The glycemic index or GI is the measurement of the blood sugar level increase in response to consuming carbohydrates (usually 50 grams) contained in foods. The value of GI is relative to that of pure glucose, which has a GI of 100. It is acknowledged that a GI of 70 or above is considered as high, a GI of 56 - 69 is medium, and a GI of 55 or less is considered as low. Foods with a higher GI number will induce a greater and more rapid the blood sugar response after ingestion; and conversely, food with a low GI will induce a more slow and steady glucose release.
The concept of GI was initially introduced by Jenkins et al. in 1981, originally designed as a food guide for people with diabetes. Due to the importance of carbohydrates and sugar in body's metabolism and function, GI has since been widely adapted by the non-diabetic population as a guide to optimize diet and performance. Here we have a brief look at the role of GI control in weight management, exercise and limitations of using GI as a sole measure for blood sugar control.
Effects of GI on weight management and health
It is estimated that there are over 1 billion overweight adults in the world and 300 million of those are obese. It has been proposed that the population's increased consumption in high energy, high fat and high sugar diet coupled with low physical activity are the main causes of overweight (Lopes da Silva and de Càssia Gonçalves Alfenas, 2011, Nutrición Hospitalaria). Being the main energy course of human diet, the amount of carbohydrates ingested thus has a significant influence on human health.
GI links to obesity and other health problems
There is evidence suggesting that low GI diets may be protective against the development and obesity of related disease such as type 2 diabetes and coronary heart diseases (Barclay et al. 2008, Amercian Journal of Clinical Nutrition). Low GI diets have been linked with improved BMI (Ma et al. 2005, American Journal of Epidemiology). High GI diets have been linked with increased body fat mass, body weight and waist circumference in women over a 6-year period, but not in men (Bare-Bruun at al. 2006, American Journal of Clinical Nutrition). A diet based on high carbohydrates, low GI foods has been found to reduce the sugar and insulin hike post-eating without increasing LDL-cholesterol (the bad cholesterol) or serum triglycerides (level of fat in blood) (Brand-Miller et al. 2009, Journal of the American College of Nutrition). Low GI diets have been found to be more effective in promoting weight loss in children and adolescents than low fat diets (Esfahani et al 2011, IUBMB Life). A meta-analysis concluded that (after analysed 6 trials) strict low GI diets resulted in significant reduction in weight, total fat mass, and BMI compared to people with normal diets (neither high GI nor low GI) in adolescent and adults (Thomas et al. 2007, Cochrane Database of Systematic Reviews). Another meta-analysis study compared results from 23 studies and found that low GI diets can effective induce weight loss (Livesey 2008 et al. 2008, American Journal of Clinical Nutrition).
There has been inconclusive evidence showing that low GI diets may reduce the risk of various cancers, such as breast cancer, prostate cancer, colon cancer and pancreatic cancer, however, the results of various studies are inconsistent and as such this is an area warrants further scientific investigation (Esfahani et al. 2009, Journal of the American College of Nutrition). Nevertheless, there is overwhelming evidence to show that low GI diets are effective in weight management and can reduce the risk of type 2 diabetes, coronary heart disease and possibly a range of other health related complications such as cancer.
GI is affected by combining different types of food
GI provides values for individual foods. However, when a food is included into a meal, the GI of the meal will not be the same as the GI of the individual food. Contents from other co-ingested food, such as protein, fat, soluble dietary fiber and even acidic compounds will have an influence on the glucose and insulin response to the meal as a whole (Bornet 2007, Apetite). It is generally agreed that the GI of a meal should be the calculated by the GI of individual food weighed by their percentage of carbohydrate contribution to the entire meal. Thus the overall GI of a meal can be reduced (without being unhealthy) by incorporating foods contain high levels of dietary fiber.
GI and exercise
The manipulation of GI for optimizing exercise performance and post-exercise recovery is an exciting new area of research in sports nutrition. However, studies to date yielded mixed results on whether GI manipulation can actually enhance performance. More research into the effects of GI on exercise is clearly required. Therefore, it is important to look at the findings of these studies in a critical manner. Here we present to you some of the recent findings regarding to GI manipulation and exercise.
Effects of GI on exercise and recovery
It has been found that the ingestion of carbohydrate before and during a workout can improve performance. For details of this please refer to the article "Are sports drinks beneficial during workouts?" from the previous issue. While a small number of studies suggested that eating low GI foods before an exercise is beneficial, as the more sustained release of glucose can maintain the energy levels of athletes during the session, the majority of the studies concluded that GI of the ingested meals before exercise has no impact on performance (Donaldson et al. 2010, International Journal of Sports Nutrition and Exercise Metabolism). It is worth noting that a portion of the small number of studies where a pre-exercise low GI meal did enhance performance, the subjects were conducting workouts at lower intensities (3 out of 4 such studies showed improvement). The catch is that this improvement of performance would diminish if the intensity was maxed at any time during the exercise (Mondazzi and Arcelli 2008, Journal of American College of Nutrition). Therefore, to benefit from a low GI pre-exercise meal from a performance point of view, one has to maintain a consistently low intensity performance throughout the entire course of the workout.
