What is the Glycemic Index?

The Glycemic Index or GI Index is a scale from 0 to 100. It ranks carbohydrates based on how they make your blood sugar rise versus glucose. Some foods have a high value and other have a low value. The value depends on a number of factors.

The basic test goes like this. A group of people are given 25-50 grams of carbohydrates. Their blood is tested over the next 3 hours to track blood sugar and insulin levels.

Blood sugar is glucose - a simple sugar. It is what your body uses for fuel and comes from the breakdown of carbohydrates. If you ate pure sugar, it would go right into your blood stream without having to be broken down or "digested".

Other carbohydrates need to be digested - in the mouth, the stomach or small intestine BEFORE they enter the blood stream.

There is a simple way to track which foods cause fast or slow rises in your blood sugar. Drippers have a low glycemic index between 0 and 55, while flooders have an index of over 70. Glucose is the reference and has a glycemic index of 100.

The higher the glycemic index, the faster your blood sugar will rise. This means that the flooders are easily digested and push glucose into the blood stream faster than drippers.

We need glucose to survive. However, if the glucose cannot get into the cells to provide fuel, then it builds up in the blood stream. When blood glucose (sugar) levels get too high, it can cause damage to eyes, kidneys, blood vessels, heart and nerves. People with diabetes have high blood sugar.

Scientists have discovered that proper blood sugar levels are better. Too high - not good. Too low - not good.

Glycemic Index Range

Carbs, based on their GI rank, are categorized to three groups, Low, Medium and High GI Foods.

What do the numbers mean?

• Low GI foods are foods with a GI less than 55. They cause a slower and lower rise in blood glucose levels. Examples are Porridge, Apple and Sweet Potato.
• Intermediate GI foods are foods with a GI between 55 and 70. They cause blood glucose levels to go up at a moderate rate. Examples are Pineapple, New Potato and Mango.
• High GI foods are foods with a GI greater than 70. They cause a rapid rise in blood glucose levels. Examples are Jasmine rice, Cornflake and Watermelon.

Glycemic index and glycemic load for 100+ foods

Glycemic index and glycemic load offer information about how foods affect blood sugar and insulin. The lower a food's glycemic index or glycemic load, the less it affects blood sugar and insulin levels.

* Compared with pure glucose, which is assigned a glycemic index of 100

The complete list of the glycemic index and glycemic load for 750 foods can be found in the article "International tables of glycemic index and glycemic load values: 2002," by Kaye Foster-Powell, Susanna H.A. Holt, and Janette C. Brand-Miller in the July 2002 American Journal of Clinical Nutrition, Vol. 62, pages 5–56.

What are the Benefits of the Glycemic Index?

Eating a lot of high GI foods can be detrimental to your health because it pushes your body to extremes. This is especially true if you are overweight and sedentary. Switching to eating mainly low GI carbs that slowly trickle glucose into your blood stream keeps your energy levels balanced and means you will feel fuller for longer between meals.

• Low GI diets help people lose and manage weight
• Low GI diets increase the body's sensitivity to insulin
• Low GI carbs improve diabetes management
• Low GI carbs reduce the risk of heart disease
• Low GI carbs improve blood cholesterol levels
• Low GI carbs can help you manage the symptoms of PCOS
• Low GI carbs reduce hunger and keep you fuller for longer
• Low GI carbs prolong physical endurance
• High GI carbs help re-fuel carbohydrate stores after exercise

How to Switch to a Low GI Diet

The basic technique for eating the low GI way is simply a "this for that" approach - i.e., swapping high GI carbs for low GI carbs. You don't need to count numbers or do any sort of mental arithmetic to make sure you are eating a healthy, low GI diet.

• Use breakfast cereals based on oats, barley and bran
• Use breads with wholegrain, stone-ground flour, sour dough
• Reduce the amount of potatoes you eat
• Enjoy all other types of fruit and vegetables
• Use Basmati or Doongara rice
• Eat plenty of salad vegetables with a vinaigrette dressing

Q: What is the difference between glycemic index (GI) and glycemic load (GL)?

A: The GI describes the type of carbohydrate in foods and its potential to raise blood glucose levels. Our actual blood glucose levels are determined by both the quality, or GI, of the carbohydrate and the quantity of carbohydrate. We can predict the effect of a food on our blood glucose level by calculating the glycemic load which is the GI x the amount of carbohydrate, divided by 100. Teaspoon of jam (GI = 51): (51 x 5 grams carb) / 100 = 2.5

Q: Can I download or can you email me a full list of all GI food values?

