FOOD NUTRITION: BLOOD SUGAR
Diagram
How fast a food raises blood sugar, and how much it raises it
Glycemic index
How fast the carbohydrate in a food raises blood sugar, compared with glucose. It ignores how much is eaten.
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Glycemic load
How much a serving raises blood sugar: the glycemic index times the grams of carbohydrate in the serving.
Figure 1: Glycemic index measures speed, while glycemic load measures speed times amount in a serving. Drawn in our own style from the glycemic index discussion in Raffetto, Glycemic Index Diet For Dummies, and in Barclay et al., The Ultimate Guide to Sugars and Sweeteners.
Introduction
Glycemic index and glycemic load both rank foods by their effect on blood sugar, but they measure different things. The glycemic index, GI, measures how fast the carbohydrate in a food is digested and released as glucose into the blood, compared with glucose itself. It says nothing about how much of the food is eaten. The glycemic load, GL, takes the glycemic index and multiplies it by the grams of available carbohydrate in a serving, so it measures the blood sugar effect of a real serving. A food can have a high glycemic index but a low glycemic load if a serving holds little carbohydrate.
Quick comparison
Table 1: Glycemic Index compared with Glycemic Load
| Aspect | Glycemic Index | Glycemic Load |
|---|---|---|
| What it measures | How fast a carbohydrate raises blood sugar | How much a serving raises blood sugar |
| Counts the amount eaten | No | Yes, the grams of carbohydrate in a serving |
| Reference | Glucose, given a glycemic index of 100 | No single reference, it is worked out from the index and the serving |
| Units | A scale from 0 to 100 | A number worked out from the index and the grams eaten |
| High or low | High GI means fast, low GI means slow | High GL means a large blood sugar effect, low GL means a small one |
| Same food, different serving | The index stays the same | The load rises with the serving size |
| Food with little carbohydrate | Can still have a high index | Has a low load, because little carbohydrate is eaten |
| Use | Comparing foods gram for gram of carbohydrate | Planning a meal and judging a real serving |
| Decided by | How fast the carbohydrate is digested | The index and the grams of carbohydrate in the serving |
| Example | White bread has a high glycemic index | A small piece of white bread has a lower glycemic load than a large piece |
Scientific explanation
Definition
The glycemic index is a ranking of carbohydrate foods by how rapidly their carbohydrate is digested and released as glucose into the blood, compared with glucose, which is given a value of 100. The glycemic load is a ranking that measures the amount of carbohydrate in a serving as well as its speed, so it gives the blood sugar effect of a real serving.
Working principle
When a carbohydrate food is eaten, it is digested into glucose, which enters the blood. The glycemic index is found by feeding a portion of the food that holds a set amount of available carbohydrate, then measuring the rise in blood glucose over time and comparing it with the rise from the same amount of glucose. The glycemic load then brings in the serving: it multiplies the glycemic index by the grams of available carbohydrate in the serving and divides by 100. That is why two foods with the same glycemic index can have different glycemic loads, and why the same food has a higher load when the serving is larger.
Apparatus
The books used here name no laboratory apparatus for measuring or comparing either term, so no apparatus list is given.
Measurement procedure
The books used here describe the test in outline, not as a laboratory procedure with volumes and steps, so no procedure is invented here. In outline, a portion of the food holding a set amount of available carbohydrate is eaten, blood glucose is measured over time, and the result is compared with the rise from glucose. The glycemic load is then worked out from the index and the grams of carbohydrate in the serving.
Factors affecting readings
For the glycemic index, the type of starch, the fiber, the cooking, and the ripeness decide the speed: a starchy food that is cooked soft digests faster than one that is firm and fibrous. For the glycemic load, the serving size decides the result as well: doubling the serving doubles the load. Fat and protein in the meal slow digestion and lower the rise, so the same food gives a different response in a mixed meal than on its own.
Formulas and calculations
Glycemic load. Glycemic load = glycemic index x grams of available carbohydrate in the serving / 100.
Worked example: take a food with a glycemic index of 70 and a serving that holds 30 grams of available carbohydrate. Then glycemic load = 70 x 30 / 100 = 21. If the serving is doubled to 60 grams of carbohydrate, the load doubles to 70 x 60 / 100 = 42, while the glycemic index stays 70. The index says the carbohydrate is fast, and the load says how much blood sugar effect the serving gives.
Industrial applications
- Glycemic index: comparing breads, rice and breakfast cereals gram for gram of carbohydrate.
- Glycemic index: choosing slow carbohydrate foods for people who need a steady blood sugar.
- Glycemic load: planning meals, where the serving size decides the blood sugar effect.
- Glycemic load: food labels and diet guides that rank real servings, not just the carbohydrate alone.
- Both: diabetes care and sports nutrition, where the speed and the amount of carbohydrate both matter.
Advantages and limitations
- Glycemic index, advantage: it is a simple scale from 0 to 100 that compares foods gram for gram of carbohydrate.
- Glycemic index, limitation: it ignores how much is eaten, so a food with a high index but little carbohydrate looks worse than it is.
- Glycemic load, advantage: it counts the serving, so it gives the blood sugar effect of what is actually eaten.
- Glycemic load, limitation: it needs the grams of carbohydrate in the serving, so it cannot be used without the serving size.
FAQ
What is the difference between glycemic index and glycemic load?
The glycemic index measures how fast a carbohydrate raises blood sugar, while the glycemic load also counts how much carbohydrate is in a serving.
How is glycemic load worked out?
Glycemic load = glycemic index x grams of available carbohydrate in the serving / 100.
Can a food have a high glycemic index and a low glycemic load?
Yes. If a serving holds little carbohydrate, the load is low even when the index is high.
Does the serving size change the glycemic index?
No. The index stays the same. The load rises with the serving size.
What is the reference for the glycemic index?
Glucose, which is given a glycemic index of 100.
References and further reading
- Raffetto, Meri. Glycemic Index Diet For Dummies. 2nd ed., Wiley.
- Barclay, Alan et al. The Ultimate Guide to Sugars and Sweeteners. 2014, chapter Understanding the Glycemic Index (GI) and Why It Matters, p. 12.
- Gropper, Sareen S., Jack L. Smith and Timothy P. Carr. Advanced Nutrition and Human Metabolism. 7th ed., Cengage, 2016.
- Google Books pages read for this article:
- Gropper, Sareen S., Jack L. Smith and Timothy P. Carr. Advanced Nutrition and Human Metabolism. 7th ed., Cengage, 2016 (Google Books page read)
