Understanding the glycemic index
IN THE DAYS when the low-fat, high-carbohydrate diet was king, very few people knew what the glycemic index or GI was all about. It was used mainly by diabetics and athletes to help them maintain stable blood sugar levels.
Today, the low-carb diets reign, and the GI has become part of weight-loss jargon. It was first introduced to the dieting public in books like "The Zone" and "Sugar Busters."
In this year's best-selling diet book, "The South Beach Diet," author and cardiologist Arthur Agatston recommends eating foods low on the GI scale to control sugar cravings and hunger.
The GI is a measurement of how fast a food with carbohydrates will make your blood sugar levels rise. The standard against which all foods are measured is glucose, which will increase your blood sugar by 100 percent in two hours. So glucose is given a numerical value of 100.
A high GI score is 70 and above, a medium GI is between 56 and 69, and a low GI is 55 and below (based on the Revised International Table of Glycemic Index and Glycemic Load Values).
To give you an idea, here is a short list of common foods and their GI.
The theory is that when you eat a high GI food, it raises blood sugar levels quickly, which triggers the pancreas to release the hormone insulin. This, in turn, lowers blood sugar levels just as quickly, which activates the hunger response.
In a study conducted by Dr. David Ludwig, overweight teenage boys were divided into three groups. Each group was given three different breakfasts with the same amount of calories. Group 1 had a low-GI breakfast of fruit and omelette, group 2 had a medium-GI breakfast of steel-cut oats, and group 3 had a high-GI breakfast of instant oatmeal.
To test what effect the different types of breakfasts had on their appetite at lunchtime, Ludwig put the boys in front of an all-you-can-eat buffet. The boys who ate the high-GI breakfast of instant oatmeal ate twice as much as the boys who ate the fruit and omelette.
The take-home lesson here is, the slower the digestion of carbohydrates, the slower the increase in blood sugar. This means that less insulin is needed, which in turn means that, later, there will be a gradual instead of a sharp drop in blood sugar and there will be less hunger at your next meal.
The less hungry you are, the less food you will want and the more in control of your appetite you will be. The end result is an easier and less painful way of losing weight.
There is also good evidence that eating a diet composed mostly of high GI foods can raise the risk of type-2 diabetes, heart disease and obesity.
Scientists believe this is because your pancreas works double-time producing insulin to deal with the high blood-sugar levels. Over time, this can lead to insulin resistance and type-2 diabetes.
The extra insulin can also increase triglyceride levels (the amount of fat circulating in the blood), lower your HDL or good cholesterol and increase the tendency of your blood to form clots.
It would seem that the GI is all you need to lose weight and stay healthy. Unfortunately, it's not that simple. The GI is not a perfect guide to choosing healthy carbohydrates. As one nutritionist put it, the GI is only part of the weight-loss jigsaw puzzle.
Here's what you need to know about the GI to get a more complete picture.
The GI tells you how fast the carbohydrates in a particular type of food will turn into sugar, but it doesn't tell you how many carbohydrates or calories per average real-life serving the food contains.
It's important to know both. Otherwise, you might think that watermelon should be avoided like the plague because it has a GI of 72.
This is where the glycemic load (GL) comes in. The GL is like an extension or expansion of the concept of the GI. It was created by Dr. Walter Willet of the Harvard School of Medicine together with Dr. Jenny Brand-Miller of the University of Sydney, who is one of the foremost authorities on the GI.
The GL takes into account how many carbohydrates there are in a serving of a particular food, not just how fast the carbs turn into sugar. It uses a different rating than the GI.
A GL of 20 or more is high, a GL of 11-19 is medium, and a GL of 10 or less is low.
A watermelon is mostly water. So one serving doesn't contain as many carbohydrates as a serving of potato, which is mostly starch. Thus, a watermelon has a GI of 72 but a GL of 4, while a baked potato has a GI of 85 and a GL of 26.
If you want a comprehensive list of the GI and GL values of 750 types of food, go to this site.
If you aren't the type to look at a long list of numbers, just remember that food with high water or fiber content will not cause a steep rise in blood sugar levels even if it has a high GI rating.
According to Brand-Miller, "Foods that have a low GI invariably have a low GL, but foods with an intermediate or high GI can range from very low to very high GL. Therefore, you can reduce the GL of your diet by limiting foods that have both a high GI and a high carbohydrate content."
High GI food with plenty of starch or sugar will usually also have a high GL, but high GI food with plenty of water and fiber will usually rank low to medium in the GL scale.
High-calorie foods that contain fat and protein as well as carbohydrates/sugar can rank lower than low-calorie foods that do not contain as much fat or protein.
This is because fat and protein slow down the effect that carbohydrates have on blood sugar.
For example, a chocolate bar has a GI of 45 while green peas have a GI of 54. But a chocolate bar contains far more calories than one serving of green peas.
The GL rating gives a truer picture: The chocolate bar has a GL of 14 while the peas have a GL of 4.
Always remember that when it comes to weight loss, calories still count.
Thus, choose carbohydrate foods based not just on their GI rating but also on the basis of their health benefits.
White rice has a GI of 56 while brown rice is 55. If you use only the GI to choose between the two, there is hardly a difference. But brown rice has more fiber and nutrients compared to white rice. Also, brown rice will make you full faster than white rice will.
The GI of a food depends on many things. Different varieties of rice have a GI ranging from 45-92 because they contain different amounts of starch. According to Paula Wart of Vanderbilt University, the amount of cooking and processing also makes a difference.
"Starches in food swell when cooked (whether it's boiled, broiled, baked, or fried)," she writes. "The starch grains in a baked potato swell to the bursting point, whereas the starch grains in brown rice remain relatively unchanged. When grains are rolled, ground, or smashed, the protective (and harder to digest) outer coating is removed. Whole oats have a lower GI than oatmeal, which is made from smashed oat grains."
When you combine a high GI food with a low GI food, you get a medium GI rating. For example, corn flakes with milk.
If you eat a high GI food with food that contains protein and fat, you slow down the absorption of the carbohydrates. An example would be bread with peanut butter or a banana with yogurt.
Acidic food like oranges, vinegar and lemon juice also slow down the digestion of carbohydrates and make you less hungry at your next meal.
This explains why grapefruits and apple cider vinegar have a reputation as "fat burners." They don't burn fat or raise the metabolism, but their acidic content blunt the rise in blood sugar.
Try drinking fresh calamansi juice (diluted with water) with your meals and see if this works for you. But don't be like my high school classmate who used to drink undiluted calamansi juice on an empty stomach because she believed it would burn her fat away. She ended up in the hospital with a very acidic stomach.
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