What affects the GI of a food?
Foods that contain fat and/or fiber affect the overall glycemic response of a food by slowing down gastric emptying which in turn slows down the digestion of carbohydrates. This is similar to the lower glycemic response observed when a meal contains fat, protein and/or carbohydrate. Cooking or processing a food expands and softens starch based foods, which speeds up the rate of digestion. Cooking or processing foods may raise the GI of that food, e.g. cooked carrots have a GI of 92 which is considered high, while raw carrots have a GI of 16 which is considered low. (3) Other factors contributing to a higher GI include ripeness and storage time, cooking method and type of carbohydrate.
The overall glycemic response of a meal may be affected by the type of food that is eaten with the carbohydrate. If a high GI food (such as white bread) is combined with a low GI food (such as low fat yogurt) blood glucose will benefit by a more balanced fluctuation of blood glucose and a lower GI.
Although GI can represent a carbohydrate-containing food’s effect on blood glucose, portion sizes are still an important consideration for blood glucose management and for losing or maintaining weight. Glycemic load measures the degree of glycemic response and insulin demand elicited by a given amount of a certain food. Glycemic load reflects both quality and quantity of dietary carbohydrates. (4) GI is one factor which may be used to evaluate the nutritional quality of foods and ranks carbohydrates based on immediate blood glucose response. (4) There are other factors that contribute to nutritional quality as well and all of these factors should be considered when determining overall healthfulness. Many healthy food choice options may have a higher GI than their less nutritive counterparts.
What types of foods are considered low, medium and high GI?
Clinical Significance of GI
The concept of GI is most frequently used in diabetes patient education and is presented in different ways, either as table values or otherwise integrated into dietary recommendations. In 2011, the American Diabetes Association (ADA) published a position statement on “Standards of Medical Care in Diabetes.” ADA states among its recommendations for medical nutrition care for people with diabetes:
“Monitoring carbohydrate intake, whether by carbohydrate counting, exchanges, or experience-based estimation, remains a key strategy in achieving glycemic control. For individuals with diabetes, the use of the glycemic index and glycemic load may provide a modest additional benefit for glycemic control over that observed when total carbohydrate is considered alone.” (5)
The significance of low GI foods to healthy people is an ongoing discussion within the scientific community and official bodies in different countries. Health problems associated with being overweight are a major concern for countries all over the world. The World Health Organization and Food and Agriculture Organization of the United Nations (WHO/FAO) have stated that, globally, overweight is a bigger problem than undernourishment. They have recommended that people in industrialized countries base their diets on carbohydrate-based low GI foods to prevent lifestyle-related diseases. (6)
Conversely, recommendations by the US Dietary Guidelines Advisory Committee currently do not support the GI concept. In 2005, it was concluded that “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.” (7) It was thought that the clinical and practical value of the GI merits to be further studied, and more evidence was needed before adding information regarding low GI food to public recommendations on an evidence-based level. About 5 years later, the 2010 US Dietary Guidelines Advisory Committee concluded that was not sufficient evidence to take the GI into account and stated: “When selecting carbohydrate-based foods, there is no need for concern with their glycemic index or glycemic load. What is important to heed is their calories, caloric density, and fiber content.” (8)
Europe has taken a different position from the US with regulations that encourage positioning foods with low or reduced glycemic properties and allow for corresponding claims. Within the current attempts to establish a list of generally accepted health claims in the Europe Union, the European Food Safety Authority (EFSA) evaluated a number of ingredients as related to blood glucose lowering effects and concluded: “The Panel considers that the reduction of postprandial glycemic responses may be a beneficial physiological effect.” Corresponding favorable opinions have been issued for the polyols, isomalt, lactitol, maltitol, mannitol, sorbitol, xylitol and erythritol, as well as for isomaltulose and a number of other carbohydrate ingredients as well as for intense sweeteners. (9)
Consistent consumption of high GI foods may increase risk factors associated with obesity, type 2 diabetes, and heart disease. Conversely, the consumption of foods that elicit low glycemic responses may help to reduce such risk factors. A lower glycemic response is thought to correspond to less insulin release, better long-term blood glucose control and a reduction in blood lipids. While there is no definitive proof that reducing glycemic impact will prevent disease on an individual basis, some research suggests that reducing the glycemic effect of the diet may reduce disease risk. A growing number of studies suggest that reducing the glycemic impact of the diet may help consumers eat fewer calories, however not all investigators and reviewers have reached the same conclusion. (10)
