The glycemic index resulted from the work of scientists who studied volunteers who ate 50 g of carbs and a control food such as white bread. Blood samples taken from these subjects determined blood glucose levels prior to eating and then at intervals over several hours after eating. They applied the data they obtained to many foods using various calculations. Then they compiled a list of foods and their corresponding value based on how fast or slow the food became glucose in the body after eating. Generally the foods with low values took longest to turn to sugar and therefore are easiest on the pancreas. This list of glycemic values gave birth to the glycemic index.
A practical limitation of the glycemic index is that it does not take into account the amount and type of carbohydrate actually consumed. Meals typically consist of a number of food items rather than a single food and servings of various foods typically do not each consist of exactly 50 g of carbohydrate.
A related measure, the glycemic load, or GL, factors this in by multiplying the glycemic index of the food in question by the carbohydrate content of the actual serving and dividing by 100 to get the glycemic load of a serving of that food. The glycemic load also takes into account the amount of fiber contained in the food. Fiber has shown the ability to slow the metabolic processing of carbohydrates, making the carbs less offensive to the body (Net Carbs are equal to the Total Carbohydrates minus Dietary Fiber).
Therefore, a food that has a moderate GI value but contains lots of fiber may actually have a low glycemic load.
Do this for all the foods in a meal and add the results together. This gives you the glycemic load of the entire meal. Mathematically the equation looks like this: (GI) x (g of carbs – g of fiber) ÷ 100.
For example, a large size carrot has a glycemic index of 47 and contains 8 g of carbohydrate. The carrot’s glycemic load is 47 times 5, or 235, divided by 100 equals 2.35, rounded to 2.
A serving of Vegan Chocolate Shakeology (44g) has a GI of 24 and contains 22 g of carbohydrates minus 5g of fiber for a net carbs of 17 g. (Tropical Strawberry Vegan has 21 g of carbs – 4g of fiber for a net carbs of 17g) Vegan Chocolate’s GL is 24 x 17, or 408/100 =4.
You can use the GL to evaluate to what extent a food will make your blood sugar levels rise after eating it. Foods with a GL value of 10 and below have a low GL and only have a small influence over your blood sugar levels. Foods with a GL value between 11 and 19 have a medium GL and result in a significant increase in your blood sugar levels. Foods with a GL value of 20 and higher have a high GL and lead to a quick and sharp rise in your blood sugar levels.
We tend to equate carbs with sugar, but this is something our society has created. Carbs in nature are usually more complex and, even when they are sugar, such as the case with fructose, they are naturally the type that digest more slowly and, hence, create less havoc with your insulin response. Fructose, as a sugar, is rarely found outside of fruit, although crystalline fructose is an ingredient in some healthy foods trying to add sugar and keep the GI low. Fructose has a GI around 24, whereas table sugar is near 100. HFCS is a mixture of fructose and glucose.
Some once-vilified foods of the GI world, like carrots and watermelon, score low on the GL scale because they simply don’t have the calories to cause the dreaded insulin spike. Potatoes and rice remain high. Whole grain rice and sweet potatoes are much lower but still reasonably high due to their high calorie nature. However, these foods are much better than their processed brethren because of their fiber content. It’s also important to note that some fairly low GI foods can, like Coke or chocolate, which score low due to their mix of ingredients, score extremely high in the GL scale. Ultimately, calories still matter.
Limitations of Glycemic Load’s Use
To calculate Glycemic Load, you must first determine the food’s Glycemic Index. food manufacturers are creating new food products at a much faster rate than GI testing can be performed. Each year, tens of thousands of new packaged-food items are added to grocery shelves, but only a few hundred foods are tested for GI. Because of this, it’s doubtful that we’ll ever reach a point in time where GI is known for all foods.
The way foods are processed, cooked and consumed together influences the glycemic effect of the meal. Food package labels seldom specify GI ranking or glycemic load and many foods are not yet ranked.
The glycemic response to a meal is somewhat complicated and not always intuitive.
A low GI food may not be the healthiest choice. You can’t rely solely on the glycemic index to decide what to eat, as some unhealthy snacks have lower GI values than much healthier foods. A candy bar high in calories and low in nutrients may have a low GI and some ice creams may have a lower GI than potatoes. You need to focus on the overall nutrition of the food, and the calories it contains, as well as the GI value, according to the American Diabetes Association.
A very important thing to realize is that when you go over the data from the GI and GL studies you see and pretty simple trend that those who eat predominately whole foods don’t have trouble and those who eat a lot of processed foods do. Nature provides us with a pretty good diet. If we stick to this we’ll pretty much be okay.
Individual differences in glycemic response
The rate at which different people digest carbohydrates also varies, so there are some individual differences in glycemic response from person to person. In addition it has been shown that one person’s glycemic response may vary from one time of day to another. And finally, different people have different insulin responses (i.e. produce different levels of insulin), even with an identical glycemic response. This fact alone means that a diabetic cannot rely completely on the Glycemic Index without monitoring his own blood sugar response. (This, of course, is a limitation of any food index, and not a specific limitation of GI.)
Reliance on GI and GL can lead to over consumption
It’s important to remember that the Glycemic Index is only a rating of a food’s carbohydrate content. If you use GI and GL values as the sole factor for determining your diet, you can easily end up over consuming fat and total Calories.
For example, Apples have a GI of 38 (as shown in the table above), and a medium-size apple, weighing 138 grams, contains 16 grams of net carbohydrates and provides a Glycemic Load of 6. This is a low GL, and most would consider the apple to be a very appropriate snack. But now look at peanuts. A 4-oz serving not only weighs less than the apple, but has a much lower GI (14), and provides an even lower GL of 2. Based on Glycemic Load alone, you would have to believe that the peanuts were a better dietary choice than the apple. But if you take a look at the Calories contained in these two foods, you’ll see that the apple contains approximately 72 Calories, while the peanuts contain more than 500! Those 400+ extra Calories are NOT going to help you lose weight.
Don’t choose foods based solely on their GI or GL ranking; it’s important to consider the nutrient density and portion size of the foods you eat.
It doesn’t matter whether you are trying to gain control of your weight, seeking to stabilize your blood glucose levels, or simply trying to increase your level of fitness. Experience has shown that the whole process is far more successful and far more permanent when it is part of a low Glycemic Load, low fat program that offers encouragement, support, and most importantly, your personal involvement in choosing what you eat and drink.
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