort to curb the growing amount of obesity seen in Western countries. The effectiveness of the intervention will be dependent greatly on the degree to which saturated fat is involved in the development of obesity. Increasing energy intake or decreasing energy expenditure results in a positive energy balance that undoubtedly plays some role in weight gain. Because fat contains nine calories per gram and is the most energy rich macronutrient (over twice the calories of protein and carbohydrate) it could be argued that reductions in the amount of fat might be a successful strategy in creating a negative energy balance that might result in weight loss.
Consuming too much saturated fat may therefore produce a positive energy balance that causes weight gain. However, over consumption of any macronutrient would have the same effect, although protein over consumption tends to produce the least weight gain because deamination of amino acids is an energy intensive process. Establishing that saturated fat has a higher involvement in obesity than that of unsaturated fat would not be possible from the scientific literature. Research has also shown that factors other than total fat intake are important in weight gain associated with Western diets. For example, the ratio of n-6 to n-3 polyunsaturated fatty acids (PUFA) is an important determinant of eicosanoid production and inflammatory pathways, and obesity is increasingly being seen as a disease of chronic inflammation. Addressing this imbalance would make more sense in terms of addressing ill health than reducing saturated fat intake.
Recent research investigating the development of obesity has tended to move away from the oversimplified positive energy balance theory, and towards a theory that centres on the development of insulin resistance and the metabolic syndrome. Metabolic syndrome is the name given to a metabolic abnormality that result in the development of insulin resistance, and ultimately can lead to diabetes and cardiovascular disease. Over consumption of fructose is implicated in metabolic syndrome, because only the liver is able to metabolise fructose, which by-passes the regulating enzyme for glycolysis resulting in uncontrolled conversion to acetyl-CoA. This then increases the formation of free fatty acids (de novo lipogenesis), which are packaged in very low density lipoproteins (VLDL) and exported to skeletal muscle where they lead to a desensitising of the insulin receptor. Fructose may also decrease activity of lipoprotein lipase further raising levels of circulating VLDL.
Mineral deficiencies may also play a role in the development of obesity. In particular, chromium deficiency has been linked with the development of insulin resistance and weight gain. Chromium is trace mineral and an essential component of the insulin receptor in humans. The insulin receptor is activated by insulin in response to a rise in blood glucose levels. The insulin receptor then stimulates glucose transporters to the surface of the cell to allow transport of the glucose molecules from the blood to the interior of the cell. However, this process is dependent on chromium, which forms a complex with the insulin receptor that is necessary for its function. As the blood sugar levels drop the chromium and the insulin receptor unbind and the chromium is excreted in the urine. If the chromium is not replaced, the insulin receptor desensitised to the action of insulin and insulin resistance develops.
The cause of weight gain is not fully understood, but the evidence that saturated fat intake is associated with the development of obesity is not established. Epidemiological evidence supports this viewpoint, as do the numerous intervention studies that have attempted to correlate dietary macronutrient intake with weight gain. While the role for dietary saturated fat in the development of obesity is inconclusive, the role of fructose over consumption in the development of metabolic syndrome through increases in circulating VLDL-triglycerides is more fully established. Fructose over consumption increases de novo lipogenesis and increases circulating levels of VLDL-triglycerides that are thought to lead to the development of skeletal muscle insulin resistance. Deficiencies of chromium in Western populations are well documents and may contribute to insulin insensitivity which leads to weight gain and obesity.
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