Evidence is increasingly linking fructose consumption to the development of Western lifestyle diseases. This is because fructose in high concentrations is a metabolic poison that can overload the liver and increase flux through the de novo lipogenesis pathway. This results in the production of lipids which accumulate in tissues and interfere with insulin signalling. The long-term consequences of this is a decrease in insulin sensitivity and the development of the metabolic syndrome, a disorder characterised by severe metabolic dysfunction. A high fibre intake is protective of metabolic syndrome, although the mechanism of protection is not known. Therefore the fibre present in natural foods containing fructose may negate the deleterious effects of the fructose. This explains why fruit is not detrimental to the health, yet fruit juice is associated with an increased risk of metabolic syndrome (here). The effects of fibre on the metabolic changes associated with fructose consumption are therefore of interest to nutritional scientists.
For example, in a study published in the American Journal of Clinical Nutrition1, researchers assessed the effects of sugar on the glucose tolerance in healthy subjects. However, unlike many other studies, the authors included the sugar as part of a high fibre diet. Subjects consumed diets that contained 50% of calories from carbohydrate, 35% fat and 15% protein for 13 weeks. During this time carbohydrate sources were only complex in nature, including oatmeal, bread and beans. Following week 13, subjects were switched to an identical diet, except that 35% of the carbohydrate content were sugars, of which 17.5 % were fructose. In men, a glucose tolerance test produced a significantly higher glucose response following the high sugar diet, when compared to similar tests during week 7 and 13 of the complex carbohydrate diet. In addition, the insulin binding was significantly lower at baseline and after the high sugar diet diet, compared to during the complex carbohydrate diet, for both men and women.
These results suggest that high sugar diets containing fructose detrimentally affects glucose tolerance and insulin binding after a single week of intake. This is supported by animal experiments showing rapid deteriorations of blood sugar control with fructose intakes (here, here, here and here). Subjects consumed 40 grams of fibre, but this was not effective at preventing detrimental glycaemic effects. The lack of a protective effect from fibre in this study may relate to the way the sugar was administered. Most of the sugar calories were provided by a single drink containing 50 grams of fructose and 50 g glucose per 2700 kcal of energy intake, whereas fibre was ingested throughout the day. This suggests that fibre may need to be present concomitantly with the sugar in order to have beneficial effects. Interestingly, the subjects had improved insulin binding following the complex carbohydrate diet compared with their baseline reading, suggesting that the subjects may already have developed poor insulin sensitivity from their diets prior to the start of the study.
