Two questions about any food
In their 2016 paper in the Journal of Insulin Resistance, Dr David Unwin, Dr David Haslam and Dr Geoffrey Livesey reduce the effect of a food on blood glucose to two plain questions. How carby is it? And how sugary is that particular carb? The first question is about the amount of digestible carbohydrate in a portion. The second is about how strongly that carbohydrate raises blood glucose. Glycaemic index answers the second question. Glycaemic load combines both.
Glycaemic index
Glycaemic index, or GI, is measured in a laboratory. Volunteers who have fasted overnight eat a portion of a food containing a fixed amount of available carbohydrate, usually 50g. Their blood glucose is measured at intervals over the next two hours, and the area under that curve is compared with the curve after the same amount of pure glucose. Glucose is set at 100. A food whose carbohydrate produces half the response of glucose has a GI of 50.
Published GI values are averages, typically from around ten people. The figures on Dr Unwin's charts come from the International Tables of Glycemic Index and Glycemic Load Values, compiled at the University of Sydney and published in Diabetes Care in 2008. By convention, a GI of 55 or less is called low, 56 to 69 medium, and 70 or more high.
GI has one limit that matters a great deal in practice: it says nothing about how much carbohydrate a normal portion contains. It is a property of the carbohydrate, not of the plate. Watermelon has a high GI, but a slice is mostly water. Broccoli has a GI of 15 and so little carbohydrate that the GI barely matters.
Glycaemic load
Glycaemic load, or GL, puts the portion back in. The formula is short:
GL = GI × grams of available carbohydrate in the portion ÷ 100
The result is expressed in grams: the amount of pure glucose that would have about the same effect on blood glucose as the portion. By convention, a GL of 10 or less per serving is called low, 11 to 19 medium, and 20 or more high.
The paper's own worked example uses a 150g baked potato:
Carbohydrate: 17g per 100g × 150g ÷ 100 = 25.5g
Glycaemic load: 25.5g × GI 86 ÷ 100 = 22g
GI and GL do not rank foods the same way
The table below lists foods from Dr Unwin's charts in order of GI, highest first, with the teaspoon figure for the portion shown. The two columns do not run in the same order.
| Food | Portion | GI | Teaspoons |
|---|---|---|---|
| Watermelon | about 3/4 cup diced (120g) | 80 | 1.8 |
| Rye bread (69% whole-grain rye flour) | 1 small slice (30g) | 78 | 4.0 |
| Sweet corn, boiled | about 1/2 cup (80g) | 60 | 4.0 |
| Peas, frozen, boiled | about 1/2 cup (80g) | 51 | 1.3 |
| Apple (Golden Delicious) | 1 small apple (120g) | 39 | 2.2 |
| Spaghetti, white, boiled | 1 plate, cooked (180g) | 39 | 6.6 |
From glycaemic load to teaspoons of sugar
A glycaemic load in grams is accurate but abstract. The paper's step was to express it in a unit people already picture: a teaspoon of table sugar. To do that, a teaspoon of sugar is put through the same two questions.
- How carby is it? A 4g teaspoon of sucrose yields 4.2g of carbohydrate once digested. Sucrose splits into glucose and fructose and picks up a molecule of water as it does, which is why 4g becomes 4.2g rather than staying at 4g.
- How sugary is that carb? Table sugar has a GI of 65. It is half glucose and half fructose, and fructose raises blood glucose very little, so its GI is lower than pure glucose.
Glycaemic load of one teaspoon of table sugar: 4.2g × 65 ÷ 100 = 2.73
Every food's teaspoon figure is its glycaemic load divided by 2.73. That single division is the whole conversion.
Worked examples
Basmati rice, 150g boiled
Carbohydrate: 40g (Unwin 2016, Table 1)
GL: 40 × 69 ÷ 100 = 27.6
Teaspoons: 27.6 ÷ 2.73 = 10.1
Brown bread, one small 30g slice
Carbohydrate: 40g per 100g × 30g ÷ 100 = 12g
GL: 12 × 74 ÷ 100 = 8.9, rounded in the paper to 9
Teaspoons: 9 ÷ 2.73 = 3.3
Apple, 120g
Carbohydrate: 16g (Unwin 2016, Table 1)
GL: 16 × 39 ÷ 100 = 6.2
Teaspoons: 6.2 ÷ 2.73 = 2.3
Why the divisor is 2.73 and not 4
A shorthand sometimes repeated online divides glycaemic load by 4, on the reasoning that a teaspoon weighs 4g. That treats the teaspoon as 4g of pure glucose with a GI of 100. Dr Unwin's published figures do not use that shortcut. With a divisor of 4, the bowl of basmati rice above would come out at 6.9 teaspoons rather than 10.1, and the slice of brown bread at 2.3 rather than 3.3. The figures on this site match the published infographics, which means they use 2.73.
Portion size is the lever
Because GI is fixed for a given food, the only thing that changes glycaemic load from one meal to the next is how much of the food is eaten. Twice the rice is twice the load, and twice the teaspoons. This is why each card in the calculator states a portion with a gram weight, and why the figures cannot be read without it. A figure for 100g of honey and a figure for one 9g poppadom describe very different amounts of food.
Limits worth knowing
- GI values for the same food vary between studies, between varieties and between cooking methods. The tables average them.
- Available carbohydrate on UK food tables is expressed as monosaccharide equivalents. US nutrition labels list total carbohydrate including fibre, so a US label figure cannot be dropped straight into the formula.
- Fat, protein and fibre in a mixed meal slow digestion. GL describes each food as if eaten alone.
- The conversion depends on table sugar's GI of 65. A different value for sucrose would shift every figure proportionally, though the ranking of foods would not change.
Each value in the calculator, with its source, is listed on the sources page. For the other side of the comparison, see how much sugar is in your blood.