Glycemic Index and Diet
Recently, there has been a lot of chatter about the glycemic index and the possible health benefits of including foods ranked low on the glycemic index into your diet. Here we will take a closer look at the glycemic index and discuss why you might not want to make food choices based solely on their low ranking on this index.
All foods consist mainly of three substances: proteins, fats, and carbohydrates. More than twenty years ago, a Canadian doctor, David Jenkins, and his colleagues at the University of Toronto, developed a numeric rating system classifying carbohydrate-rich foods based on their effect on blood sugar levels following meals. This development arose from the observation that although all carbohydrates have 4 calories per gram, (proteins 4 per gram; fats 9 per gram) the human body’s response to ingesting carbohydrates varies significantly from one food item to another, and from person to person when certain diseases are present.1
Prior to formation of the glycemic index, carbohydrates were defined within duo-classification systems such as simple or complex, sugars or starches, and available or unavailable. The glycemic index offers researchers different insight into the specific nutritional benefits and system affects of various carbohydrates.1,2 While originally intended as a research tool, the index has since found its way – perhaps inappropriately – into our everyday lexicon through fad diets and some nutritional guidelines.
How does the glycemic index work?
The digestive system breaks down carbohydrates into their simplest form, glucose, before this nourishment can enter the bloodstream. Glucose is the universal energy source for our cells.
Foods found to have a high glycemic value break down quickly; causing blood glucose levels to rise sharply, prompting the pancreas to produce more of the hormone insulin which causes a subsequent fall in blood glucose levels. These processes lower blood concentrations of glucose and fatty acids, the body’s two primary fuels, resulting in increased hunger, and possibly triggering overeating, weight gain, and obesity.
On the other hand, foods found to have a lower glycemic value break down slowly, causing a steady release of glucose into the bloodstream, allowing for more even levels, thereby stabilizing the appetite and potentially averting inappropriate appetite triggers.5
While, generally speaking, simple carbohydrates are higher on the glycemic index than complex ones,5 scientists have discovered a few interesting surprises, along with some mitigating factors influencing a particular food’s position on the index. It is interesting that white bread has a higher rating than table sugar, and that fruit leather is higher than a typical chocolate bar!
Glucose, or dextrose, is the final form of sugar once it reaches the bloodstream.
The Disease Link
The most significant factor driving use of the glycemic index for food choices is that several research studies reveal a connection between high-carbohydrate, high glycemic diets and some diseases.3 It is unknown if disease results from the carbohydrates themselves eaten in large quantities, or if their sharp ‘rise and fall’ effect on blood sugar levels contributes to an environment within the body from which diseases arise. It is important to keep in mind that this is still very new science without long-term assessment and verification.
On the plus side, the rapid rise in glucose levels provided by a snack that is high on the glycemic index may be beneficial for post endurance-exercise recovery.2 Some of the research shows that eating lower-glycemic index foods could stabilize blood glucose, lower cholesterol levels, and help with appetite control.4 The theory is that these actions could reduce the risk factors for a number of conditions, including:1,5
- obesity
- heart disease
- type 2 diabetes
- gastric cancer
- breast and colorectal cancer
Moreover, as diseases are often inter-connected, a lower-glycemic diet might exert a secondary influence on the following digestive conditions, possibly due to weight-control and the typically higher fibre content of some lower glycemic foods:
- Gastroesophageal reflux disease (GERD)
- Irritable bowel syndrome (IBS)
- Gallstones
Challenges with the glycemic index
Two Different Scales
When first published, the creators of the index designated glucose with a value of 100, representing the top end of the scale, and calculated all other foods in relation to it. However, a second scale is now also in use, designating white bread as 100, and re-calculating all other foods in relation to it. For example, on the glucose-based scale, glucose is 100 and white bread is 72 while on the white bread-based scale, glucose is 138 and white bread is 100.5 Therefore, comparing foods is confusing because you might not know from which scale the ranking is derived.
Usable Carbs vs. Serving Size
Most food catalogues for nutrients, such as calories or carbohydrates, compare items by typical serving size. However, the glycemic index compares 50 grams of usable carbohydrate per item, not serving size, further confounding the comparison process. For example, carrots, at 92, rank high on the glucose-glycemic index but since there are far fewer than 50 grams of carbohydrates in a typical serving of carrots, the actual glycemic response per serving is relatively low. In this case, the index ranking calculation is for a greater quantity of carrots than will likely be eaten at any one time.
A newer concept, glycemic load, is a measurement that accounts for a food item’s glycemic index value in relation to a typical serving size. According to some, the glycemic load is a more accurate measure of the glycemic effect.1 For examples, see the accompanying chart showing various foods with their ranking on the glucose-glycemic index and their calculated glycemic load.
Unequal Comparisons
Once you learn a food’s numeric glycemic value on whichever of the two glycemic indexes you choose, and adjust for serving size, your comparisons still might not be equal. Although the concept originated in Canada, researchers all over the world are analyzing carbohydrates and reporting their findings. However, living in Canada, you really cannot rely on figures from other countries. In Australia for example, where they are avid proponents of the glycemic index, foods often have differing ingredients and processing methods than in Canada, even when marketed under a familiar brand.1 Furthermore, even when only using Canadian data, there are so many variations in food growing and processing across this country that the glycemic index level indicated might not be accurate for a specific food you choose to consume.
