Understanding glycaemic index and glycaemic load

Carbohydrates, sugar, insulin and diabetes

When we eat food containing carbohydrates, such as bread, potatoes, rice and pasta, our digestive system breaks down the digestible carbohydrates into sugar, which then enters our blood and gives us energy.

When our blood sugar level spikes, the pancreas – located near the stomach – releases extra insulin to lower it. If extra insulin is required regularly, for instance if we take in too much sugar too often in our diet, the pancreas produces too much insulin which may then result in ‘insulin resistance’. This resistance can potentially lead to diabetes, especially in families where there is already a history of the illness. Diabetes can in turn increase other health risks, like stroke and heart attack.

We shouldn’t stop eating carbohydrates – they are an important part of a balanced diet.

We need to try to:

eat more ‘good carbs’, such as wholegrain cereal, fruits, vegetables and wholemeal products
eat fewer ‘bad carbs’, such as free sugars, refined cereals and sweetened beverages

Which foods contain carbohydrates?

Carbohydrates provide energy for our body and are found in many types of food, including:

Starchy foods like rice, noodles, pasta, yams, potatoes and cereals
Sugar, sweetened drinks and other sweet foods
Crisps, pies, biscuits, cakes and pastries
Dairy products such as milk and yoghurt
Fruit and fruit juices

All these foods contain carbohydrates either as hidden sugar (‘intrinsic sugar’) like bread, potatoes, milk and cereals, or as free sugar (‘extrinsic sugar’) such as honey, sugar syrup, fruit juice or ordinary sugar.

The glycaemic measures: GI and GL

We can measure our ‘glycaemic response’ to food – the effect that food has on our blood sugar (blood glucose) levels – using the glycaemic index (GI) and glycaemic load (GL). Both measures can help us manage our diet and reduce the risk of diabetes.

Glycaemic index (GI)

GI is a value between 0 and 100 (glucose = 100) that shows us whether a food raises blood glucose levels quickly, moderately or slowly after we’ve eaten it.¹

Based on their GI value, foods are classified into three categories:

High GI foods release sugar (glucose) quickly, resulting in a rapid rise in blood sugar levels; a sugar level spike. In contrast, low GI foods release sugar gradually over a number of hours, resulting in less insulin being produced.

Food is likely to have a higher GI if it has been processed or refined, if it has been finely ground, if it has a low fibre content and if (in the case of fruits and vegetables) they are ripe.

GI is not the only factor to be considered when deciding if a food is healthy or not. Some foods that are high in fat, such as chocolate, can have a lower GI index because fat slows down the absorption of carbohydrates. Similarly, some foods with beneficial nutrients, such as watermelons, can have a higher GI index.

Glycaemic load (GL)

Research suggests that it is the amount of carbohydrate (the load) you eat, rather than its GI rating, which has the biggest effect on blood glucose levels after a meal. The Glycemic Load (GL) measure includes not only how much carbohydrate there is in a portion of food but also the GI of that food.²

So, what foods should I eat?

Foods with low to moderate GI and GL include:

*Fruits from warmer countries tend to have a higher GI and medium GL, but oranges and grapefruits have low levels.

Some foods have a high or medium GI but a low GL, so the recommendation is to eat them in moderation as part of a balanced diet; for example, watermelon (high GI) and couscous (medium GI).

And what foods should I eat less of?

Try to eat fewer foods with high GI and high GL. For example, white rice, potatoes without skins (potato skins are a good source of fibre), old potatoes, doughnuts, bagels and pizza.

You might have noticed that (with the possible exception of red meat) most low GI/low GL foods are also ones that appear regularly on lists of the kind of foods to include in a healthy diet.

What diseases might a low GI/GL diet help protect against?

Changing to a regular low GI/GL diet can help protect you against several diseases:

Type 2 diabetes. Studies typically show that a high GI diet is associated with increased likelihood of type 2 diabetes. This includes a 2024 study which looked at 128,000 adults aged 35–70 years from 20 countries and found a significant association between a high GI diet and higher diabetes risk.3

Cardiovascular diseases. Foods that cause a higher glycaemic response are associated with a greater risk of coronary heart disease.4 A recent study of 138,000 people found that a high GI diet was associated with increased risk of major cardiovascular event or death.5

Obesity. It has been suggested that a low GI/GL diet helps you feel fuller for longer. This may be a result of higher fibre and protein in these foods. This may mean that you’re less likely to overeat and become obese. This seems to be supported by a 2021 study of more than 2,000 participants undergoing a weight loss programme. The study showed that individuals on a low GI/high protein diet experienced significant decreases in hunger compared with those on a moderate GI diet.6

Some cancers. Recent studies have shown possible associations between various forms of cancer and high GI or GL diets, but the quality of evidence is low7. Associations between a high GI intake and lung cancer8, colorectal, breast and bladder cancers4, and between dietary GL and ovarian cancer risk9 have been reported.

