Glycemic Index: Weight Loss Sham or Sensation?
Research: 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).
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The glycemic index can be a useful tool to help exercise enthusiasts select the right type of carbohydrates to eat before, during and after exercise. Selecting foods with a high- or low-GI score can speed up or slow down the availability of carbohydrate (Manore et al. 2004). For the exercise enthusiast, a low-GI pre-exercise snack—such as apple slices with peanut butter, or low-fat yogurt and half a bagel—results in better maintenance of blood glucose concentrations during exercise, as well as a slightly higher rate of fat oxidation or burning (Bernard et al. 2005). Research has established that endurance is improved when subjects consume a low- versus high-GI meal prior to exhaustive exercise (DeMarco et al. 1999).
Moderate- and high-GI foods are recommended during and after exercise (Beavers & Leutholtz 2008). Higher-GI foods are easily consumed, digested and absorbed by the body, making energy rapidly available. Examples of high-GI foods commonly used during exercise include sports drinks and energy gels or bars. Eating a high-GI snack immediately (within 45 minutes) after exercise elevates plasma glucose concentrations (Ivy & Portman 2004). Postexercise muscle glycogen resynthesis is a high metabolic priority for the exercised muscles (to replenish depleted glycogen stores). The elevated glucose levels from a high-GI food also stimulate insulin secretion. Postexercise, insulin helps promote glycogen storage. Insulin also increases protein synthesis by increasing amino acid uptake by the muscle. Last, insulin enhances blood flow into muscle, thus facilitating removal of the metabolic byproducts of exercise (lactate and carbon dioxide) (Ivy & Portman 2004).
The general guidelines detailed above work well for most, but not all, exercise participants. Human beings differ greatly in how well they digest and process foods. Experiment with meal timing and food choice to find out what works best for your clients. One important exercise rule: never try new foods prior to an important competition or training session.
Myth #1. The glycemic
index is the best way to
determine the amount of
carbohydrate (sugar) in a
The GI describes the rate at which glucose is released into the bloodstream; it says nothing about carbohydrate content. The more grams of carbohydrate you consume, the higher the glycemic response, because the glycemic load is greater. If you eat two different foods with a similar GI, the blood glucose response will be greater for the food eaten in higher concentration.
Myth #2. White foods, such as pasta and potatoes, should be avoided because they have a high GI score.
White foods are not necessarily high on the GI. A food’s GI score is affected by several factors—cooking method, amount of processing, fiber content, physical form (liquid or solid) and meal composition—but not by food color. For example, the GI of boiled potatoes is substantially lower (56) than that of microwaved potatoes (82). What many don’t realize is that pasta is a low-GI food (40–50). Also, we usually eat foods in combination, so the fat and protein consumed at mealtime serve to lower the overall GI of some meals.
Myth #3. The GI can be used to assess whether foods are healthy or unhealthy. GI level does not indicate whether a food is healthy or not. Take milk, for example. Whole milk has a GI of 27, while skim milk has a GI of 32. Lower GI does not always mean a healthier product.
Myth #4. All simple sugar is high GI. Not all sugar is created equal. Fruit, for instance, contains the simple sugar fructose. This simple sugar has a slower rate of digestion and absorption than glucose and therefore produces a lower glycemic response. The GI for most raw fruit is in the 30–50 range.
Myth #5. I can eat as much low-GI food as I want and maintain low insulin levels. Some people feel that low-GI eating entitles them to consume as much as they want. It is possible to get high insulin responses with low-GI eating. Remember, glycemic response is determined by GI × carbohydrate concentration (glycemic load), so more grams of carbohydrate consumed will result in higher insulin levels.
Jerry Mayo, PhD, RD
Certification: NSCA less
Beavers, K., & Leutholtz, B. 2008. Glycemic load food guide pyramid for athletic performance. Strength and Conditioning Journal, 30 (3), 10–14.
Bennard, P., Imbeault, P., & Doucet, E. 2005. Maximizing acute fat utilization: Effects of exercise, food, and individual characteristics. Canadian Journal of Applied Physiology, 30 (4), 475–99.
Brand-Miller, J., et al. 2002. Glycemic index and obesity. The American Journal of Clinical Nutrition, 76 281S–85S.
Das, S., et al. 2007. Long-term effects of 2 energy-restricted diets differing in glycemic load on dietary adherence, body composition, and metabolism in CALERIE: A 1-yr randomized controlled trial. The American Journal of Clinical Nutrition, 85, 1023–30.
DeMarco, H., et al. 1999. Pre-exercise carbohydrate meals: Application of glycemic index. Medicine & Science in Sports & Exercise, 31 (1), 164–70.
Foster-Powell, K., Holt, S., & Brand-Miller, J. 2002. International table of glycemic index and glycemic load values: 2002. The American Journal of Clinical Nutrition, 76, 5–56.
Gaesser, G. 2007. Carbohydrate quantity and quality in relation to body mass index. Journal of the American Dietetic Association, 107, 1768–80.
Howlett, J., & Ashwell, M. 2008. Glycemic response and health: Summary of a workshop. The American Journal of Clinical Nutrition, 87, 212S–16S.
Ivy, J., & Portman, R. 2004. Nutrient Timing: The Future of Sports Nutrition. Laguna Beach, CA: Basic Health Publications Inc.
Jenkins, D., et al. 1981. Glycemic index of foods: A physiological basis for carbohydrate exchange. The American Journal of Clinical Nutrition, 34, 362–66.
Maki, K., et al. 2007. Effects of a reduced-glycemic-load diet on body weight, body composition, and cardiovascular disease risk markers in overweight and obese adults. The American Journal of Clinical Nutrition, 85, 724–34.
Manore, M., Mason, M., & Skoog, I. 2004. Applying the concepts of glycemic index and glycemic load to active individuals. ACSM’s Health and Fitness Journal, 8 (5), 21–23.
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