July 18, 2006
With the rise in obesity and related diseases, particularly diabetes, the food industry's interest in glycemic response and glycemic index (GI) as a means to provide healthier products is growing. The market is growing to serve diabetics' need to control glycemic response and insulin levels, and to help curb the general population's excessive consumption of high-glycemic foods, which leads to weight gain by increasing fat storage and adipose tissue.
The concept is deceptively simple: provide low-glycemic food products that make it convenient and eliminate the guesswork. But as with many simple concepts, execution can be anything but simple. For companies that want to tap into the market, a general understanding of the GI concept and the ingredients that contribute to low-glycemic formulating is crucial.
GI in general
GI ranks carbohydrates according to their effect on blood glucose levels after consumption. The Food and Agriculture Organization of the United Nations (FAO) defines glycemic index as: "the incremental area under the blood glucose response curve of a 50 grams carbohydrate portion of a test food expressed as a percent of the response to the same amount of carbohydrate from a standard food taken by the same subject. For most glycemic index data, the area under the curve has been calculated as the incremental area under the blood glucose response curve (IAUC)." Glucose has a GI value of 100 and is generally considered the standard. When tests use white bread as the standard, they give GI values about 1.4 times higher than those obtained with a glucose standard.
Carbohydrates with a high GI require less energy to be converted into glucose, and are digested more quickly, resulting in a spike in blood glucose. Foods with a low GI break down more slowly, and release glucose gradually into the blood stream. This results in a lower insulin demand.
As far as GI, foods fall into three general categories:
Low GI: less than 55;
Intermediate GI: between 56 and 69;
High GI: higher than 70.
Australian products can participate in the GI Symbol Program (run by Glycemic Index Limited, a nonprofit company formed by the University of Sydney, Diabetes Australia and the Juvenile Diabetes Research Foundation). Foods that meet specific nutritional criteria and use an approved method to measure GI can display a symbol and list the GI value on their labels. High, medium and low GI foods as outlined above are eligible.
The concept is gathering a following in Europe also, especially in Germany and the United Kingdom. However, in the United States the glycemic theory is still met with caution. For example, the Dietary Guidelines Advisory Committee decided to pass over GI in its most recent guidelines, saying: "Although the use of food with a low-glycemic index may reduce postprandial glucose, there is not sufficient evidence of long-term benefit to recommend general use of diets that have a low glycemic index."
Dialing in on GI
The reluctance to embrace GI may be due to the complicated nature of the beast. GI is not a simple measurement like fat content or calories. A number of factors and characteristics influence a food's glycemic response. These include:
Differences in composition caused by factors such as growing conditions, ripeness, etc.;
Amount of carbohydrates;
Type of monosaccharide;
Type of starch (amylose has a slower absorption than amylopectin);
Starch-nutrient interaction or physical entrapment;
Type and level of resistant starch;
Degree of starch gelatinization;
Mechanical and thermal processing;
Particle size (faster absorption of smaller particles);
Other food and meal components, especially protein, fat and dietary fiber (which can inhibit or decrease the rate of carbohydrate absorption);
Individual differences in intestinal absorption;
Content and timing of the previous meal.
For example, rice might be generally considered a high-GI ingredient. However, researchers across the globe have measured a large range of GI values that occur due to botanical differences in rice from country to country. High-amylose types might have a GI in the high 30s to 50s, while other varieties might score over 100.
In addition, some researchers believe that using a weighed average calculation provides a good correlation between meal GI and the observed glycemic responses of meals of equal nutrient composition. (Values for each food equal the proportion of total glycemic carbohydrate multiplied by the food GI. The sum of these values is the meal GI.) But again, a value of the GI for every food in the diet or meal needs to have been assigned, and the accuracy of the calculation depends on the accuracy of the GI values ascribed to foods.
The GI doesn't tell a food's whole story. Glycemic load (GL) takes into account both the amount of carbohydrate in the food and the impact of that carbohydrate on blood sugar levels. A food's glycemic load is determined by multiplying its GI by the amount of carbohydrates in it. Both a low-GI/high-carbohydrate food and a high-GI/low-carbohydrate food can have the same GL. The lower the glycemic load, the less likely blood sugar will spike. GL is categorized as follows:
High GL: 20 and higher;
Moderate GL: 11-19;
Low GL: 10 or less.
Low-glycemic ingredient toolbox
Confounding factors aside, it's best to formulate with ingredients that have or contribute to a low GI or GL. In general, that means avoiding high levels of "glycemic carbohydrates"--ingredients high in sugar, paying particular attention to glucose, and high in starch, especially amylopectin, or any other compound that easily converts to glucose.
Fruit and fruit ingredients. In general, fruit contains a relatively high level of fructose (10% to 15% of the total carbohydrate), a sugar with a lower GI than glucose, and fiber. For example, apple juice might be a better choice as an ingredient than grape juice because it has a higher fructose content than grape juice. Most fruits have a low GI. Fresh bananas, pineapple and watermelon are among the exceptions. However, based on the carbohydrate content per serving, even these have a low or moderate GL.
