Understanding Glycemic Index

Originally developed as a research tool for evaluating the effects of foods on blood glucose control in diabetics, the glycemic index (GI) concept now forms the basis for a number of dietary strategies for weight and disease management. However, controversy on its validity and practicality abounds with U.S. and international scientists and public-policy formulators differing in their interpretation and application of the GI. Most U.S. health organizations and authorities do not endorse the use of GI for diet management, while FAO/WHO and diabetes organizations in Australia, Europe and Canada include it in their guidelines. In fact, Australian nonprofit Glycemic Index Limited, North Sydney, offers manufacturers the opportunity to market nutritionally qualified products with a GI symbol.  

GI ranks the effect of carbohydrate-containing foods on blood glucose compared to a standard high- glycemic reference food on a gram-for-gram basis. It is determined by measuring the blood-glucose values over 2 to 3 hours after eating 50 grams of a food and after 50 grams of either glucose or white bread. The value for the test food is then expressed as a percentage of the reference food, which is given a value of 100. For example, the average GI value for baked russet potatoes is 85±12 (glucose) or 121±16 (white bread) according to the 2002 International Table of Glycemic Index and Glycemic Load (American Journal of Clinical Nutrition, July 2002). High-GI foods, those with a value of 70 or higher, are quickly digested and absorbed, creating a higher spike in blood glucose while low-GI foods, those having a value of 55 or less, are slowly digested and absorbed, resulting in a lower, prolonged glycemic response.

One criticism leveled against GI is that it has little relation to the way people eat because it is not based on typical portions and measures glycemic response from individual foods versus complete meals. Additionally, certain variables affect digestion, and therefore, GI: cooking method (gelatinized starch is more easily digested); processing method (reduced particle size and fiber removal make digestion easier); type of starch (amylose is harder to digest than amylopectin); fiber (viscous, soluble-fiber foods slow digestion); fat (slows digestion and consequently absorption of food components) and acidity (also slows stomach emptying and therefore the digestion rate).

Glycemic load (GL) helps put a practical spin on glycemic response to specific foods. Derived by multiplying the GI by the grams of carbohydrate in a serving, GL quantifies the potential glycemic impact of foods. Thus, the GL of a 150-gram baked russet potato with 30 grams of carbohydrate is 26 (using its GI based on the glucose standard.) High-GL foods score 20 or more and low-GL foods are 10 or less. While the baked potato classifies as high GI and GL, this is not the case for all foods. For example, some juices, such as orange and apple, fall into the low-GI category but get a medium rating for GL. Overall, GL is more relevant for meal planning and dietary management.

Hormonal response to glucose is at the core of the theory of how GI/GL impacts health. Insulin, glucagon and other hormones help regulate metabolism and body composition. In healthy individuals, the low, prolonged blood glucose rise after eating low-GI foods is easily matched with insulin. However, high-GI foods create a high peak in blood glucose that triggers a proportional insulin release. In theory, the insulin surge causes a rapid drop in blood glucose to below-normal levels that leads to the release of glucagon and other hormones that raise blood glucose. Along with these hormones comes decreased ability to use fat as an energy source and increased free-fatty-acid concentrations and fat storage. Additionally, low blood sugar triggers the hunger response, stimulating the appetite.

This cascade of reactions is believed to initiate excessive eating causing obesity, insulin resistance and metabolic syndrome; abnormal blood lipid levels causing heart disease; and impaired pancreatic insulin production causing type 2 diabetes. Also, athletes who consume high-GI foods during endurance activities may experience fatigue because the excess insulin pushes glucose into storage rather than allowing its use for needed energy.

With the popularity of low-carbohydrate diets, food scientists frequently consider GI during product development. Unfortunately, current data on the GI of foods are inconsistent and only provide clues to a formula's glycemic impact. Commercial laboratories have begun offering GI testing services based on internationally standardized methods that use healthy human volunteers. These services offer an advantage to international companies that want to incorporate GI into their marketing and labeling programs in countries that include GI in dietary guidelines. Technically, current U.S. regulations do not allow GI statements on products. However, a Net/Impact Carbs statement is a limited and controversial surrogate for GI that identifies the potential glycemic effect of foods. While this calculated value quantifies the carbohydrates that may affect blood glucose by subtracting the grams of fiber and polyols from the total carbohydrate, it does not rate a food's actual GI/GL.

On the ingredient side, the GI issue is a mixed bag. For some, such as potatoes and white flour, GI/LG is a negative while others find expanded roles and new life in a low-GI food world. The generally low GI of protein opens up new opportunities for whey protein and soy protein ingredients. Many new beverage and cereal-based products incorporate soy protein as a carbohydrate-lowering strategy. Dried plums with 15% sorbitol, 7.5% dietary fiber and no sucrose actually classify as low GI/GL according to the 2002 GI/GL table. Research sponsored by the California Dried Plum Board, Sacramento, will soon determine the GI of ingredient products, such as purée, powder and juice concentrates.

Given current consumer interest in low-carb diets, addressing the GI/GL of foods seems prudent. However, its staying power as a tool for healthy eating will depend on future clinical and food science research into its true impact and significance on health and disease risk.

Angela M. Miraglio, M.S., R.D., ([email protected]) is a Fellow of the American Dietetic Association from Des Plaines, IL. Her firm, AMM Food & Nutrition Consulting, provides communications and technical support to the food and beverage industry.

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