On the other hand, there's good evidence suggesting that consuming pre-exercise low GI foods can prevent hypoglycemia (low blood sugar levels) that are usually experienced towards the end of prolongs exercise, as compared to consuming high GI pre-exercise foods. Preliminary research also showed that consuming low GI foods before exercise can induce greater plasma free fatty acids levels in comparison to consuming high GI foods. Free fatty acids are used as energy fuel and can be oxidized by the muscle during exercise, therefore support better energy metabolism. Even though the performance enhancing property of a low GI pre-exercise meal is highly questionable, there is enough evidence to show that low GI foods can at least enhance energy metabolism and thus can be deemed as more beneficial in comparison to high GI foods as pre-exercise meals.
Effects of GI on post exercise recovery
In contrast, it's almost universally agreed that high GI foods/meals can increase muscle glycogen resynthesis, replenish glucose storage after a workout session. It has been shown in one study that the increase in muscle glycogen concentrate from the end of exercise to end of 24 hour recovery period was 50% greater in people consumed high GI foods in comparison to those who had low GI foods (Burke et al. 1993, Journal of Applied Physiology). Consuming high GI foods after a workout can speed up immediate post-exercise recovery.
The limitations of GI and glycemic load
Many manufacturers advertise their food as having a "low GI" and this may have created misconceptions in the public that low GI = healthy. Well, here is something for you to think about: a super supreme pizza from a popular fast food franchise has a GI of 36, whereas green peas have a GI of 51; here is another one, the GI of premium full cream vanilla ice cream is 38, and the GI of grapes is 59; and there is one more, the GI of watermelon is 71, whereas the GI of microwave chicken nuggets is 46. In each case, the junk food wins the low GI contest hands down compared to natural fruits and vegies. But would you really honestly think a super supreme pizza is healthier than fresh green peas? Our examples show that lower GI doesn't necessary mean healthier, what is it then?
GI is originally created as a simple tool to measure of how quickly the blood sugar level would rise after consuming 50 grams of pure carbohydrate equivalent from a specific food. And that is that, it doesn't measure fat content, or calories, or additives, it is only an index to predict sugar levels in blood in response to consuming carbohydrates. This means that food contains pure trans fat (no carbohydrates) would probably have a GI of close to 0, and trans fat is certainly not healthy. On top of that, GI doesn't tell you how much carbohydrate there is in a specific type of food either, and this is the real limitation of using GI as a sole gauge of sugar control. As we stated before, GI is established by measuring the change in blood glucose levels after consuming 50 grams of carbohydrates. Now, carrots and super supreme pizzas have a similar GI, however, it would require a lot of carrots to equate 50 grams of carbs and it is unlikely for anyone to eat that many carrots in one serving. This means the carbs in carrots would have a much-reduced effect on blood sugar in real life situations than the published GI. In contrast, pizzas contain a much higher percentage of carbs and it would not take much to consume the 50-gram portion. You see the problem? It is okay to compare GI of similar types of foods. But comparing GI dissimilar foods without knowing their carbohydrate content can be very misleading, it's like comparing the weight of people without knowing their heights and gender. People in the profession realized this limitation of GI and hence another form of measurement was developed, this is called glycemic load, GL.
GL combines GI and the amount of carbohydrates contained in a serving of a particular food, gives a much fuller picture on how foods can affect our blood sugar. A GL of less than 10 is considered as low, 11-19 is considered as medium and 20 or more is considered as high. Watermelon has a GI of 71, that is quite high, but as it only contains around 5% carbs, that gives each serving (120g) a GL of 4, which is low. In contrast, even though s super supreme pizza has a much lower GI compared to the watermelon, each serving (100g, just under 2 slices) would yield a much higher GL of 9, that's not too bad actually, considering how unhealthy we think fast food pizzas are.
This brings me to the final point that I want to make: both GI and GL are measurements of the effects of carbohydrate on blood sugar ONLY, and they can only measure foods that contain carbs. Pure fat without any carbs would have a GI and GL of both 0, but it is by no means healthier than fruits and vegies with much higher GI/GL. Two slices of super supreme pizza may have a GI and GL in the relatively lower range, but it also contains quite a bit of fat and is quite high in calories, both are not accounted for in the GI and GL measurements. That being said, GL is a more sensible way of measuring how carbohydrates in a particular food can affect blood sugar levels compares to GI, and should be used in conjunction with other criteria to assess the quality of a particular food.
The infamous last words
The GI/GL are nifty tools to predict blood sugar response after ingesting food. They can be used to optimize diet in order to achieve a better health. Use it wisely, don't abuse it.