Glycemic Index and Glycemic Load
|These terms are becoming better known to athletes who read books, magazines and journals, but there are still millions of people who have no idea what these terms mean or why it's important to understand their significance.
Comprehension of the glycemic response of food is just as important as knowing its micronutrient content, allergenicity, connection to blood type, caloric value, fiber content or pH. Successful body composition management relies on this knowledge and for athletes, it is crucial to performance. For example, a low glycemic food consumed 1-2 hours before running a marathon will provide the legs with a slow-burning source of fuel for more endurance. In contrast, a high glycemic meal or protein shake taken right after a morning workout can restore muscle glycogen quickly in time for an afternoon soccer game.
The glycemic index (GI) of carbohydrates was developed originally for diabetics, as knowing the effect of food on blood sugar is important for monitoring a diabetic's glucose levels. Foods that lead to a slow increase in blood glucose have a low glycemic index. Those that induce a rapid rise in blood sugar have a high glycemic index. The higher your blood sugar level is after eating a certain food, the higher the glycemic index or glycemic load.
Glycemic index measures the extent to which blood glucose increases after eating a 50g portion of carbohydrate. This increase is then compared to glucose, which is given the value of 100. After an overnight fast, a test food providing 50 grams of carbohydrate is consumed. Then, on the result sheet, the incremental area under the blood glucose curve created by the test food is measured and compared to the area under the blood glucose curve affected by the reference food, which in this case is 50 grams of pure glucose.
Understanding the GI of different fruits, vegetables and starchy foods is important because a rapid rise in insulin, except after a workout, can ruin your bodyfat-reducing objectives, increase serum cholesterol levels, encourage the onset of fatigue, intensify joint inflammation and set you up for hyperinsulinemia. Athletes need blood sugar stability. We all do.
Carrots, white potatoes, ripe bananas, white bread, rice cakes and white rice can cause a rapid rise and fall in blood sugar. These foods should be avoided before a workout, especially if eaten alone. Pre-workout meals should consist of foods that have a low to medium glycemic index such as slow-cooked oats, lentils, green apples, cherries and plums, or a yam/protein combination. The slow rate of digestion helps maintain a steady stream of blood glucose throughout your workout, especially towards the end of exercise, when muscle glycogen tends to run out.
Fruits that have had more time to ripen are sweeter due to conversion of sugar polymers (chains) to simpler forms of sugar. This raises the GI of the plant, which would otherwise be lower, provided the fruit was eaten in a starchier, less ripened state. A good example is the banana. Eaten under ripe it has a GI of 30, but if eaten over ripe it can jump up to 80.
Low GI foods have been proven to extend endurance when eaten alone 1-2 hours before prolonged strenuous exercise. In a study reported in the International Journal of Sports Nutrition, cyclists who ate lentils as a pre-event meal were able to cycle at 65% of their maximum 20 minutes longer compared to cyclists on a pre-event meal of regular white potatoes. Cyclists on the lentils had higher levels of blood sugar and insulin after 90 minutes of exercise. This demonstrates that the carbohydrate sugars were still being absorbed.
You can reduce the glycemic response of almost any food item by combining it with an essential fat or a high quality protein, both of which are typically low on the GI scale. Fiber also lowers the GI of food, but this depends on the form of fiber. Soluble fiber, which is thick and viscous and found in oats, legumes and psyllium, slows digestion and generally slows down and modifies the release of sugar into the blood, whereas finely ground, insoluble fiber like the type found in wheat flour often doesn't. This is why there isn't much difference in the GI between white and whole meal flour, or between whole-wheat pasta and white flour pasta, although differences in their micronutrient density and nutrition value are significant.
Fresh lemon juice and red wine vinegar are both known to modify glycemic index when added to food in the form of a salad dressing. Dipping raw vegetables in lemon juice and extra virgin olive oil has a similar effect. The acidity of lemon juice and red wine vinegar slows stomach emptying which increases the time food takes to leave the gut. Slow release of food from the gut means longer digestion time and less insulin spiking. The natural carbohydrate sugars in sourdough bread are also modified by the action of lactic acid or proprionic acid. Wholegrain sourdough is a better choice for extending endurance and stamina than regular bread. Sourdough takes longer to digest and absorb and is more satisfying.
