Fruit contains high amounts of sugars, particularly fructose and sucrose. Sucrose is a disaccharide made up of a molecule of glucose joined through a glycosidic bond to a molecule of fructose. Therefore fructose is the primary sugar in fruit, its name actually meaning fruit sugar. Fructose has been shown to cause insulin resistance in mammals including mice, rats, monkeys and humans in a short amount of time, perhaps a few weeks, when consumed in high concentrations. The amount of fructose used in studies to induce insulin resistance in animals has been shown to be equivalent to the high intakes of fructose associated with soft drinks. Soft drinks are rich in fructose, either in the form of sugar or high fructose corn syrup, in Europe and the United States, respectively. Soft drinks are associated with obesity and have been shown to cause insulin resistance in humans. However, fruit does not cause insulin resistance, despite the fact that it contains high amounts of fructose.
The reason that fruit does not cause insulin resistance, as mentioned previously, is that it contains fibre. Fibre slows the digestion and absorption rate of any concomitantly eaten carbohydrates and this reduces the nutrient overload experienced by the cells. Although fruit does not cause insulin resistance, fruit juice does, and this is a reflection of the fact that the fibre has been removed. Studies show that fruit juice is no less detrimental to the health than soft drinks and high intakes of fruit juice may induce insulin resistance and lead to obesity. Many people consume fruit juice and give it to their children under the impression that it is a healthy food, but increasingly it is being shown that the decision to consume fruit juice is not a sound nutritional one. Consuming fruit juice with high fibre foods may negate some of the detrimental effects as the fibre in the gut can limit the absorption of the fructose and inhibit nutrient overload in the cells. However, the nutritional literature suggests that consumption of fruit juice should be severely limited.