|Glycemic Index: Weight Loss Sham or Sensation?
Jerry J. Mayo, Ph.D., R.D. and Len Kravitz, Ph.D.
The list of 'proposed' low glycemic index (GI) benefits seems endless. Supporters of this eating plan suggest this strategy will help people lose and control weight, improve diabetes control (for those who have it), reduce hunger and improve one's health profile. What is the truth? This article will analyze and discuss evidence-based research on the GI and its touted weight loss claims as well as resolve how the GI may be used to properly support exercise performance.
What is the Glycemic Index?
GI 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 is has been consumed. By contrast, 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 diabetics (Jenkins et al., 1981). They had subjects consume 50 grams of food and monitored blood glucose response for 2 hours. This response was then compared to 50 grams of a reference food, either glucose or white bread. 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 either low (below 55), medium (56-69), or high (greater than 70). More recently, extensive GI tables have been developed (Foster-Powell et al., 2002).
Several factors affect a food's GI such as physical form (liquid or solid), the amount of fiber, and preparation method (raw or cooked) (Manore et al., 2004). In general, highly processed foods containing refined sugars (such as 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's important to test foods for yourself to determine their effects.
What are the similarities/differences in a Low Carbohydrate Diet and a Low GI Diet?
The supposition of low carbohydrate diets is that throughout the day insulin levels are also lower, allowing (or promoting) greater use of fat as the source of fuel. This is a similar hypothesis of what is occurring with low GI diet plans. However, low GI diets essentially do not restrict carbohydrates; they are just very selective of the carbohydrates chosen for consumption.
What is the Glycemic Load?
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: GL = (GI x the amount of carbohydrate) divided by 100. Let's calculate the glycemic load of an apple. It has a GI of 40 and it contains 15 grams of carbohydrate. The GL = 40 x 15g/100 = 6. Now, let's calculate the GL of a small baked potato? Its GI is 80 and it contains 15 g of carbohydrate. The GL = 80 x 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.
Impact of GI on Weight Loss: A Research Summary
The ability of a low GI eating plan to facilitate weight loss is heavily debated among scientists. There is no consensus that eating low GI foods are any better for weight reduction than a traditional diet. Those who advocate eating low GI foods believe this eating plan 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
), length of investigation, as well as the methodological differences between studies make interpretation of results challenging. These issues present real barriers that could have affected study outcomes.
In a one-year randomized control trial, Das and coworkers (2007) tried to eliminate many of these methodological problems. In their research, subjects were either assigned to a low or 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 these eating plans on their own. Both groups lost weight after one year (8.04 ± 4.1% and 7.81 ± 5.0% for high and low GI, respectively) but there was no differences between groups.
It's interesting that one study reviewed found weight loss at six months but no differences after a year (Maki et al., 2007). This could mean that eating low GI isn't something that can be maintained for a long period of time.
Based on the lack of substantial evidence, the stance of the American Dietetic Association (ADA) is that low GI diets 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 at this time there is limited evidence to support the role of a low GI diet for weight loss (Howlett and Ashwell, 2008). In addition, the summary reported that success from low GI eating has more to do with the high fiber content of the low GI plan. Independent of GI, fiber intake has been well correlated with body weight reduction and maintenance as well as lower energy intake (Gaesser, 2007; Howlett and Ashwell, 2008).
Who Might Benefit From a Low GI Diet?
There is only weak evidence to support the use of low GI diets in healthy individuals. However, these diets have been shown to be quite effective in improving glucose control for 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 need to be performed. (Howlett and Ashwell, 2008).
The GI has several practical applications not only for exercise enthusiasts wanting performance benefits but also for those looking to improve glucose control. Currently, research doesn't 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).
Side Bar #1. How relevant is the Glycemic Index To Exercise Performance?
The GI can be a useful tool to help exercise enthusiasts select the right type of carbohydrates to eat before, during, and after exercise. Selecting foods that are high or low GI can speed up or slow down the availability of carbohydrate (Manore et al, 2004). For the exercise enthusiast, a low GI pre-exercise snack results in better maintenance of blood glucose concentrations during exercise, and a slightly higher rate of fat oxidation or burning (Bernard et al, 2005). Research has established that endurance is improved when subjects consumed 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 and Leutholtz, 2008). Higher GI foods are easily consumed, digested, and absorbed by the body allowing rapid availability of energy. Examples of high GI foods commonly used during exercise include sports drinks and energy gels/bars. Eating a high GI snack immediately (with 45 minutes) after exercise elevates plasma glucose concentrations (Ivy and Portman, 2004). Post-exercise muscle glycogen resynthesis is of high metabolic priority for the exercised muscles (to replenish depleted glycogen stores). The elevated glucose levels from the consumed GI food also stimulate insulin secretion. Post-exercise, insulin helps to promote glycogen storage. Insulin also increases protein synthesis by increasing amino acid uptake by the muscle. Lastly, insulin also enhances blood flow into muscle, thus facilitating the removal of metabolic byproducts from exercise (lactate and carbon dioxide) (Ivy and Portman, 2004).
The general guidelines detailed above work well for most, but not all, exercise participants. There is a great deal of individual differences in how well one digests and processes foods. Experiment with meal timing and food choice to find out what works best for your clients. One important exercise rule is never try new foods prior to an important competition or training session.
Side Bar #2 Five Myths Related to the Glycemic Index
Myth #1 The Glycemic Index is the best way to determine the amount of carbohydrate (sugar) in a particular food. Glycemic index describes the rate glucose is released in the bloodstream and says nothing about carbohydrate content. The more grams of carbohydrate consumed the higher the glycemic response because there is an increased glycemic load. For instance, you could eat two different foods with a similar GI but the blood glucose response will be greater for the food eaten in higher concentration.
Myth #2 Avoid foods that are white such as pasta and potatoes because they are high GI foods. The white color of foods doesn't necessarily mean it has a high GI. As mentioned previously, cooking method, amount of processing as well as meal composition affect GI, not food color. For example, the GI of boiled potatoes is substantially lower (56) than when 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 Glycemic Index Can Be Used to Assess How Healthy vs. Unhealthy foods. The GI doesn't indicate whether a food is healthy or not. One example of this is milk. Whole milk has a GI of 27 while skim milk has a GI of 32. Lower GI doesn't 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, therefore it produces a lower glycemic response. The GI for most raw fruit is between 30-50.
Myth #5 I Can eat as much Low GI food as I want and maintain low insulin levels
Some 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 a combination of GI x carbohydrate concentration (glycemic load), so the more grams of carbohydrate consumed will result in higher insulin levels.
American Dietetics Association Evidence Analysis Library. Effective of consumption of low glycemic foods in weight loss and maintenance. https://www.adaevidencelibrary.com/conclusion.cfm?conclusion_statement_id=250240 accessed September 13, 2008.
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