A: Sorry but we have no such list available. Food testing for GI is expensive and can vary greatly depending on where the food comes from and how it is prepared.

Q: Can you tell me the GI of alcoholic beverages (beer, wine and spirits)?

A: Alcoholic beverages contain very little carbohydrate. In fact, most wines and spirits contain virtually none, although beer contains some (3 or 4 grams per 100 mL). A middy of beer (10 ounces) contains about 10 grams of carbohydrate compared with 36 grams in the same volume of soft drink. For this reason, a beer will raise glucose levels slightly. If you drink beer in large volumes (not a good idea really) then you could expect it to have a significant effect on blood glucose.

Q: Why does some variability occur in the GI for the same food types? For example, baked potato has GIs that vary from 56 to 111, a factor of 2.

A: The GI database confirms the reproducibility of GI results around the world. White and whole meal bread, apples, cornflakes, breakfast cereals etc give the same results wherever/whoever tests them. Where there is variability, there are four possible explanations:

1) Some GI testing groups are not as experienced or as accurate. They use venous blood which gives more variability than capillary blood. If we test a product over and over again, we get the same result ± 5%. That's as good as nutrient data such as protein, fat, fiber etc.

2) The variability among different types of potatoes, rices, and oats is REAL. They contain different types of starch (amylose, amylopectin) and that affects the degree of starch gelatinisation. When it comes to sugars like fructose, the concentration of the solution makes a difference to the rate of gastric emptying and therefore the glycemic response.

3) Sometimes the manufacturer may change the formulation of their product by reducing the fat content for example. Reducing the fat can increase the GI. Manufacturers may have their products retested if they make significant changes to the formulation or source ingredients from different suppliers.

4) Some foods have been tested in people with type 2 diabetes. These values may be higher than that seen in the normal population. Follow the food links in the database to find more information on the testing setup.

Q: Why does pasta have a low GI?

A: Pasta has a low GI because of the physical entrapment of ungelatinised starch granules in a sponge-like network of protein (gluten) molecules in the pasta dough. Pasta is unique in this regard. As a result, pastas of any shape and size have a fairly low GI (30 to 60). Asian noodles such as hokkein, udon and rice vermicelli also have low to intermediate GI values.

Q: Some vegetables appear to have a high GI. Does this mean a person with diabetes should avoid eating them?

A: Definitely not, because, unlike potatoes and cereal products, these vegetables are very low in carbohydrate. So, despite their high GI, their glycemic load (GI x carb per serve divided by 100) is low. Vegetables contain only small amounts of carbohydrate but loads of micronutrients and should be considered as "free foods". Eat them all you like!

Q: Some high fat foods have a low GI. Doesn't this give a falsely favorable impression of that food?

A: Yes it does, especially if the fat is saturated fat. The GI value of potato chips or french fries is lower than baked potatoes. A large amount of fat in foods tends to slow the rate of stomach emptying and therefore the rate at which foods are digested. Yet the saturated fat in these foods will contribute to a much increased risk of heart disease. It is important to look at the type of fat in foods rather than avoid it completely. Good fats are found in foods such as avocadoes, nuts and legumes while saturated fats are found in dairy products, cakes and biscuits. We'd all be better off if we left the cakes and biscuits for special occasions.

Q: Do I need to eat low GI foods at every meal to see a benefit?

A: No, because the effect of a low GI food carries over to the next meal, reducing its glycemic impact. This applies to breakfast eaten after a low GI dinner the previous evening or to a lunch eaten after a low GI breakfast. This unexpected beneficial effect is called the "second meal effect". But don't take this too far, however. We recommend that you aim for at least one low GI food per meal.

Q: Most breads and potatoes have a high GI. Does this mean an end to my favorite foods?

A: Potatoes and bread, despite their high GI, can play a major role in a high carb/low fat diet, even if your goal is to reduce the overall GI. Only about half the carbohydrate needs to be exchanged from high to low GI to derive health benefits. Of course, some types of bread and potatoes have a lower GI and these should be preferred in order to lower the GI as much as possible.

Q: Why doesn't the GI of beef, chicken, fish, tofu, eggs, nuts, seeds, avocadoes, many fruits and vegetables, wine, beer and spirits appear in GI lists?

