Glycemic Index
Allulose StorePage audition
The glycemic index (GI) is a scientifically based measure that expresses the blood sugar-raising effect of a food's carbohydrate content compared to glucose, as a percentage.
This concept plays a fundamental role in food science, research into sugar metabolism disorders, and the development of nutritional recommendations.
Scientific analysis of the glycemic index and its nutritional-physiological relationships
1. Introduction
The glycemic index (GI for short) is a measure of the blood sugar-raising effect of the carbohydrate content of foods, which shows a percentage value compared to glucose.
The concept was introduced by University of Toronto researchers Jenkins and colleagues in the early 1980s, with the aim of characterizing the metabolic effects of carbohydrates more accurately than the traditional “simple–complex” grouping.
Using the glycemic index, it has become possible to identify that certain foods – such as refined wheat bread or potatoes – cause a rapid rise in blood sugar levels, while others, such as lentils or oats, cause a more moderate rise.
This realization fundamentally shaped diabetes treatment, sports nutrition, and health-conscious diet development.
2. The concept and measurement principle of the glycemic index
The GI is determined by examining the blood sugar-raising effect of a given food under standardized conditions after consuming 50 grams of digestible carbohydrates.
The test subjects' blood sugar levels are usually measured at 15-30 minute intervals for two hours.
The resulting area under the glycemic response curve (AUC) is compared to the AUC of the same amount of glucose – the latter is given a reference value of 100.
The glycemic index can be interpreted on a scale between 0 and 100:
- Below 55: low GI
- 56–69: medium GI
- Above 70: high GI
The GI therefore shows the qualitative characteristics of carbohydrates, but does not provide information about the quantity of carbohydrate content in the food.
3. Physiological relationships between blood sugar levels and insulin response
The body regulates blood sugar levels through a complex hormonal system – primarily through the counterbalancing effects of insulin and glucagon.
Eating foods with a high GI value causes rapid glucose absorption, causing the pancreas to suddenly release large amounts of insulin.
This can lead to rapid hypoglycemia following hyperglycemia and increased hunger.
In the long term, repeated insulin overproduction contributes to the development of insulin resistance, metabolic syndrome, and type 2 diabetes.
4. Low glycemic index (GI) and glycemic load (GL)
Carbohydrates with a low glycemic index (GI) and glycemic load (GL) result in a slower, more gradual rise in blood sugar levels, as illustrated in Figure 1:
Foods with different GI and GL values, thus affecting blood sugar and insulin levels, over time
The glycemic index reflects how quickly a food raises blood sugar levels, while the glycemic load (GL) takes into account both the quality (GI) and quantity of carbohydrates, giving a more complete picture of its effect.
Foods with high GI (≥70) and high GL (≥20), shown by the red curve, cause a sudden rise in blood sugar levels, peaking approximately 30 minutes after consumption.
This rapid rise triggers a significant insulin response, which then rapidly lowers glucose levels, often below baseline, potentially causing reactive hypoglycemia:
This blood sugar 'roller coaster' is common with refined or processed carbohydrates that are absorbed quickly.
Foods with a medium GI (56–69) and medium GL (11–19), shown by the yellow curve (Figure 1), produce a more moderate rise in glucose and insulin levels, with a lower peak and a slower return to baseline.
This intermediate response is typical of foods with a balanced carbohydrate composition, which provide a sustained glucose supply and avoid extreme fluctuations.
Foods with a low GI (≤55) and low GL (≤10), illustrated by the green curve (Figure 1), have only a minimal impact on blood sugar and insulin levels.
The gradual increase and sustained availability of glucose , characteristic of foods high in fiber and complex carbohydrates, helps stabilize blood sugar levels and minimize excessive insulin release, supporting metabolic health.
5. Factors affecting GI
The glycemic index can be modified by several factors:
- Carbohydrate structure:
Starches rich in amylose (e.g. basmati rice) are digested more slowly, while foods with amylopectin dominance (e.g. corn) result in a faster glucose release.
- Fiber content:
Soluble fiber (e.g. in oatmeal, legumes) reduces the rate of absorption.
- Effect of fat and protein:
both slow down gastric emptying, thus reducing blood sugar spikes.
- Technological processing:
Heat treatment and pureeing increase it, while cooling (through the formation of resistant starch) reduces the GI.
- Maturity and food combinations:
for example, ripe bananas have a higher GI than unripe ones; fibrous or acidic components (lemon juice, vinegar) can reduce the overall GI.
6. The concept of glycemic load (GL)
GI alone does not show how much a given serving actually causes a rise in blood sugar, which is why the concept of glycemic load (GL) was introduced.
Calculation method
GL = GI × carbohydrates (g/serving) : 100
GL categories
- low: <10
- medium: 11–19
- high: ≥20
For example, watermelon has a GI of 74, but because it has a high water content and low carbohydrate content, its glycemic load is only 4 – so the measured blood sugar effect is low.
7. Classification of foods according to glycemic index
According to databases based on the glycemic index:
- High GI (above 70):
white bread, mashed potatoes, cornflakes, rice pudding, honey, grapes, puffed rice.
- Medium GI (56–69):
cooked rice, oatmeal, sweet potato, banana, pineapple.
- Low GI (0–55):
dairy products, legumes, some fruits (apples, pears), oil seeds, vegetables.
Such classifications are relevant not only in clinical dietetics, but also in food industry development, for example for the development of products with reduced GI (functional foods).
8. The impact of the glycemic index on health
8.1 Diabetes and insulin resistance
Several clinical trials have shown that low-GI diets reduce HbA1c levels, reduce postprandial (after-meal) glucose fluctuations, and increase insulin sensitivity.
A high-GI diet, on the other hand, is associated with an increased incidence of type 2 diabetes.
8.2 Cardiovascular risk
A low-GI diet improves the lipid profile: triglyceride and LDL cholesterol levels decrease, while HDL levels increase.
At the same time, the concentration of C-reactive protein (CRP) also decreases, indicating a reduction in inflammatory processes.
8.3 Weight control and satiety
The slow rise in blood sugar levels and gradual insulin response reduces hunger and promotes body fat loss.
A low GI diet provides a more stable energy supply, which is also beneficial in weight loss programs.
8.4 Sports physiology aspects
For athletes, GI plays a key role in the timing of energy intake.
A low-GI meal before exercise is recommended to maintain sustained energy levels, while high-GI meals immediately after exercise improve glycogen replenishment.
9. Scientific limitations and criticisms of GI
Although GI is a useful indicator of carbohydrate quality, several limiting factors are known:
- Individual glucose responses can vary by up to 25–30% for the same food.
- The combination of foods can modify the actual blood sugar effect.
- Laboratory measurements do not always reflect the true glycemic response per meal.
- It does not take into account the nutrient density or micronutrient content of the food; for example, low GI chocolate is not necessarily a healthy choice.
10. Future research directions
New directions of research include:
- the use of continuous glucose monitoring (CGM) to accurately measure real-life GI responses;
- exploring the relationship between the gut microbiome and glycemic response;
- the creation of personalized GI profiles that take into account genetic and lifestyle factors;
- AI-based diet planning , where personalized glycemic patterns can be created by analyzing large data sets.
11. Summary
The glycemic index is one of the most significant developments in nutrition science, allowing for the qualitative assessment of carbohydrates based on their blood sugar effect.
A low-GI diet has been proven to improve metabolic parameters, aid weight control, and reduce the risk of chronic diseases.
However, GI alone is not sufficient for nutritional decisions — only a complex assessment of glycemic load, composition, nutrient density, and individual response can truly create an effective diet.
