Nutrient overload is one of the primary, if not the primary drivers, of insulin resistance. Too much energy in the form of fructose can overload the hepatocytes of the liver with energy and this stimulates the de novo lipogenesis pathway. The resulting fatty acids may accumulate in skeletal muscle and liver tissue where they cause insulin resistance by interfering with the insulin signal cascade. In addition, too much energy in too short a period of time overloads the cells with energy, including glucose and fatty acids, and as a result oxidation of the energy proceeds at an accelerated rate leading to the generation of free radicals. As a compensatory mechanism, the insulin receptor is desensitised in order to prevent further nutrient uptake by the cells. Refined carbohydrates, including refined crystalline sugar and refined crystalline fructose contribute to this nutrient overload as the fibre in the original plant material, which normally controls the rate of digestion and absorption, is absent.
Low fibre diets high in refined carbohydrates are therefore one of the contributory factors in nutrient overload. This explains the association between the typical Western diet, which is high in refined carbohydrate, and insulin resistance and obesity. Fibre is protective of insulin resistance because it can limit the rate at which the starch in carbohydrate foods is digested and absorbed. In this way fibre slows the rate of absorption of the glucose that is digested from the starch, and this prevents nutrient overload. Soluble fibre appears to be particularly effective in this regard because when in the gut it absorbs water and forms a gel. This gel acts as a physical barrier between the starch and the digestive enzymes required for its digestion, slowing the rate of digestion considerably. In addition this gel creates a barrier on the walls of the gut to inhibit the passage of glucose from the gut to the blood. The fibre in plant foods may also slow the gastric emptying rate, further reducing the rate of starch digestion.
Low fibre diets such as the typical Western diet are associated with obesity and insulin resistance. Therefore adding more fibre to the diet should provide benefits to weight loss because dietary fibre has the potential to improve insulin sensitivity. Soluble fibre is present in high concentrations in oats and legumes, and this may explain the weight loss, blood glucose and insulin lowering, as well as the insulin sensitising effects of these foods. Fruits are high in fructose, which would suggest they may contribute to insulin resistance. However, fruits also contain the soluble fibre pectin, which may neutralise the damaging effects of fructose in fruits. Vegetables also contain soluble fibre in the form of pectin, as well as cellulose and other insoluble fibres. Whole plant foods such as fruits and vegetables and whole grains have been shown to produce weight loss effects when they replace refined carbohydrates in the diet. Supplemental fibre may not be as effective as that from whole plant foods.