The words are bandied about all over — glycemic index, glycemic load, low-glycemic diet — but who really gets it? It’s pretty scientific stuff, and it’s not exactly intuitive. Maybe you heard that carrots are high-GI? (They’re not, as it turns out.) Are all high-carb foods also high-glycemic? It’s time to get the lowdown.
The glycemic index (GI) is a measurement that indicates a food’s effect on blood sugar levels. Foods with carbohydrates that break down quickly during digestion have high GI values. A person’s blood-glucose response to these foods is both rapid and high.
The way scientists come up with GI values is through clinical tests. A volunteer is given a portion of food that contains 50 grams of carbohydrate. Over the next two hours (or three if the volunteer has diabetes), they test the volunteer’s blood glucose every 15 minutes for the first hour, and then every 30 minutes thereafter.
These readings are compared to that volunteer’s readings after consuming pure glucose. If the food raised the volunteer’s blood sugar 60 percent as much as glucose did, the GI value would be a 60. A food’s GI value is an average of volunteers’ responses.
Technically speaking, the glycemic index is the area under the post meal glucose curve that compares how the same person in a fasting state (no food or caloric drinks for 8 hours) responds to the same 50 grams of digestible carbohydrate from a standard food (usually white bread or glucose).
Let’s visualize this. You wake up with nothing in your system. You take 50 grams of sugar. The glycemic index measures how long the sugar stays in your system, not how rapidly glucose values peak. You do this a few other days to see the consistency with which the body responds.
Research shows a significant variability between individuals and food types. For example, rice has a standard deviation of plus or minus 38, meaning, the same person having the same 50 grams of rice will have a wide range of glucose values. When was the last time you only ate 50 grams of one type of food? We normally eat foods with other foods. So to cut through the jargon, here’s a quick primer to help you understand the science behind the glycemic index.
Blood Sugar Response in a Nutshell
Quickly digested, high-glycemic-index (GI) carbohydrates (known as “gushers”) enter the bloodstream rapidly, causing a surge of glucose. This “gush” of glucose is met with an equalizing amount of insulin to move the glucose into muscle and fat cells.
The result is an overcompensation –– and low blood glucose. Looking for a quick fix to the unpleasant hypoglycemic feelings, people often choose another high-GI food, another glycemic surge occurs, and a roller coaster pattern emerges. This leaves the body feeling hungry and fatigued, while inducing fat storage rather than fat burning.
Slowly digested, low-GI carbohydrates (“tricklers”) are also metabolized into glucose but their entry into the bloodstream is stretched over an extended period of time. Instead of a glycemic surge, there’s a glycemic “trickle.” The body responds to a slower, smaller, infusion of glucose with a reduced insulin release.
Rather than having too much and then too little glucose circulating in the blood, the body experiences a steady supply of readily available energy. With no hyper- or hypoglycemic conditions to correct, there are no hunger cravings, no energy lulls, and no fat storing. The body has found its comfort zone and rides through it in “cruise control.” Once this internal homeostasis is set, the body naturally seeks to perpetuate these conditions, and low-GI eating becomes an easily acquired lifestyle rather than a “diet.”
Since its first mention in 1981, the glycemic index (GI) has received erratic support from mainstream organizations. The American Diabetes Association (ADbA) has acknowledged, at times, that various carbohydrates have different glycemic responses and that low-GI foods may reduce blood glucose levels.
The ADbA has conceded in recent years that low-GI diets provide a 0.43 percent decrease in A1C over and above the reduction achieved by diets in which only the amount of carbohydrate consumed is considered. The 2007 Nutrition Recommendations for the Management of Diabetes from the ADbA identify monitoring the amount of carbohydrate consumed as key to achieving glycemic control, but also mentions the additional benefit provided by use of the glycemic index.
The Joslin Clinical Oversight Committee recommends reducing both the quality (GI) and the quantity (glycemic load or GL) of carbohydrate choices for overweight and obese adults who have type 2 diabetes, prediabetes or are at high risk for developing type 2 diabetes.
The American Dietetic Association (ADA) continues to adhere to the traditional exchange/carbohydrate-counting mindset, maintaining that, “for people with diabetes, monitoring total grams of carbohydrate remains the key strategy.” The controversy over a robust endorsement of the glycemic index for blood sugar management continues.
Gushers vs. Tricklers
How does one identify gushers and tricklers? Several factors affect the digestibility of a carbohydrate and its GI value. The most important GI determinant is the physical state of the starch. The development of grinding, milling, and other food processing techniques over the past 200 years has had the greatest impact on the quality of the carbohydrates in our current diet.
Removing outer fibrous layers and reducing the particle size of grains such as wheat enhances quick digestion. This is why breads, crackers and other bakery products made from finely milled enriched flours have high GI values, while stoneground products that are dense and grainy have lower GI values.