Cholesterol level problems. Studies of populations from Mexico10 and Iran11 suggest that higher GL/GI is associated with lower levels of “good cholesterol” (HDL) and higher odds of “bad cholesterol” (LDL), respectively.

Key points to consider

A healthy, balanced diet is likely to contain many foods with low GLs.
GL is a more realistic measure than GI of how foods affect our sugar levels, as it includes the amount of carbohydrates we consume in a meal.
Low GI/GL foods include most vegetables and fruits that grow in the UK, as well as pulses, fish, nuts and dairy or soya milk.
A low GI/GL diet is associated with a reduced risk of type 2 diabetes, stroke, obesity, some cancers and coronary heart disease.
A low GI diet may help reduce LDL-cholesterol (“bad cholesterol”).
GL and GI should not be considered the sole measures of a healthy diet.

Reviewed and updated by Daniel Piears, October 2025.

Read these Age Watch articles next

Diet: Diet
Diet: Healthy foods
Diet: Healthy eating on a budget
Diet: Cruciferous vegetables – kale, cabbage and broccoli
Diet: Nutritional value of Top 20 foods
Diet: Vegetarian diet
Illnesses: Type 2 diabetes
Tackling obesity: Are carbs really bad for your diet

References

Diabetes UK. Glycaemic index and diabetes. https://www.diabetes.org.uk/living-with-diabetes/eating/carbohydrates-and-diabetes/glycaemic-index-and-diabetes
Diabetes.co.uk. Glycemic Load. 2023. https://www.diabetes.co.uk/diet/glycemic-load.html
Miller M, Jenkins DA, Dehghan M, et al. Associations of the glycaemic index and the glycaemic load with risk of type 2 diabetes in 127 594 people from 20 countries (PURE): a prospective cohort study. Lancet Diabetes Endocrinol. 2014. https://pubmed.ncbi.nlm.nih.gov/38588684/
Jayedi A, Soltani S, Jenkins D, et al. Dietary glycemic index, glycemic load, and chronic disease: an umbrella review of meta-analyses of prospective cohort studies. Crit Rev Food Sci Nutr. 2022. https://pubmed.ncbi.nlm.nih.gov/33261511/
Jenkins, DA, Dehghan, M, Mente, A, et al. Glycemic Index, Glycemic Load, and Cardiovascular Disease and Mortality. N Engl J Med. 2021. https://pubmed.ncbi.nlm.nih.gov/33626252/
Zhu R, Fogelholm M, Larsen TM, et al. A High-Protein, Low Glycemic Index Diet Suppresses Hunger but Not Weight Regain After Weight Loss: Results From a Large, 3-Years Randomized Trial. Front Nutr. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8203925/
Long T, Liu K, Long J, et al. Dietary glycemic index, glycemic load and cancer risk: a meta-analysis of prospective cohort studies. Eur J Nutr. 2022. https://pubmed.ncbi.nlm.nih.gov/35034169/
Zhu L, Shu Y, Liu C, et al. Dietary glycemic index, glycemic load intake, and risk of lung cancer: A meta-analysis of observational studies. Nutrition. 2022. https://pubmed.ncbi.nlm.nih.gov/35576876/
Zhu L, Shu Y, Ran J, et al. Glycemic load, but not glycemic index, is associated with an increased risk of ovarian cancer: A systematic review and meta-analysis. Nutr Res. 2024. https://pubmed.ncbi.nlm.nih.gov/38281319/
Castro-Quezada I, Nunez-Ortega PE, Flores-Guillen E, et al. Glycemic Index, Glycemic Load and Dyslipidemia in Adolescents from Chiapas, Mexico. Nutrients. 2024. https://www.mdpi.com/2072-6643/16/10/1483
Soltani M, Gerami S, Ghaem Far, Z, et al. Higher Glycemic Index and Load Could Increase Risk of Dyslipidemia. Int J Nutr Sci. 2023. https://ijns.sums.ac.ir/article_49316_ef958f4c36a8e5b4e9fbf09f67b1010e.pdf