Carbohydrates constitute a big part of the American diet, representing more than half of the calorie intake. Carbohydrates are found in grains, flours, legumes, starchy vegetables, fruits, some dairy products, juices, soft drinks, sugar, sweets and desserts. Some carbs are definitely healthier than others and the glycemic index is one of the tools that can be used to measure their quality.
Glycemic Index
The glycemic index is a concept that was first proposed by Dr. David Jenkins at the University of Toronto in 1981 to help people with diabetes better estimate the impact of different types of carbohydrates on their blood sugars. It is measured by giving a serving of food containing the equivalent of 50 g of available carbohydrates to a minimum of 10 participants. Their blood sugar levels are tracked during the 2 hours following the consumption of the food and compared against the glycemic response obtained after eating 50 g of pure glucose. A GI of 55 or less is low, between 56 and 69 is medium, and 70 and greater is high.
Fiber Content
The fiber content is another way to evaluate to what extent a carbohydrate has been refined. Processed foods are usually fiber-less, while unprocessed foods tend to have a higher fiber content. For example, 1 cup of cooked instant white rice contains only 1 g of fiber, while a cup of cooked brown rice has 3.5 g. Another good example is barley. A cup of cooked hulled barley provides 27.2 g of fiber, while pearled barley, which has its outer layer removed, provides only 6 g of fiber. Fiber is one of the constituents of carbohydrates, along with starches and sugars, but has the particularity of not impacting blood sugar levels. Moreover, a higher fiber intake is associated with a healthy body weight, improved gastrointestinal health as well as a reduced risk of developing heart disease and type 2 diabetes.
Glycemic Load
The glycemic load, or GL, constitutes a more complete concept, which was proposed in 1997 by Harvard University, and accounts not only for the quality of the carbohydrates, but also the quantity consumed. The GL is measured by multiplying the GI of a food by the amount of available carbohydrate a serving of food contains and dividing the result by 100. Available carbohydrate corresponds to the portion of carbohydrates able to influence blood sugar levels and can be calculated by subtracting dietary fiber from total carbohydrates. For example, a large ripe banana contains 31.1 g of carbohydrates and 3.5 g of fiber, which corresponds to 27.6 g of available carbohydrates. Because bananas have a GI of 51, a large banana has a GL of 14.1. A GL below 10 is considered low, between 11 and 19 is medium and above 20 is high.
Carbohydrates and Health
The 2010 "Dietary Guidelines for Americans" recommend that you eat between 45 percent and 65 percent of your calories as carbohydrates and that you include at least 14 g of fiber per 1,000 calories. The Harvard School of Public Health recommends selecting low GI carbohydrates to maintain or improve your health. The best quality carbohydrates come from low GI choices, such as quinoa, barley, whole grain pasta, brown rice and large oat flakes. Reduce or eliminate your consumption of high GI carbohydrates, such as potatoes, white bread, white rice and other refined grains and foods made with refined flours.
References
- Harvard Health Publications: Carbohydrates and Health: Not that Simple...or that Complex
- University of Sydney: The Glycemic Index
- "American Journal of Clinical Nutrition"; International Table of Glycemic Index and Glycemic Load Value; Kaye Foster-Powell et al; 2002
- Harvard School of Public Health: Glycemic Index and Glycemic Load for 100+ Foods
- USDA National Nutrient Database: Nutrient Data Laboratory
- Dietary Guidelines for Americans 2010: Carbohydrates