Two papers in Obesity Reviews addressed the question of whether obese patients should be advised to follow a low glycemic index diet. Pawlak et al. concluded that obese patients should be advised to follow a low GI diet based on a concern that the reduction in fat intake widely advocated in the prevention and treatment of obesity has the potential to encourage an increase in the consumption of high GI carbohydrates. (11)
Raben concluded that obese patients should not be counseled to follow a low GI diet because there is no evidence at present that low-GI foods have a better or different effect than high-GI foods when considering long-term body weight control. However, long-term studies where ad libitum intake and fluctuations in body weight are permitted, and the diets are similar in all variables except GI, have yet to be conducted. (12) Raben noted that low GI is recommended in the management of diabetes. A systematic review was undertaken of intervention studies comparing high and low GI foods and diets on appetite, food intake, energy expenditure and body weight. Of 31 short-term studies, low GI was associated with a greater feeling of fullness or reduced hunger in 15. No difference was seen in 16. In 20 longer term studies, weight loss occurred in four low GI and two high GI trials, however it should be noted that many of the diets had the same number of calories.(12)
A 2007 literature review, “Glycaemic response to foods: Impact on satiety and long-term weight regulation,” published in Appetite, analyzed 32 short and long-term studies relating to GI and satiety. The review concluded, “While there is evidence from short-term studies that low-glycaemic foods (low-GI in particular) have higher satietogenic properties than high-glycaemic foods, the available long-term studies do not allow conclusion about the regulation of energy intake and body weight. The demonstration of the long-term benefit of low-GI foods and diets with respect to their effect on satiety requires further studies.” The results further conclude, “Based on these data, one can legitimately say that wordings such as, ‘This is a low-GI food. Low-GI foods help one to feel fuller for longer than equivalent high-GI foods’ are substantiated health claims. By contrast, claims on a longer-term health benefit of low-glycaemic diets based on energy intake and body weight regulations remain unsubstantiated.” (13)
ILSI Europe, a regional group of the International Life Sciences Institute (ILSI), published a concise monograph on “Food, Glycemic Response and Health” in 2011, which reviewed the scientific evidence for an influence of the glycemic response on health indicators related to blood glucose management, insulin resistance, plasma lipids, satiety and body weight. This review concludes that “there is a growing body of scientific evidence that compositional changes affecting the glycaemic response to foods is associated with beneficial outcomes related to key public health priorities, including type 2 diabetes, weight management and CHD”. Dietary interventions that lower the GI and/or GL of the diet have been shown to improve fasting blood glucose in individuals with impaired blood glucose control, e.g. diabetics. Similarly, glycated proteins (associated with tissue damage) are reduced, with the effect being greater in subjects with poorer glycaemic control. Insulin sensitivity can also be improved in non-diabetics, overweight subjects, obese subjects and patients with type 2 diabetes, particularly in association with higher unavailable carbohydrate intake. (14)
In summary, the low GI diet has emerged as in interesting tool in reducing risk of lifestyle diseases. The evidence supporting a potential protective role of low GI foods against chronic diseases is still judged differently in different parts of the world, like a comparison of the US and Europe shows, while the still growing body of evidence will help to further evaluate and strengthen its clinical significance and value in healthy people. Finally, it should always be kept in mind that reductions in the GI and/or GL of the diet are often associated with changes beyond just the available carbohydrate source (e.g. changes in fat and protein levels and dietary fiber). Therefore, modifying the glycaemic response of the diet should not be seen as a stand-alone strategy but rather as an element of an overall balanced diet and lifestyle.
The Role of Specialty Carbohydrates
Future Challenges and Opportunities
Traditionally, GI has been used as a tool to assist people with diabetes in controlling their blood glucose. In recent years, it has gained popularity among the general population as an effective means of monitoring carbohydrate intake.
The “GI concept” should not be seen as a diet, but rather a tool to compare carbohydrates and carbohydrate-based foods for their glycemic properties and a strategy to achieve quality nutrition that becomes part of a healthy lifestyle. With a reduction in the glycemic impact of the diet, a reversal in the trend towards ‘lifestyle’ related diseases may be observed. A low GI diet may play a key role in the prevention of lifestyle related diseases and conditions as well as help maintain steady post meal blood glucose levels. More well-controlled clinical trials are needed to establish the association with greater certainty.
Selected Glossary
Available Carbohydrate – Carbohydrate that is actually digested and absorbed into the blood and metabolized.
Glycemic Effect, Glycemic Response, Glycemic Impact, Glycemic Challenge– Are not formally defined, but generally refer to the changes that happen to blood glucose after consuming of a carbohydrate-containing food.