After a workout in the gym, when your metabolic window is wide open and glycogen stores are depleted, indulge yourself with a delicious smoothie blended with carbs and protein. Mix 50 - 100 grams of medium to high glycemic carbs with up to 25% of your total daily requirement of high-quality protein. Use 1 gram of carbohydrate per kilogram of your weight as a general rule. I use mango, berries, banana, raw honey and dates. I also add creatine, ribose, glutamine, calcium ascorbate and liquid EFAs. This approach will encourage glycogen restoration much faster than eating steak and eggs or a tuna salad sandwich. It promotes quick recovery, suppresses cortisol activity and enhances the metabolic function and sensitivity of insulin, IGF-1, testosterone and human growth hormone.
Sports research scientists at the Australian Institute of Sport in Canberra showed that high GI foods definitely result in faster replenishment of glycogen back into fatigued muscles. High glycemic carbs taken immediately after training also restores hepatic glycogen required to fuel and protect the liver.
Here's a tip for those who like to consume alcohol after playing rugby or soccer or any sport for that matter. Alcohol interferes with glycogen resynthesis and lowers blood glucose levels. Try not to drink every day if you do or you'll never get your glycogen levels up high enough to really enjoy your best strength, endurance and exercise performance. I've watched athletes pile drive alcohol after a big game or practice for years, and usually with hamburgers and fries. Excess alcohol reduces testosterone and insulin sensitivity. The key is timing, balance and control.
In addition to glycemic indexing, it's possible to predict what impact carbs will have on the body and what effect a specific carb load will have on your blood sugar before consuming it. This knowledge can help you select the right foods so you can time the effect according to your performance and recovery needs.
Regardless of the food or the amount eaten you can get a clear picture in advance by knowing both its glycemic index and glycemic load. Wholemeal pasta for example has a moderate GI of 45 but is very carb dense, so the glycemic "load" or carb impact is relatively high compared to an equal portion of cucumber or squash. In these two latter examples most of the weight in grams when weighed on a scale is water, whereas most of the weight of pasta is carbohydrate or sugar.
The glycemic load (GL) of a food is calculated by multiplying its glycemic index by its digestible carbohydrate content in grams and dividing by 100. Indigestible fiber doesn't count. In scientific terminology, each unit of the glycemic load represents the equivalent blood glucose-raising effect of 1 gram of pure glucose or white bread.
How individual foods are rated on the Glycemic Index scale varies a bit depending on which standard you use. I classify foods with a GI of 39 or less as Low, 40-60 as Medium and anything above 60 as High.
Let's say we want to measure the glycemic load of a Golden Delicious apple. First we need to know the glycemic index of that particular kind of apple which is 40. Believe it or not, 7500 varieties of apples are grown throughout the world and each one has its own GI. Then based on the weight or portion size we're going to eat which in this case is 120g, we need to know how many grams of digestible carbs that amount of apple actually contains. The answer is 20g of digestible carbohydrates.
So to calculate the Glycemic Load of a Golden Delicious Apple you multiply the GI (40) by its carbohydrate density or digestible carbs (20). 40 x 20 is 800. Next we divide by 100. The end product is 8. So the Glycemic Load of the apple is 8.
Glycemic Load also has a scale. Low is 10 or less, Medium is 11-19 and 20 or greater is considered High.
Another aspect of glycemic indexing or glycemic value relates to knowing how to predict what affect an entire meal or mix of foods eaten together at one time will have on your blood sugar. To calculate the predicted GI outcome of an entire meal you must first know both the carbohydrate content of each individual food in the meal plus the total carbohydrate content of the entire meal.
You don't have to become a dietician to gain control of your diet; however, you do need a true desire to be well. Once you've come to terms with that, the desire to understand what you put in your mouth and why is only a means to that end.