Glycaemic index: what it tells us and what it is used for

What is the glycaemic index and how is it calculated?

The glycaemic index is a scale that measures how quickly the carbohydrates in a food enter the bloodstream as glucose. To calculate it, a group of healthy volunteers eats a portion of the food containing 50 g of available carbohydrate, excluding fibre. Their blood glucose is then measured over two hours. This curve is compared with the one produced by the same amount of pure glucose, which has a value of 100 [1].

According to the international glycaemic index tables, a value of 55 or less is low, 56 to 69 is medium, and 70 or more is high [1]. A high value means that the starch or sugars are digested and absorbed quickly. A low value means glucose enters the bloodstream more gradually.

This number measures speed alone. It does not tell you how much carbohydrate is in the portion you serve yourself or whether the food is nutritious.

Why is glycaemic load more useful than the index on its own?

The glycaemic index is always measured using 50 g of available carbohydrate. For bread, that is about 100 g, an amount people commonly eat. For rice, it is a plateful of about 180 g once cooked. But with fruit and vegetables that contain a lot of water, reaching 50 g of carbohydrate would mean eating an amount no one would have at once.

Watermelon is the clearest example. For years it was considered a high-index food. In the 2021 tables, the average for four varieties is 50, and the value varies by variety [1]. Watermelon is more than 90% water and contains about 6 g of carbohydrate per 100 g of flesh. A 150 g slice provides about 9 g.

Glycaemic load accounts for this: it is the glycaemic index multiplied by the grams of carbohydrate in the portion, then divided by 100 [1]. A value up to 10 is low, 11 to 19 is medium, and 20 or more is high [1]. A slice of watermelon has a glycaemic load of around 4 or 5, which is low even using the highest values attributed to it. There is no reason to leave it out of your diet.

How cooking and food combinations affect the rise in blood glucose

The glycaemic index of a food changes depending on how you prepare it, how long you cook it and what you eat it with.

Fat, protein and soluble fibre slow stomach emptying and starch digestion. That is why a plate of white rice with chicken, vegetables and olive oil produces a gentler glucose curve than the same rice on its own.

Cooking matters too. When pasta or potatoes are overcooked, heat and water gelatinise the starch. In other words, they break down its structure and make it more accessible to digestive enzymes. Al dente pasta keeps a firmer structure and is digested a little more slowly.

Cooling helps a little. If you cook potatoes, sweet potatoes, rice or pulses and leave them in the fridge overnight, some of the starch reorganises into resistant starch. This starch is not digested in the small intestine and reaches the colon, where gut microbiota bacteria use it. Its effect on blood glucose is modest and partly remains if you reheat the food.

Finally, physical structure matters. Whole fruit retains fibre within its cells and requires chewing. Juicing removes that structure, making it easy to take in the sugar from several pieces at once.

Can following a low-glycaemic-index diet help you lose weight?

An idea became popular for many years: high-index foods were said to cause insulin spikes that make the body store fat and trap you in a cycle of hunger. This is an oversimplification that current evidence does not support.

The European Food Safety Authority (EFSA) reviewed the issue when setting its reference values for carbohydrates. It concluded that lowering glycaemic index and load may improve some risk factors, such as blood lipids. The evidence for their role in weight management, however, is inconclusive [2]. In people with diabetes, a review of trials found small weight reductions with this type of eating pattern [3].

Fat loss depends mainly on energy balance, eating enough protein and preserving muscle through strength exercise. Read our guide to weight loss diets.

How is the glycaemic index used in diabetes and metabolic health?

In type 1 diabetes, type 2 diabetes and prediabetes, blood glucose after eating is an important clinical measure. An analysis of randomised trials found that low-glycaemic-index or low-glycaemic-load eating patterns reduce glycated haemoglobin by about 0.3 percentage points compared with control diets [3]. Glycated haemoglobin reflects average blood glucose over the previous three months.

Based on this evidence, European recommendations for eating with diabetes consider low-glycaemic-index or low-glycaemic-load eating patterns appropriate [4]. They must also meet the other recommendations: enough fibre, little sugar and good-quality fats. The glycaemic index can help you choose between carbohydrate foods, but it does not replace looking at portion size, fibre and the meal as a whole.

There is one situation in which rapidly absorbed carbohydrate is exactly what is needed: low blood sugar in people treated with insulin or medicines that can lower blood sugar too much. When blood glucose is below 70 mg/dL, the guidance is to take 15 to 20 g of glucose and measure again after 15 minutes [5]. Glucose tablets work, or, if these are unavailable, juice or water with sugar. In this situation, rapid absorption is what protects the person.

For cardiovascular health and high cholesterol, the priority is an eating pattern based on minimally processed foods, pulses, vegetables and healthy fats. Monitoring the glycaemic index comes second.

What the number does not tell you

The glycaemic index is useful in research, but it has limitations when applied to meals.

The first is variability. One study measured the glycaemic index of white bread several times in the same people. It varied by an average of 20% from one measurement to another and by 25% from person to person [6]. Sleep, stress, time of day and what you ate beforehand all play a part.

The second is that we rarely eat a food on its own. A meal combines ingredients that affect one another, and the index of the whole meal cannot be calculated by simply adding them up.

What mistakes do we see in consultations?

These are the most common pitfalls when someone starts using the glycaemic index as a guide.

• Avoiding fruit because of its glycaemic index. Watermelon, bananas or grapes disappear from the shopping basket even though their glycaemic load is not high in a normal portion.
• Trusting a “low glycaemic index” label on the packet. A bag of crisps or a chocolate bar may have a lower index than a boiled potato because its fat slows digestion. They are still highly calorific and low in nutrients.
• Looking at the index and forgetting the portion. Switching to wholemeal bread but eating twice as much cancels out much of the change, because glycaemic load also rises with the amount.

That is why we do not ask you to keep checking tables or classify food as good or bad based on a number. Metabolic health is built through sustainable habits, minimally processed foods and a relaxed relationship with food.