Karen Collins, MS, RD, CDN
Categorizing foods based on their glycemic index (GI) provides information about how they affect blood sugar. While some scientists believe that GI principles may hold the answers to lowering risk for a handful of chronic diseases, research has yet to conclusively find specific health benefits gained from GI beyond what is achieved by following general healthy eating guidelines. For most people, focusing on a plant-based diet that is high in fiber may prove just as helpful and may be more likely to influence healthful food choices.
Carbohydrate with a high GI is digested quickly, resulting in a rapid elevation of blood sugar and a swift rise in insulin. Most sweets, but also potatoes, white rice and refined cereals and breads fall into this category. Foods with a low GI are digested and absorbed more slowly, causing a more gradual rise in blood sugar and preventing rapid spikes in insulin levels. These foods include whole-grain breads, pasta, beans and most fruits and vegetables.
The relationship between a food’s GI value and its effect on insulin levels is critical to understanding the application of GI principles to disease prevention. Consistently elevated insulin levels seem to lead to development of diabetes and other chronic diseases. Many experts believe that finding ways to control insulin levels is a means of promoting long-term health.
An analysis of data from 45 studies, recently published in the American Journal of Clinical Nutrition, showed that lower-GI diets tend to reduce fasting blood sugars and certain markers of overall blood sugar. This effect seems to occur primarily in people with diabetes or those who already have elevated levels of insulin (insulin resistance or pre-diabetes). According to the research, low-GI, high-fiber diets may also reduce the likelihood that overweight individuals – a group more likely to develop insulin resistance – actually develop elevated insulin levels. An important finding as elevated insulin levels can be a precursor to the development of type-2 diabetes.
A 2007 study in the Archives of Internal Medicine looked specifically at the relationship of GI values to the incidence of type-2 diabetes among Chinese women – a group whose primary carbohydrate was white rice. According to the study, those women who ate a high-carbohydrate, high-GI diet were over 20 percent more likely to develop diabetes than those eating diets with the lowest GI value. The impact was even greater among those subjects who were more overweight.
Some people believe that limiting the GI of one’s diet can help with weight control. In a study of obese young adults published last year in the Journal of the American Medical Association, subjects on either low-glycemic or low fat diets showed no difference in overall weight or body fat loss. Importantly, among those participants who began the study with higher levels of insulin, a low-glycemic diet led to greater weight loss than a low fat diet. Other research suggests that overweight or obese subjects may lose more weight on low-GI diets.
A review of how carbohydrate choice and amount affect weight control notes that eating more whole grains is usually linked to healthier weights, whereas eating more refined grains is not. This is significant, because whole grain choices, including grains like oatmeal, bulgur and brown rice, are not necessarily low-GI.
In theory, keeping in mind the GI of foods to lower risk of diabetes, overweight, and perhaps even heart disease and cancer makes sense. However, focusing on an exclusively low-GI diet could lead some people to adopt a less healthful eating pattern. This is particularly concerning if people exclude some higher GI, yet healthful foods and include excessive amounts of high-calorie, fatty foods or alcohol just because they are low-GI.