Along with milling and the like, simply cooking in water can have a big impact on the physical state of a starch. The longer a starch is exposed to water, the more gelatinized its particles, the more swollen it becomes, and the faster it turns into glucose. Thus, overcooking any type of pasta, for example, will increase its GI value. The presence of soluble fiber (rolled oats, beans, lentils, apples), acids (sourdough bread, pickled vegetables), and fat and protein are all factors that slow digestion and release glucose more slowly into the blood.
The following examples are adapted from Jennie Brand-Miller’s The New Glucose Revolution: The Authoritative Guide to the Glycemic Index (Marlowe and Company, 2003). Check out the Official Site of theGlycemic Index, to determine the GI of just about any food that you eat.
Contrary to popular belief, pasta is a low-glycemic food! Although it’s relatively high in carbohydrates, pasta actually has a low GI: One cup of spaghetti has 43 grams of carbs and a GI of 44 (55 and under is considered low). According to http://www.glycemicindex.com (the official website of the GI), this is “because of the physical entrapment of ungelatinised starch granules in a sponge-like network of protein (gluten) molecules in the pasta dough.”
Pasta should be cooked al dente (“to the bite”). The texture should be slightly firm and chewy. Overcooking boosts the GI. A half-cup of al dente pasta combined with a protein and plenty of vegetables may have only a moderate effect on your blood sugar. Of course, everyone is different, so you should always test.
Indeed, even white table sugar does not have the highest GI value of any food! Sugar, or sucrose, has an average GI of only 68. (Values between 56 and 69 are considered medium, and 70 and above are high. GI values range from 0 to 100, with 100 being the GI of straight glucose solution.) Many foods rank higher than table sugar, with some potatoes, cereals, and white rice rating in the 90s. And white jasmine rice, surprisingly, has a GI of slightly over 100.
Similarly, the highest GI fruit tested to date is NOT the mango! Mangoes, like some other super-sweet foods, are one of the lowest-GI fruits at 51 (55 and under is considered low). The highest-GI fruit? Watermelon — which has a GI of 72! However, there are only 6g of carbs in a 4-ounce serving. So it may work fine within your meal plan. Monitor your blood sugar closely to see how your body responds.
And the lowest GI fruits tested to date are cherries. Cherries have a GI of 22. Grapefruit is the second lowest of the fruits that have been tested, with a GI of 25. Apples, pears, plums, and tomato juice are next — all of which are under 40.
Again, it is a myth that Carrots are a high-GI vegetable. When carrots were originally tested more than 20 years ago, only five volunteers were used and there was great variation in their results. The GI value averaged out to 92. In the early days of GI testing, fewer volunteers were used and foods were only tested once.
When carrots were tested again more recently, ten volunteers were used and the test was done twice. The average value of 32 emerged, with little variation. Unfortunately, the early mistake had many people up in arms over the idea that healthy carrots should be limited or excluded from any diet, and gave the GI approach a bad rap. So, go enjoy carrots — they’re good for you, of course!
Sourdough bread, too, does not have a high GI value. However, whole-grain pumpernickel is the number-one lowest, with a GI of 51, and sourdough is a very close second at 52. In the process of making sourdough bread, lactic acid and propionic acid are produced. These acids can reduce blood glucose levels by 22 percent compared to normal bread.
Other acids, such as those found in vinegar and lemon juice, also have powerful blood-glucose lowering effects. Keep this in mind when using vinaigrettes, marinades, and sauces with these ingredients. Other breads with lower GI values are those that are “stoneground” (as in Indian wheat flour) and “100 percent whole” grain and also those made with sprouted grains.
It is a myth that the higher the fiber content of a food, the lower the GI value. High-fiber does not guarantee low GI. Whether or not the fiber in a food slows digestion and moderates blood-sugar effect depends on the type of fiber and the state it’s in.
Finely ground wheat fiber, which is in whole-wheat flour, doesn’t slow digestion or moderate blood-glucose response. This is why any product made with whole-wheat flour will have a GI value similar to that of its white-flour counterpart.
Both the method of processing and the method of cooking affect the GI of foods, too. Some whole grains have a high GI value, despite their fiber content, because of how they’re cooked. Puffed wheat, for example, because its grains are so well cooked, has a GI of 80. If the fiber is still intact, as it is in whole grains that are not finely ground or overly cooked (cereal such as All Bran), it can act as a physical barrier during digestion, and this reduces the impact on blood sugar.
Viscous, soluble fiber — found in legumes, oats, psyllium, and apples — thickens the mixture of food entering the digestive tract, which slows digestion. The end result is a lower blood sugar response, so these foods have lower GI values.
