Glycaemic index — what the science says

In a nutshell

The glycaemic index (GI) compares how a food raises blood sugar with pure glucose. Low is 55 or less, high is 70 or more.¹
In people with diabetes, a low-GI diet slightly improves long-term blood sugar.²
For preventing disease in healthy people, fibre and whole grains matter more than the GI number itself.³

1What it is

Volunteers eat a portion of food with 50 g of available carbohydrate and researchers follow how their blood sugar changes. The result is compared with the same amount of pure glucose, which has a GI of 100.^1,4

Definition ≤ 55 Foods with a value of 55 or less have a low glycaemic index, 70 or more a high one. Most legumes, pasta, fruit and dairy products are low.¹

But the GI says nothing about portion size. That is what the glycaemic load is for: the GI multiplied by the grams of carbohydrate in a portion, divided by 100.¹

Calculation 7 vs. 30 Watermelon has a GI of 76, white rice 73.¹ A portion of watermelon (120 g), however, holds only about 9 g of carbohydrate, a portion of cooked rice (150 g) about 42 g.⁵ The glycaemic load of the watermelon comes out at about 7, the rice at about 30.

2Surprising numbers

Data 78 Boiled potatoes have a GI of 78 — more than white bread (75) and white rice (73). White spaghetti has 49, an apple 36 and lentils 32.¹

Data 40 Chocolate has a low GI of 40 and potato crisps 56 — yet they are not the healthier choice. The authors of the international tables therefore warn that the GI should not be used on its own, but together with the energy and make-up of the food.¹

3Everyone responds differently

Moderate evidence 62 ± 15 White bread measured in 63 healthy volunteers had an average GI of 62. But it varied by about a quarter between people, and repeated measurements in the same person varied by about a fifth. The authors therefore do not consider the GI a good guide to choosing food.⁴

Moderate evidence A study of 800 people whose blood sugar was monitored continuously for a week across almost 47,000 meals found large differences in how people responded to the same meals.⁶

4Does eating by GI help?

Strong evidence −0.31% In people with type 1 and type 2 diabetes, a low-GI diet lowered long-term blood sugar (HbA1c) by 0.31 percentage points on average, and slightly lowered LDL cholesterol, weight and blood pressure. This is a summary of 29 comparisons from randomised trials with 1,617 people.²

Strong evidence 25–29 g A summary of 185 long-term and 58 clinical studies: people who ate the most fibre had a 15–30% lower risk of death, heart disease, stroke, type 2 diabetes and colorectal cancer. The greatest benefit was seen at 25–29 g of fibre a day. Diets with a low GI showed smaller or no associations.³

What to take from this

You don't need to count GI. Fibre does more — wholegrain bread, legumes, vegetables and fruit. They usually lower the GI of your whole diet too.³
Watch the portion, not just the type of side dish: how much your blood sugar rises also depends on the amount of carbohydrate on your plate.¹
Have diabetes? A low-GI diet can help — agree on it with your doctor or dietitian.²
ForkRep shows fibre and carbohydrates for each day, so you can easily see whether you are getting close to 25 g of fibre a day.

Sources

Written on 9 Oct 2026, based on studies available at that date. Newer research may change the conclusions.

Atkinson F. S., Foster-Powell K., Brand-Miller J. C. (2008). International tables of glycemic index and glycemic load values: 2008. Diabetes Care 31(12):2281–2283. doi.org/10.2337/dc08-1239
Chiavaroli L. et al. (2021). Effect of low glycaemic index or load dietary patterns on glycaemic control and cardiometabolic risk factors in diabetes: systematic review and meta-analysis of randomised controlled trials. BMJ 374:n1651. doi.org/10.1136/bmj.n1651
Reynolds A. et al. (2019). Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet 393:434–445. doi.org/10.1016/S0140-6736(18)31809-9
Matthan N. R. et al. (2016). Estimating the reliability of glycemic index values and potential sources of methodological and biological variability. Am J Clin Nutr 104:1004–1013. doi.org/10.3945/ajcn.116.137208
USDA FoodData Central – SR Legacy (2018): watermelon (FDC 167765), white long-grain rice, cooked (FDC 169757); available carbohydrate excluding fibre. fdc.nal.usda.gov
Zeevi D. et al. (2015). Personalized nutrition by prediction of glycemic responses. Cell 163:1079–1094. doi.org/10.1016/j.cell.2015.11.001

This is general information and does not replace a doctor or dietitian. If you have diabetes, discuss changes to your diet with your doctor.