Fructose is a naturally occurring sugar found particularly in fruit and honey. A number of well designed studies have shown that feeding high amounts of refined crystalline fructose to animals induces insulin resistance within a few weeks. In particular, studies involving rats have shown that intakes of fructose equivalent to the upper intakes of humans can induce insulin resistance and an associated weight gain in just two weeks. Studies in humans have been slower to emerge, and initially the rat studies were dismissed because of the high amounts of fructose involved. However, subsequently, human studies involving fructose have shown that lower intakes over longer time periods are also effective at inducing insulin resistance in humans. This ability of fructose to induce insulin resistance explains the obesogenic nature of sugar. Sugar is a molecule that contain both glucose and fructose, and so its regular consumption supplies high amounts of fructose, and this can induce insulin resistance and subsequent weight gain.
Fructose is known to cause insulin resistance in animals. However, mounting evidence also suggests that fructose induced insulin resistance, and weight gain, in healthy human subjects. This explains the obesogenic nature of sugar (which is composed of a molecule of fructose bound to a molecule of glucose).
Researchers have assessed the effects of fructose on humans by analysing previous research and reporting on its findings as a whole. In one such meta-analysis, 29 studies were included in an appraisal of the effects of fructose on healthy normal-weight humans. The authors reported that the data from the studies showed that exchanging other carbohydrates for fructose, in an energy matched manner, promoted the development of hepatic insulin resistance. If the intake of fructose was increased such that energy intake increased ~ 25 % above that required, hepatic insulin resistance and raised levels of plasma insulin were observed. That these effects were seen in small samples sizes and over a duration of less than 60 days, highlights the efficiency at which fructose may induce insulin resistance and cause detrimental changes to the insulin system. Avoiding sugar and products containing sugar should therefore be a dietary priority if good health and correct body weight are to be maintained.
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