ructose is increasingly being seen as a major contributory cause of the insulin resistance that characterises the metabolic syndrome. Fructose is present naturally in nature in foods such as fruits, but evidence suggests that consumption of fruit does not lead to deleterious metabolic problems. This is likely due to the low concentration of fructose in fruit. In fact evidence suggests that a high intakes of fructose, such as might occur from excessive soft drink consumption, is required for fructose to become problematic. This is because fructose can only be metabolised in hepatocytes, and high intakes overload the ability of the liver to process fructose to glycogen. Instead, high intakes of fructose increase flux through the de novo lipogenesis pathway, increasing production of fatty acids and leading to lipid accumulation in the liver and skeletal muscles. This accumulation of lipids is thought to be the primary cause of insulin resistance.
The ability of fructose to cause metabolic abnormalities in mammals is well reported in the nutritional literature. Studies investigating the digestion and absorption of fructose have also been performed. For example, one such study1 investigated the effects of pure crystalline fructose to 103 healthy subjects. Administration of 50 g of fructose in one dose with water caused an increase in hydrogen breath concentration in 58 % of the subjects, suggesting that they were not able to absorb the fructose from the small intestine, and that bacterial fermentation in the colon had occurred. Decreasing the dose administered to 25 g fructose resulted in only 19 % of the non-absorbers continuing to produce increased hydrogen in breath. Addition of glucose to the fructose solution significantly reduced the hydrogen production in the non-absorbers, suggesting that glucose facilitates the absorption of fructose.
These results suggest that high intakes of fructose cause saturation of absorption rates in some individuals and the excess fructose is subsequently fermented in the colon. In the subjects in this study, this resulted in side effects such as abdominal cramps, diarrhoea and flatulence, as might be expected. Fructose is absorbed via a transport system distinct from that of glucose and is thought to operate less efficiently. These results also showed that the absorption of fructose is dose dependent, which may have implications when considering the digestion of fructose from fruit and from soft drinks. However, in nature and in soft drinks, fructose is usually found in association with glucose, as the molecule sucrose. In this study, addition of glucose facilitated the absorption of the fructose, which may suggest that sucrose is more able to overload the liver with fructose when compared to pure fructose.
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