Vegetables. Like fruit, most high-fiber vegetables have a low to moderate GI. Starchy root vegetables, like potatoes and parsnips, however, typically fall into the high-GI arena. But even that can vary, according to the U.S. Potato Board, which cites studies that compared the GIs of potatoes prepared in a variety of different ways (including mashed, baked, reheated, boiled, boiled and cooled, and fried). The GI values varied significantly, depending on both the variety and cooking method used, and ranged from intermediate GI (boiled red-potatoes, consumed cold: 56) to moderately high (roasted California white potatoes: 72; baked U.S. Russet potatoes: 77) to high (instant mashed potatoes: 88; boiled red potatoes: 89).
Beans and legumes. Though "starchy," legumes tend to have GIs less than 40 because they are high in amylose. Their phytate content may also slow the breakdown of starch.
Whole grains. In unrefined grains, fiber protects the starchy carbohydrates from rapid digestion and slows the release of sugar into the bloodstream. Rolling or refining disrupts the outer germ layer and increases GI. White rice has a GI of approximately 64, vs. 55 for brown. Milling reduces the particle size and can increase GI. Otherwise, GI varies with the starch and fiber content; for example, pearl barley has a 33 GI and cornmeal is 70.
Fiber. Conventional wisdom says dietary fiber lowers the GI of a carbohydrate-rich food, but the actual impact of fiber on the GI of foods remains somewhat controversial. There are conflicting studies on the effects of insoluble and soluble fiber's impact on glycemic response, with reports sometimes saying that one, but not the other, tends to lower GI. (The one that lowers GI varies.) In theory, though, soluble fibers should slow digestion and slow the availability of the starch to digestion. Also keep in mind that gums that are classified as soluble fibers make a low-glycemic alternative to starch thickeners. Although not officially classified as fiber, resistant starch and maltodextrin have much the same properties and are considered low-GI ingredients.
Nutrititive sweeteners. Glucose serves as the benchmark in many measurement methods, with a GI of 100. Compared with glucose, the GI of fructose only gives a value of 20. Sucrose and honey have similar sugar compositions, and moderate GI values that range from 55 to 60; high fructose corn syrup (HFCS) would be expected to have a similar GI because its composition is comparable.
Polyols. These low-glycemic sweeteners (from a GI of 0 with erythritol and mannitol, to approximately 35 with maltitol, although the syrups are higher) have a long use in replacing higher-GI nutritive sweeteners in a wide range of applications.
High intensity sweeteners. Whether they carry a caloric load or not, high intensity sweeteners such as acesulfame K, aspartame, sucralose and saccharine are used in minute quantities to replace nutritive sweeteners that provide a much higher GI/GL. For example, one brand of low-fat fruit yogurt sweetened with aspartame was found to have 14 GI vs. the sugar-sweetened version that scored 33.
Other sweeteners. Inulin and FOS are reported to have a GI of nearly 0. Testing an oligofructose ice-cream showed a glycemic response of 13, vs. a 44 for the traditionally formulated product. It can also provide the body and mouthfeel of higher GI sweeteners and provide some stabilization in place of starch. D-tagatose, a low-calorie monosaccharide, is also considered a low-glycemic sweetener. Tests show that it too, lowers the glycemic index of foods that include it as an ingredient.
Protein and fat. Without carbohydrates, both of these macronutrients do not increase blood glucose and, in fact, lower a food's or meal's glycemic response. However, with fat at least, this technique should be approached with caution--while potato chips and French fries have a lower GI than a baked potato, no one should mistake them for health foods. Nutritionally, protein doesn't offer the same pitfalls. Both fat and protein can provide body and texture when high-glycemic carbohydrates are removed.
It bears repeating: formulating for glycemic response isn't a simple matter. A food scientist can't just plug a formula into a database and come up with a valid number--at least not as the science now stands. However, the industry can find justification for taking the concept into account.
"Carbohydrates in human nutrition" (FAO Food and Nutrition Paper--66), a report of a Joint FAO/WHO Expert Consultation, recommends the following role of GI in food choice:
"That for healthy food choices, both the chemical composition and physiologic effects of food carbohydrates be considered, because the chemical nature of the carbohydrates in foods does not completely describe their physiological effects.
"That, in making food choices, the glycemic index be used as a useful indicator of the impact of foods on the integrated response of blood glucose. Clinical application includes diabetes and impaired glucose tolerance. It is recommended that the glycemic index be used to compare foods of similar composition within food groups.
"That published glycemic response data be supplemented where possible with tests of local foods as normally prepared, because of the important effects that food variety and cooking can have on glycemic responses."
GI may be difficult, but product designers may find that using the concept as a roadmap could lead to a healthier bottom line as well as a healthier consumer.
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