What is it exactly, and is it right for your clients?
The list of “proposed” benefits associated with a low glycemic index (GI) diet seems endless. Supporters of this eating plan suggest it helps people lose and control weight, improves control of diabetes (in those who have it), reduces hunger and enhances one’s health profile. What is the truth? This article analyzes and discusses evidence-based research on the GI and its touted weight loss claims, and resolves the question of how the GI can be used to properly support exercise performance.
The glycemic index is a numerical ranking system used to measure the rate of digestion and absorption of foods and their resultant effect on blood glucose. A food ranking high on the GI produces a large, momentary spike in glucose after it has been consumed, while a food with a low GI causes a slower, sustained rise in blood glucose.
The concept of GI was first established in 1981 by Jenkins and colleagues as a way to classify carbohydrate-containing foods for improvement of glucose control in people with diabetes (Jenkins et al. 1981). Subjects consumed 50 grams (g) of various foods individually after overnight fasts, and researchers monitored the blood glucose responses to the foods for 2 hours. Each response was then compared, as a percentage, with the response to 50 g of a reference food, either glucose or white bread, both of which have rankings of 100. Jenkins et al. used the research findings to establish a table ranking 62 common foods based on glycemic response—and the GI was born. GI scores are classified as low (below 55), medium (56–69) or high (greater than 70). More recently, extensive GI tables have been developed (Foster-Powell, Holt & Brand-Miller 2002).
Several factors affect a food’s GI, among them physical form (liquid or solid), fiber content and preparation method (raw or cooked) (Manore, Mason & Skoog 2004). In general, highly processed foods containing refined sugars (e.g., crackers and corn syrup) will have a higher GI. It should also be noted that the GI for any food may vary significantly between individuals, so it is important to test foods for yourself to determine their effects.
With a low-carbohydrate diet, the supposition is that throughout the day insulin levels are also lower, allowing (or promoting) greater use of fat as the fuel source. With a low-GI plan, the hypothesis is similar. However, a low-GI diet essentially does not restrict carbohydrates; it is just very selective in the carbohydrates chosen for consumption.
For example, people following a low-carbohydrate diet count grams of carbohydrate at meals to ensure that the intake is small. On a low-GI eating plan, the focus is not on counting grams of carbohydrate but on choosing foods that are low GI. Someone on a low-carbohydrate diet may set a goal to consume no more than 150 g of carbohydrate per day, while an individual following a low-GI plan would attempt to keep food intake at a GI level of 50–55.
Glycemic load (GL) combines both the quality and quantity of a carbohydrate in one number. It’s an excellent way to predict blood glucose values of different types and amounts of food. The formula is as follows:
GL = (GI × the amount of carbohydrate) ÷ 100.
Let’s calculate the glycemic load of an apple. It has a GI of 40 and contains 15 g of carbohydrate. The GL = 40 × 15g/100 = 6. Now, let’s calculate the GL of a small baked potato. It has a GI of 80 and contains 15 g of carbohydrate. The GL = 80 × 15g/100 = 12. So this indicates that the potato will have twice the metabolic effect of an apple. The GL is the amount of carbohydrate in a food “adjusted” for its glycemic influence. The GL concept is useful in scientific research where the quantity and quality of foods are being studied.
The ability of a low-GI eating plan to facilitate weight loss is heavily debated among scientists. There is no consensus that a low-GI food plan is any better for weight reduction than a traditional diet. Those who advocate eating low-GI foods believe it will increase the rate of fat utilization and promote satiety (feelings of fullness) (Brand-Miller et al. 2002). However, determining the true effect of low-GI eating on weight loss is quite complex. When reviewing the literature, factors such as subject demographics, health status (diabetic, obese, etc.) and length of investigation, along with the methodological differences between studies, make interpretation of results challenging. These issues present real barriers that could have affected study outcomes.
In a 1-year randomized, controlled trial, Das and coworkers (2007) tried to eliminate many of these methodological problems. In their research, subjects were assigned to either a low- or a high-GI eating plan and used a 30% calorie restriction to promote weight loss over time. Food preferences were established, and then researchers provided subjects with GI education and all of their food for the first 6 months of the study. This allowed subjects to “learn” proper eating principles prior to attempting to follow the eating plans on their own. Both groups lost weight in 1 year (-8.04 ± 4.1% and -7.81 ± 5.0% for high and low GI, respectively), but there were no differences between groups.
It’s interesting that one study found weight loss after 6 months but no differences after a year (Maki et al. 2007). This could mean that people find it impossible to maintain a low-GI diet for a long period of time.
Based on the lack of substantial evidence, the stance of the American Dietetic Association (ADA) is that a low-GI diet should not be recommended for weight loss (ADA evidence analysis). A recent published summary from a workshop on glycemic response and health is in agreement, suggesting that there is limited evidence at this time to support the role of a low-GI diet for weight loss (Howlett & Ashwell 2008). In addition, the summary reported that weight loss observed by those eating a low-GI diet is due to the higher fiber content and lower caloric intake, not to the diet plan itself. Independent of GI, fiber intake has been well correlated with body weight reduction and maintenance, as well as with lower energy intake (Gaesser 2007; Howlett & Ashwell 2008).
There is only weak evidence to support the use of a low-GI diet by healthy individuals, yet this type of plan has proved quite effective in enhancing glucose control in those with impaired glucose metabolism and diabetes. Also, in short-term studies some beneficial effects have been found on cardiovascular disease risk factors. However, more definitive, well-controlled, long-term studies are needed (Howlett & Ashwell 2008).
The GI has several practical applications for exercise enthusiasts wanting performance benefits and for those looking to improve glucose control. Currently, research does not support low-GI eating as a magic bullet for weight loss. It comes back to eating less, being more active and consuming a healthy diet containing plenty of whole grains and other fiber-containing foods (e.g., fruits and vegetables).