It is important to remember that the glycemic index and the glycemic load are NOT essentially the same thing. The Glycemic Index (GI) is a measure of the effects of carbohydrates on blood glucose levels. Carbohydrates that break down quickly during digestion, releasing glucose rapidly into the bloodstream (like those found in white bread), have a “high GI” (70 or higher); carbohydrates that break down slowly, releasing glucose gradually into the bloodstream (like those in whole grains and legumes), have a “low GI” (55 or lower).
The Glycemic Load (GL) is a ranking system for the glycemic impact of foods, based on their carbohydrate content, portion size, and Glycemic Index. Low = 1 to 10; Medium = 11-19; High = 20 or higher.
The GL was developed by Harvard researchers, who posited that eating a small amount of a high-GI food would have the same effect on blood sugar as would eating large amounts of a low-GI food. Another issue with looking only at the GI of a food is that it’s tied to the number of grams of carbohydrates in that food and, obviously, that number varies by large amounts.
Watermelon is a good illustration of this problem. Watermelon’s GI is high, 72. The GI, however, is based not on a normal portion, but on 50g of carbohydrates — whatever the food . To get 50g of watermelon carbs, you’d have to eat almost 5 cups. GL combines both the quality and the quantity of the actual carbohydrates consumed — and provides one “number.” The GL of one cup of watermelon is about 9, which is low.
This leads us to our next step: understanding the math.
Not to worry, we’re not talking about advanced algebraic equations; rather, the formula for calculating GL requires only some basic multiplication and division.
GL = (GI x the amount of carbohydrate) divided by 100.
Using an apple as an example, let’s input the fruit’s GI score of 38 and its 13 grams of carbohydrates into the equation and see what the resulting GL turns out to be.
GL = (38 x 13) / 100
GL of an apple = 5
Easy enough, right? Well then, how about we try something a bit starchier, like a baked potato (GI = 85; Total Carbs = 14)?
GL = (85 x 14) / 100
GL of a baked potato = 12
Based on these simple calculations, we see that the potato will have twice the glycemic effect on blood sugar as the apple will. Knowing this can help you make better decisions toward keeping blood sugar steady — and reducing the chances of unexpected spikes.
So, GI or GL? The Answer is Both
To keep your blood sugar in a healthy range, the best strategy is to consume a lot of low-GI carbohydrates but watch your portion sizes and use the GL calculation as a reality check on your choices. And remember, these tools are for carbohydrate foods — grains, starches, legumes, fruits, and veggies.
Other rules apply when making choices about proteins and fats, of course. And even if you master the GI and GL approach, remember that diabetes is very individual, and the only way to know how foods will truly affect your blood sugar levels is to test yourself.
In short, you can think of GL as the amount of carbohydrate in a food “adjusted” for its glycemic potency, according to Jennie Brand-Miller, Ph.D., the foremost expert on the glycemic index.
Besides controlling your blood sugar, eating a low-glycemic diet could prevent you from losing your eyesight as you age. Scientists at the Laboratory for Nutrition and Vision Research have found that consuming a diet with a high glycemic index (but not high in total carbohydrates) increases the risk of developing early age-related macular degeneration (AMD), and the severity of AMD increases as the average glycemic index of a person’s diet goes up.
AMD is an eye disease that typically develops after middle age, and it is one of the leading causes of blindness. People with diabetes are at an increased risk for AMD. These findings are important because the number of people in the United Sates with vision impairment from AMD is expected to double by 2020, according to these researchers. One of the study authors said that 20 percent of the cases of advanced AMD might have been prevented if those people had consumed a lower-glycemic index diet.
Low-glycemic foods tend to be more satiating and help manage hunger. Low-GI foods spend a longer time in the intestines so you feel full longer. Also, because many low-GI foods are high in fiber, they have more bulk and water, which also increases satiety. Last, high-GI foods cause a blood-sugar spike, which is followed inevitably by a dive. The stress hormones released when the body is in this mild, hypoglycemic state stimulate appetite, in an attempt to correct the situation.
But, as as you can guess reading the above, the glycemic index can be very misleading. “Regular fizzy drinks (sodas) have a low glycemic index! Potatoes have a higher glycemic index than pizza. Pizza has many more carbohydrates per serving than do potatoes. The end result is a glycemic load (or overall glucose effect) that is much lower for potatoes than pizza,” Theresa Garnero, a certified diabetes educator, points out.
“Evidence shows no significant positive effect of eating low glycemic index foods. Over 20 studies show the benefits of choosing a variety of carbohydrates and keeping them consistent. Whether a person is a vegetarian or a carnivore, the key is to focus on total carbs. Glycemic index by itself has limited value and application,” she concludes.
But obviously, we haven’t heard the last of this yet.