Glucose and fructose are two monosaccharide sugars that are present in a number of foods commonly consumed by humans. Both glucose and fructose are present naturally in high amounts in various fruits, and also added artificially to manufactured foods and drinks as sweeteners. Fructose and glucose are also present in varying quantities in the different forms of high fructose corn syrup that is added to soft drinks, cakes and sweets. Evidence in the literature is stating to suggest that the differing metabolism of glucose and fructose can have a profound effect on the insulin sensitivity, adipose distribution and lipid metabolism in humans. In particular, studies have suggested that fructose is uniquely metabolised by the liver, which causes metabolic changes not associated with ingestion of glucose. This evidence adds weight to the argument that energy balance is not the only consideration in weight loss.
For example, researchers fed overweight to obese human subjects a high fructose or high glucose diet for 8 weeks, consumed as sweetened drinks that supplied 25 % of energy needs on top of their normal ad libitum diets1. The subjects in both groups were in positive energy balance throughout the study and gained an average of ~1.4 kg. However, the subjects consuming the fructose drinks exhibited increased fasting glucose and insulin concentrations and showed signs of the development of insulin resistance. It is thought that fructose consumption results in an increased hepatic triglyceride accumulation, because the triglyceride production rate (de novo lipogenesis from fructose) occurs at a faster rate that the free fatty acid oxidation rate. This results in an increase in very low density lipoprotein (VLDL) particles which are secreted to the circulation and cause resistance of tissues to insulin.
In addition to increased insulin resistance, those consuming the fructose drinks demonstrated a different pattern of fat accumulation to those consuming the glucose drinks. Computerised tomography demonstrated that those on the high fructose drinks gained the majority of their ~1.4 kg weight gain in the intra-abdominal (visceral) fat area. In contrast those consuming the high glucose drink gained the majority of their ~1.4 kg in the extra-abdominal (subcutaneous) area. This suggests that fructose consumption may preferentially distribute fat to the visceral adipose tissue. It is known that visceral fat accumulation is a risk factor for cardiovascular disease and so fructose consumption may be associated with adverse cardiovascular effects. The mechanism of this observation is not understood, but may be related to the production of VLDL production with fructose or the effects of fructose on insulin sensitivity.
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