Non-Standardized Testing
Testing facilities and methods are not standardized. Some centres analyze blood from veins while others use fingertips, even though tests comparing venous and capillary blood show significantly different values. Some researchers look at levels two hours after a meal while others use a three-hour interval. In addition, the subjects in these studies may include both healthy individuals and those with impaired carbohydrate metabolisms, such as type 1 or type 2 diabetes; differences that might affect the glycemic response and the resulting glycemic index value.1,2
Other Factors
Several other factors affect a food’s position on the glycemic index. Since we rarely eat only carbohydrates at one meal, we must take into consideration how the other two substances mentioned above, fats and proteins, interact. Also relevant are food preparation and cooking methods, meal size, and several other factors.
Influences that decrease glycemic response
Fat, Protein, and pH value
Eating carbohydrates along with fats, proteins, and more acidic foods slows the digestive process, lowering the glycemic load of the carbohydrates, thereby helping to stabilize blood glucose.4, 5 Surprisingly, this means that a serving of potato chips (51) is lower on the index than a baked potato (85)! But, of course, we’re not suggesting you eat high-fat foods!
Fibre
Fibre shields carbohydrates from immediate and rapid absorption.5 It also makes the digestive system work harder. This is a good thing; think ‘work-out’ for your digestive tract. Soluble fibres, such as those found in fruits and vegetables, also help avoid extreme digestive outputs such as diarrhea or constipation.
Smaller Meals
As peristalsis – regular digestive contracting – carries food through the digestive system, there are many influences. If there is less food in your stomach, then less passes at any one time into the small intestine, where most carbohydrate digestion occurs. Even if what you eat is high on the glycemic index, eating smaller quantities at one time has a lower glycemic effect.
Influences that increase glycemic response
Processing
Processed foods digest more quickly than unprocessed foods, so they are usually higher on the index.4 Apple juice is higher than an apple; white bread higher than whole grain bread; instant rice higher than regular rice.
Cooking
Longer cooking time makes some carbohydrates easier to digest thereby increasing their availability to your body and raising the glycemic load. For example, pasta cooked al dente has a lower glycemic load than overcooked pasta.
Larger Meals
See smaller meals, above, and think opposite. Bigger meals mean a greater glycemic load.
Putting it all together
Until those examining the glycemic affects of foods address some of the issues discussed above, and determine global standards, then strictly relying on a glycemic index might not result in the healthiest dietary choices. In all fairness to its originator, he designed the glycemic index as a research tool, not as a guide for choosing food. It is important to remember that foods are complex and offer more than one component. For example, if you relied solely on a diet that eliminated sources of fats then you would be missing the essential fatty acids found in such foods as almonds and walnuts.
Lessons learned from this discussion of the glycemic index illustrate and reinforce much of the common sense dietary advice advocated for disease prevention given to us by medical and nutrition professionals. These dietary recommendations include:
- Taking care when choosing food to ensure a balanced diet (use Canada’s Food Guide as a reference)
- Selecting foods based on their overall nutritional content
- Looking at the big picture for your dietary health and taking an active role in learning how food affects you
- Reducing the amount of processed food and increasing the quantity of whole foods in your diet
- Eating smaller meals at regular intervals to help keep your blood glucose levels stable
- Making the connection between healthy eating and potentially avoiding some diseases
If in doubt, a registered dietitian can be of assistance to you in making wise dietary choices, which might include reducing your glycemic load. Always consult with your physician before making significant dietary changes.
Glycemic Index vs. Glycemic Load |
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Food (source, location provided when available) |
Glycemic Index (glucose=100) |
Glycemic Load |
| Apple (golden delicious, Canada) |
39 |
6 |
| Apple Juice (unsweetened, Allen’s, Toronto, Canada) |
41 |
12 |
| Baked Potato (Russet, mean of 4 studies, Canada & USA) |
85 |
26 |
| Banana(Canada) |
62 |
16 |
| Brown Rice(Canada) |
66 |
21 |
| Coca-Cola® (Atlanta, USA) |
63 |
16 |
| Corn Flakes®(Kellogg’s, Canada) |
80 |
21 |
| Cornmeal (boiled in salted water, 2min McNair Products, Toronto, Canada) |
68 |
9 |
| Fruit Leather Roll-ups(Uncle Toby’s, Australia) |
99 |
24 |
| Kidney Beans (Canada) |
29 |
7 |
| Peanuts |
13 |
1 |
| Potato Chips (plain and salted, Canada) |
51 |
12 |
| Raisins |
64 |
28 |
| Snickers Bar |
68 |
23 |
| Sweet Corn (Canada) |
59 |
11 |
| Sweet Potato(Canada) |
48 |
16 |
| Watermelon |
72 |
4 |
| White Rice |
72 |
30 |
Table Adapted From: Foster-Powell K, Holt SHA, Brand-Miller JC. International table of glycemic index and glycemic load values. The American Journal of Clinical Nutrition. 2002;76:5-56. |
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