The Standard Clinical Testing Procedure
Determining a food's glycemic index (GI) is a rigorous scientific process conducted in a laboratory setting with human volunteers, rather than a simple mathematical calculation based on a nutrition label. The internationally recognized protocol involves several key steps to ensure accuracy and consistency.
Preparing for the Test
The process begins with selecting healthy volunteers, typically a group of at least 10 individuals. Participants must observe a 10 to 12-hour overnight fast before each testing session. This ensures that their baseline blood glucose levels are stable and not influenced by recent food intake. On the test day, participants consume a portion of the test food containing a specific amount of available carbohydrate, usually 50 grams, over a fixed period.
Measuring the Blood Glucose Response
Once the test food is consumed, blood samples are collected at regular intervals over the next two hours. Common time points include 0 (fasting baseline), 15, 30, 45, 60, 90, and 120 minutes. A trained technician typically collects capillary blood from a finger prick using a device like the Hemocue 201+ Glucose analyzer, a reliable method for blood glucose analysis. These measurements track how the test food influences the volunteers' blood sugar levels over time.
Calculating the Incremental Area Under the Curve (iAUC)
The blood glucose data is then plotted on a graph, and the incremental area under the curve (iAUC) is calculated. The iAUC represents the total rise in blood glucose above the fasting baseline over the two-hour test period. To ensure only the relevant data is used, any area below the fasting baseline is ignored. Geometric methods, such as the trapezoidal rule, are used to precisely calculate this area.
Comparing to a Reference Food
To establish a standard, the same volunteers are tested on a separate occasion with a reference food. This reference food is typically pure glucose (given a GI of 100) or white bread, also containing 50 grams of available carbohydrate. The iAUC is calculated for the reference food, and for accuracy, the reference test is often repeated multiple times for each participant to get a reliable average.
The Final GI Formula
The final glycemic index value for the test food is a ratio comparing its iAUC to the iAUC of the reference food, multiplied by 100.
$GI = (\frac{\text{iAUC of Test Food}}{\text{iAUC of Reference Food}}) \times 100$
This calculation is performed for each individual subject, and the final reported GI value is the average of these individual results.
Factors That Influence a Food's Glycemic Index
While the testing method is standardized, a food's GI value is not fixed and can be affected by various factors.
Processing and Cooking: More processed foods tend to have a higher GI. For instance, instant oatmeal has a higher GI than steel-cut oats, as the processing breaks down the starches, making them easier to digest. Similarly, the cooking time and method can alter GI, with softer-cooked pasta having a higher GI than 'al dente' pasta.
Fiber Content: Foods with high fiber content, particularly soluble fiber, tend to have a lower GI. Fiber slows down digestion and the absorption of glucose into the bloodstream.
Fat and Protein Content: The presence of fat and protein in a meal also slows down stomach emptying and digestion. This results in a lower overall glycemic response for that meal, even if it contains high-GI components.
Ripeness: The ripeness of fruits can affect their GI. An unripe banana, for example, has a lower GI than a fully ripe one.
Beyond the GI: Understanding Glycemic Load
While GI ranks the quality of a carbohydrate, it doesn't account for the quantity typically consumed. This is where the concept of Glycemic Load (GL) becomes important. GL provides a more complete picture of a food's real-life impact on blood sugar by combining its GI with the portion size.
$GL = (\frac{\text{GI of food}}{100}) \times \text{available carbohydrate (g) per serving}$
For example, watermelon has a high GI (76), but a standard serving contains very few carbohydrates, resulting in a low GL. Conversely, a bowl of white rice has a GI comparable to watermelon but has a significantly higher carbohydrate load, leading to a much higher GL. Focusing on a food's GL, along with its GI, provides a more practical guide for managing blood sugar.
Comparing High and Low Glycemic Impact Foods
Choosing foods with a lower glycemic impact can help maintain more stable blood sugar and energy levels throughout the day. Here are examples of low, medium, and high GI foods:
- Low GI (55 or less): Beans, lentils, non-starchy vegetables (like broccoli, spinach), most fruits (apples, pears), oats, and grainy bread.
- Medium GI (56-69): Wholemeal bread, basmati rice, couscous, sweet potatoes, and raisins.
- High GI (70 or more): White bread, most processed cereals, white rice, russet potatoes, and rice cakes.
Glycemic Index vs. Glycemic Load Comparison
| Food (Standard Serving) | Glycemic Index (GI) | Available Carbs (g) | Glycemic Load (GL) | Note |
|---|---|---|---|---|
| Watermelon (1 cup) | 76 | 11 | 8 | High GI, but low GL due to low carb density |
| Baked Potato (1 medium) | 111 | 30 | 33 | High GI and high GL |
| Apple (1 medium) | 39 | 15 | 6 | Low GI and low GL |
| White Rice (1 cup) | 66 | 53 | 35 | Medium GI, but high GL due to large serving size |
| Lentils, Boiled (1 cup) | 29 | 24 | 7 | Low GI and low GL |
The Role of Glycemic Science in Nutrition
Understanding glycemic science is particularly important for individuals with diabetes, as it provides a valuable tool for managing blood glucose levels. Choosing lower GI foods can help avoid the sharp spikes and crashes in blood sugar associated with high GI foods. For the general population, a diet focused on lower GI foods has been associated with a reduced risk of developing type 2 diabetes and cardiovascular disease. Furthermore, since low GI foods are digested more slowly, they can promote a longer-lasting feeling of fullness, which can be beneficial for weight management. While not the only tool for healthy eating, GI and GL offer powerful insights into how food affects our bodies.
Conclusion: The Nuance of Glycemic Response
The clinical calculation of a food's glycemic index is a complex process, involving human subjects, controlled feeding, and meticulous blood glucose monitoring. The result provides a reliable, standardized ranking of how carbohydrate-containing foods affect blood sugar. However, the GI of a food is not the sole determinant of its health value, nor is it a constant. It can be influenced by preparation, fiber, and other macronutrients in a meal. By considering both the GI and the glycemic load (which accounts for portion size), individuals can make more informed and practical dietary decisions. Embracing low GI principles generally means prioritizing whole, less-processed foods, leading to a diet that promotes stable energy levels and long-term health benefits.
For more information on GI values of common foods, consult reputable sources such as the University of Sydney's GI website.
