Glycemic index (GI) is a value used to rate or categorize the impact that a carbohydrate-containing food has on blood glucose levels.
The use of GI as a weight-loss tool has been accepted in consumer publications and debated by the scientific community.
Foods have been categorized as low, moderate, or high, compared to the reference food, which is glucose or white bread. Below are examples of foods based on their GI.
High GI ≥ 70
- White bread or bagel, graham crackers, dry fruit
- Cornflakes, puffed rice, bran flakes, instant oatmeal
- Short grain white rice, rice pasta, macaroni and cheese from mix
- Russet potato, pumpkin
- Pretzels, rice cakes, popcorn, saltine crackers
- Melons and pineapple
Medium GI 56-69
- Rye bread
- Shredded Wheat
- Ice cream
- Refined pasta
- Brown, wild or basmati rice
Low GI ≤ 55
- Sweet potato, corn, yam, lima/butter beans, peas, legumes, and lentils
- Most fruits, non-starchy vegetables and carrots
- Plain yogurt, strawberries, all bran
- Oatmeal, oat bran, muesli
- Pumpernickel bread or 100% stone-ground whole wheat
- Pasta, converted rice, barley
What affects the glycemic index of a food?
The challenge lies in the fact that determining a food’s GI is difficult without testing. Fat and fiber tend to lower the GI of a food. As a general rule, the more cooked or processed a food, the higher the GI; however, this is not always true.
Below are a few specific examples of other factors that can affect the GI of a food:
- Ripeness and storage time — the riper a fruit or vegetable is, the higher the GI
- Processing — juice has a higher GI than whole fruit; mashed potato has a higher GI than a whole baked potato, stone ground whole wheat bread has a lower GI than whole wheat bread.
- Cooking method — how long a food is cooked (al dente pasta has a lower GI than soft-cooked pasta)
- Variety — converted long-grain white rice has a lower GI than brown rice but short-grain white rice has a higher GI than brown rice.
The GI of food also does not reflect its overall nutritional value. A high glycemic food is not necessarily “unhealthier” or less nutritious, nor is a low glycemic food necessarily “healthier” or more nutritious.
It should also be noted that when these carbohydrates are combined with other nutrients, such as protein or fat, it changes the glycemic effect. For this reason, some experts consider the glycemic load to be a more realistic approach.
What is glycemic load?
Glycemic load is a measure of the glycemic index, multiplied by the number of carbohydrates (CHO) consumed, divided by 100.
Glycemic load = Glycemic index x CHO (g)/100
The glycemic load is useful in that it represents how much a given amount of food will affect blood sugar levels. The thinking behind GI and glycemic load are that if blood sugar is rapidly increased, insulin levels will rise quickly and lead to increased fat deposition.
GI or carbohydrate counting?
There is no one diet or meal plan that works for everyone with diabetes. The important thing is to follow a meal plan that is tailored to personal preferences and lifestyle and helps achieve goals for blood glucose, cholesterol and triglycerides levels, blood pressure, and weight management.
Research shows that both the amount and the type of carbohydrate in food affect blood glucose levels. Studies also have shown that the total amount of carbohydrate in the food, in general, is a stronger predictor of blood glucose response than the GI.
Based on the research, for most people with diabetes, the first tool for managing blood glucose is some type of carbohydrate counting.
Carbohydrate counting, or “carb counting,” is one of many meal planning options for managing blood glucose levels, most often used by people who take insulin twice or more times a day.
Because the type of carbohydrate can affect blood glucose, using the GI may be helpful in “fine-tuning” blood glucose management. In other words, combined with carbohydrate counting, it may provide an additional benefit for achieving blood glucose goals for individuals who can and want to put extra effort into monitoring their food choices.
Is there a link between GI and weight loss or weight gain?
A study conducted by Anderson & Woodend, showed that this relationship leads to speculation that even though carbohydrates, as a whole, promote a feeling of fullness, high-glycemic foods that cause a spike in blood glucose may instead lead to overeating and subsequent weight gain.
When the GI is investigated for its effect on weight loss, the research is inconclusive at best. The ADA, in its position paper on weight management, suggests that low GI foods can be incorporated, but are not essential for a diet to be efficacious in managing weight.
Sloth et al. evaluated the influence of GI on appetite and body weight, as well as risk factors for T2DM over a 10-week intervention. After dividing the groups into high-glycemic and low-glycemic diet programs, researchers assessed diet intake, body weight, and blood samples for lipids, glucose, and insulin.
Upon completion, no differences in energy intake, body weight, or fat mass were detected. The low-glycemic group showed a significantly greater decrease in low-density lipoproteins (LDL) as the only change in blood levels. The results fail to support the assumption that a low-glycemic diet will be more satiating or that high-glycemic diets promote an increase in caloric intake.
The lack of difference in body weight and fat mass implies that the glycemic index alone is an ineffective tool for weight maintenance. In fact, experts agree that the evidence is insufficient to support the use of GI as an agent for weight loss.
Glycemic load on weight loss and cardiovascular risk reduction
A controlled clinical trial was initiated to determine if diets with a low GI and high level of protein had a beneficial effect on weight loss in 129 overweight or obese young adults. The subjects were randomly assigned to one of three diet regimens:
- A reduced fat, high fiber diet
- A high carbohydrate diet with either high or low GI carbohydrates
- A high protein diet with either high or low GI carbohydrates
The results showed that although all participants lost similar amounts of weight (on average), there was a higher proportion of individuals that lost 5% or more of their body weight in the high carbohydrate (55% of total calories), low GI (40 GI, 75 glycemic load) and the high protein (25% of total calories), high GI (57 GI, 87 glycemic load) diets.
These same two groups showed a greater loss of fat mass than those on the high carbohydrate, high GI diet.
In a review of the biomarkers for cardiovascular disease risk, the high carbohydrate, low GI diet group showed a decrease in LDL cholesterol, although no differences were seen in lipid profiles between the other diet groups.
The results of this study should be considered when encouraging people to consume more quality whole grains, thus moving toward a lower GI diet composition.
Researchers in both studies agree that further investigation is warranted.
What is carbohydrate loading?
Individuals training for long distance endurance events lasting more than 90 minutes, such as a marathon or triathlon, may benefit from carbohydrate loading in the days or weeks prior to competition.
Eating more carbohydrates helps muscles store more carbohydrates in the form of glycogen. If more glycogen is stored, it will take longer to deplete the body’s preferred energy source during a prolonged workout.
This effort to maximize available glycogen on race day is the same reason that fitness professionals advise people to taper their workout duration as they approach an event. Those who are carbohydrate loading may gain a few pounds because carbohydrates require water for storage.
Those individuals who are serious about optimizing sports performance may consider a consultation with a sports nutritionist to help them adopt the most appropriate dietary plan and carbohydrate-loading regimen.
While various carbohydrate-loading regimens exist, the following is a one-week sample plan:
- Days 1-3: Moderate-carbohydrate diet (50% of calories)
- Days 4-6: High-carbohydrate diet (80% of calories). This equates to about 4.5 grams of carbohydrate per pound (0.45 kg) of body weight. For a 170 pound (77.2 kg) man, that is 765 grams, or a whopping 3,000 calories from carbohydrates per day.
- Day 7 Competitive event: Pre-event event (typically dinner the night before the event) with >80% of calories from carbohydrates.