Macronutrients in Health and Disease: Refined vs. Unrefined Carbohydrates
Refining is a process by which the fibrous outer bran coating of grains is removed. By this process, brown rice is converted to white rice, for example, or whole wheat is converted to white flour, greatly reducing fiber content. Note that a food can be rich in complex carbohydrate but also be refined. White rice and white bread, for example, are refined grain products, but retain their complex carbohydrate.
Glycemic Index. The glycemic index was first presented in 1981 as a means of quantifying the effects of carbohydrate–rich foods on blood glucose concentrations.1 The glycemic index of a food is determined by feeding a portion containing 50 g of carbohydrate to 10 healthy people after an overnight fast. Blood glucose is tested at 15– to 30–minute intervals over the next 2 hours, and the results are compared to feeding the same amount of glucose. A glycemic index below 100 means the food has less effect on blood sugar, compared with glucose. A higher number means the test food has a greater effect.1
Distinctions between various kinds of carbohydrate are clinically important. Diets high in sugars and refined carbohydrate may cause elevations of plasma triglyceride concentrations. However, diets high in low–glycemic index foods and fiber tend to have the opposite effect, leading to significant reductions in triglycerides.2
In studies of individuals with diabetes, a meta–analysis of 14 prior studies with a total of 356 participants showed that diets emphasizing low–glycemic–index foods reduce hemoglobin A1c (the principal clinical measure of long–term blood glucose control) by about 0.3 to 0.4 percentage points. In some studies, the difference was as much as 0.6 points.3 Studies showed a similar benefit for both type 1 and type 2 diabetes.
Diets that are high in carbohydrate and fiber and low in fat and cholesterol have clinical utility in prevention and management of several diseases, including obesity and weight–related conditions such as diabetes and hypertension. Dietary fiber promotes satiety, and its intake is inversely associated with body weight and body fat.4
Although carbohydrate intake is the main determinant of plasma glucose, available evidence indicates that people who consume approximately 3 servings per day of whole grain foods have a 20% to 30% lower risk of developing type II diabetes than individuals consuming <3 servings per week.5 Low–fat, high–carbohydrate, high–fiber diets also significantly reduce the need for insulin and oral hypoglycemic agents in patients with type II diabetes.6 Such diets are also associated with significant improvements in blood lipid concentrations, blood pressure, and indices of atherosclerosis (see Hyperlipidemia) and appear to be useful for preventing and treating some intestinal disorders (see Constipation, Inflammatory Bowel Disease, Peptic Ulcer Disease, and Gastroesophageal Reflux Disease).
Clinicians should be aware that patients sometimes mistakenly blame carbohydrate for weight or health problems, based on the tenets of popular low–carbohydrate weight–loss diets. Patients may need to be reminded that carbohydrate is essential to human health. Complex carbohydrates, in unprocessed or minimally processed forms, are staple foods in the diets of countries where chronic diseases are rarely seen. In areas of changing dietary patterns, where carbohydrate–rich foods become displaced by fat– and protein–rich foods, several chronic diseases become much more common. In Japan, for example, the Westernization of the diet occurring in the latter half of the twentieth century meant a sharp decrease in rice consumption and an increase in meat and total fat intake, with corresponding increases in obesity, diabetes, cardiovascular disease, and other health problems.7 In studies of Japanese adults over the age of 40, diabetes prevalence was between 1% and 5% prior to 1980. By 1990, that number had gone up to 11% to 12%.7
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