Refine carbohydrates are the staple starch source in the Western diet. While traditional diets contain carbohydrates sources as whole grains, Westerners have developed a taste for refined carbohydrates. This is problematic because refined carbohydrates are strongly implicated in the aetiology of Western lifestyle diseases. In particular refined carbohydrates can cause hepatic and peripheral insulin resistance which is the likely cause of metabolic syndrome (syndrome X). Metabolic syndrome is a group of related metabolic dysfunctions characterized by abdominal fat accumulation, lipoprotein irregularities and aberrations to normal glycaemic control. The development of metabolic syndrome increases the risk of developing non-alcoholic fatty liver disease (NAFLD), type 2 diabetes and cardiovascular disease. The detrimental effects of refined carbohydrates can be seen in the health benefits conferred from their absence. This is evidenced by the absence of Western lifestyle diseases in those who consume traditional whole grain diets.
Refined carbohydrates are produced by stripping the bran and germ layers from grains in order to leave the starchy endosperm. The bran and germ layer contain the majority of the fibre and micronutrients, whereas the endosperm is almost completely starch. This leaves a more tasty product for the consumer, which is comprised almost exclusively of carbohydrate. Fibre is necessary in the digestive process because it slows the digestion rate of starch which allows absorption of the subsequent glucose at a rate slow enough for the liver to process it. Removing the fibre increases the rate of absorption of the glucose and this causes excessive insulin release which can decrease insulin sensitivity. The benefits of fibre are so great to human health some argue it should really be classified as an essential nutrient. The lack of micronutrients is also problematic in refined carbohydrates. Chromium for example is essential to the function of the insulin receptor and refined carbohydrates do not supply adequate chromium for this purpose.
However, refined carbohydrates do not just include the complex carbohydrates that we call starches. More simple carbohydrates, the sugars, can also be refined from their original plant material. The most common refined sugar in the human diet is sucrose and this is often referred to as table sugar. Sucrose is a refined carbohydrate processed from sugar beet or sugar cane, where it is extracted from its original plant material and crystallised to form a fibre and micronutrient free product. Fructose is another refined carbohydrate that is extracted from various fruits. Both sucrose and fructose are favoured by food manufacturers because they are appealing to humans due to their sweet tastes. As a result of this they are frequently added to products to make them more desirable to consumers. Increasingly, artificial sugars such as high fructose corn syrup (HFCS) are also being added to foods and drinks for the same reason. However, unlike refined starches, refined sugars also have other effects which may be even more detrimental to the health.
The metabolism of fructose is surprisingly similar to that of alcohol. It is processed almost exclusively by the liver. Small quantities of fructose are no problem for the liver just as small quantities of alcohol are also metabolised without problem. Fructose when consumed in such low amounts can be used to increase hepatic glycogen stored and later be released to circulation as glucose. However, high intakes of fructose overload the glycogen synthesis pathway and lead to liver overload syndrome. Under such circumstances the liver diverts the incoming fructose from glycogen synthesis to fatty acid synthesis. This de novo lipogenesis pathway (literally the creation of new fat) produces fatty acids and triglycerides that are stored in hepatic tissues and skeletal muscle where they interfere with the insulin receptor. Such fructose induced insulin resistance can develop surprisingly quickly, with reductions in insulin sensitivity seen in just a few weeks of starting a high fructose intake. Sucrose and HFCS both contain fructose and are equally bad.
That refined carbohydrates are implicated in blood sugar disorders is evidenced in the nutritional literature. Numerous clinical trials have reported that ingestion of whole grain foods reverses the insulin resistance in those consuming high intakes of refined carbohydrates. This data is supported by results from epidemiological studies showing that consumption of whole grains is associated with a reduction in the risk of blood sugar disorders. For example, in a recent prospective study1, researchers investigated whether higher intakes of whole grains protect from a deterioration of glucose intolerance to pre-diabetes or type 2 diabetes. The results showed that consuming ~60 grams of whole grains per day was associated with a 34 % reduced risk of deteriorating glucose intolerance when compared to a ~30 gram per day intake. The intake of whole grains in the subjects was also inversely associated with their degree of insulin sensitivity. Therefore higher intakes of refined carbohydrates may increase the risk of disease significantly.
Studies looking at the impact of fructose on blood sugar disorders are also numerous in the nutritional literature. Much of the work in their field has been performed on rodents, and results from these studies show deleterious effects of fructose intakes in just a few weeks (here and here). Human studies support the data from animal experiments and show deleterious effects of fructose on metabolic regulation through reductions in insulin sensitivity. In this regard soft drink consumption has been extensively researched. In a recent study published in the American Journal of Clinical Nutrition2, researchers concluded that soft drink consumption was associated with a higher risk of developing type 2 diabetes. However, coffee consumption by comparison was associated with a reduced risk. That habitually high fructose intakes are associated with blood sugar disorders is also evidence through research showing a link between high uric acid levels and type 2 diabetes3. Fructose raises levels of uric acid because it causes ATP depletion in the liver.
Evidence therefore supports the contention that refined carbohydrates are detrimental to the health. Traditional diets do not contain refined carbohydrates and the populations who consume such diets are almost completely free from Western lifestyle diseases (here). The benefits of fibre in the diet have been extensively reported in the nutritional literature, and these benefits explain the health benefits of consuming legumes, whole grains and nuts, all of which are good sources of fibre. It should not be forgotten that refined carbohydrates are also a marker for a low quality diet. Those who consume refined carbohydrates are also likely consuming other foods that have detrimental health effects. Increasing the intake of plant foods that contain their natural fibre such as nuts, legumes and whole grains should be considered by anyone wishing to improve their insulin sensitivity and reduce their risk of developing a blood sugar disorder. In this regard adopting a traditional diet, absent from refined carbohydrates, may be a particularly wise decision.