A: These foods contain no carbohydrate, or so little that their GI can't be tested according to the standard methodology. Bear in mind that the GI is a measure of carbohydrate quality, not quantity. Essentially, these types of foods, eaten alone, won't have much effect on your blood glucose levels.

Q: Why not just adopt a low carbohydrate diet (like the Atkins diet) to keep my blood glucose levels and weight down?

A: Recent studies show that low carb diets such as the Atkins diet produce faster rates of weight loss than conventional low fat diets. The probable mechanism is lower day-long insulin levels - allowing greater use of fat as the source of fuel - the same mechanism underlying the success of low GI diets. We believe that low carb diets are unnecessarily restrictive (bread, potato, rice, grains and most fruits are restricted) and may spell trouble in the long term if saturated fat takes the place of carbohydrate. Low GI diets strike a happy medium between low fat and low carb diets - you can have your carbs, but must choose them carefully.

Q: If testing continued long enough, wouldn't you expect the areas under the curve to become equal, even for very high and very low GI foods?

A: Many people make the assumption that since the amount of carbohydrate in the foods is the same, then the areas under the curve will finally be the same. This is not the case, however, because the body is not only absorbing glucose from the gut into the bloodstream, it is also extracting glucose from the blood. Just as a gentle rain can be utilized better by the garden than a sudden deluge, the body can metabolize slowly digested food better than quickly digested carbohydrate. Fast-release carbohydrate causes "flooding" of the system and the body cannot extract the glucose from the blood fast enough. Just as water levels rise quickly after torrential rain, so do glucose levels in the blood. But the same amount of rain falling over a long period can be absorbed into the ground and water levels do not rise.

Q: Does the GI increase with serving size? If I eat twice as much, does the GI double?

A: The GI always remains the same, even if you double the amount of carbohydrate in your meal. This is because the GI is a relative ranking of foods containing the "same amount" of carbohydrate. But if you double the amount of food you eat, you should expect to see a higher blood glucose response - i.e., your glucose levels will reach a higher peak and take longer to return to baseline compared with a normal serve.

Q: I have recently been diagnosed with celiac disease (gluten sensitivity) on top of diabetes. It's extremely hard to find both low GI and wheat-free foods. Any suggestions?

A: This is not as hard as you may think? If you like Asian food - Indian dahls, stir-fries with rice, sushi, noodles - you're in luck, because they are all low GI. Choose vermicelli noodles prepared from rice or mung beans and low GI rices such as basmati. Use sweet potato instead of potato; use all manner of vegetables without any regard for their GI. Choose fruits and dairy for their low GI. If you can tolerate dairy products, then take advantage of them for their universal low GI. If lactose intolerance is a problem, reach for live cultured yoghurts and lactose-hydrolyzed milks. Even ice-cream can be enjoyed if you ingest a few drops of lactase enzyme first.

Q: Is a low GI diet suitable for vegetarians?

A: The low GI diet is just as easy for a vegetarian to follow - in fact, teaching vegetarians to follow the low GI diet can be easier because most are eating many of the best low GI foods already.

For the vegetarian, the same principles apply: substitute your plant protein sources for the meat. Eat more beans, lentils and other legumes - all among the lowest GI foods we have tested. Quorn is also a great meat substitute with no GI as it has almost no carbohydrate (2 g/100 g).

Some additional points:

• The GI only applies to foods containing significant amounts of carbohydrate. Most vegetables have small amounts of carbohydrate and those that provide more usually have a low GI, with the exception of potatoes. You can therefore tuck into your veggies without considering the GI for every one – and benefit from antioxidants and all the micronutrients they supply!

• Legumes should be a daily part of any vegetarian diet for your protein – happily these are also a mainstay of a low GI diet.

• Almost every low GI food we talk about in the book is suitable as part of a vegetarian diet. Animal products are usually high in fat, protein or both and therefore do not have a GI.

• The range of protein and carb intake that is healthy is fairly broad – as a vegetarian you will inevitable have a higher carb intake and slightly lower protein intake. This makes the GI important for you, but easy to adapt if you choose wholegrain cereal products and legumes as your carbohydrate base.

• Coffee has no carbohydrate (unless you add sugar and/or milk and the GI response comes from these foods) and hence it is not in the GI tables. Neither does it contain calories so has little impact on weight control.

Dr. Syal MD, FACOG