Carbohydrate grams and portions

Most often in grams, because grams appear on food labels, in food composition tables and in apps. The second option is the carbohydrate portion, a fixed unit that groups several grams together. Both describe the same thing, namely the amount that becomes glucose in the blood after the meal. Food labels show carbohydrate values per 100 g or 100 ml of product [1].

The difference is one of convenience, not of substance. Grams give you accuracy and match the product label. Portions give you speed, because you work with small numbers and can do the sums in your head. You can switch between them at any time, with a simple division or multiplication. What matters is always knowing which unit the table or app you use is working in [2].

It is a fixed amount of carbohydrate, used as a unit of measurement. It does not mean how much you eat, but how much of the substance that raises blood glucose — glucose itself — your meal contains. Foods that provide the same amount of carbohydrate can be swapped for one another, even if they look completely different. That is why the system was called "exchanges" for a long time [3].

The American system works with portions of 15 g of carbohydrate, also called carbohydrate choices. In this system, a slice of bread, a medium piece of fruit, a glass of milk or 50 g of cooked pasta each provide about one portion. You learn these reference points once and they serve you for years. The kitchen scale remains useful, but you cannot pull it out of your pocket at every meal [1].

It depends on the country and on the material you are reading. In the United States, a portion means 15 g. In the United Kingdom and in many European countries, a portion is 10 g. The unit used in Germany is the exception, at 12 g of carbohydrate, and it applies to any food. The 10 g unit is, however, gaining more and more ground in Germany, which helps to harmonize the advice given across the EU [1] [4].

None is more correct than another, because they are all simply conventions. The real mistake is to mix them in the same calculation. Check which unit your app, your table or your doctor uses, then stay with it so that you all speak the same language. If you change system, the insulin-to-carbohydrate ratio changes too, when it is expressed per portion. The final insulin dose must stay the same, whatever system you use [5].

You divide the grams by the size of the portion. If the portion is 15 g, you divide by 15; if it is 10 g, you divide by 10. A meal with 60 g of carbohydrate means four portions in the American system and six portions in the European one. The other way round, you multiply the number of portions by the same value. The calculation is generally done in your head, without opening an app just for that [6].

The result rarely comes out round, and that is not a problem. A snack with 21 g of carbohydrate means roughly two 10 g portions or one and a half 15 g portions. You round to the nearest half portion and move on. The difference left over is smaller than the error you make when estimating the carbohydrate on your plate [7].

Because they come from an old German tradition, from before apps and detailed labels. The bread unit (BE) took its name from a thin slice of white bread, which provides about 12 g of carbohydrate. It was even defined that way in German law, until 2010. After that wording was withdrawn, it survived only as a habit in the textbooks [4].

The carbohydrate unit (KE) appeared as a simpler 10 g version, precisely because dividing by ten is easier. More and more tables, apps and recipes in Germany and Austria have now moved to this system. If you come across such a resource, look at the value of the unit first. This German example teaches you to check carefully which system is currently used in your own country [1].

Both options are equally good. The choice depends on how you want to work out your dose and on what is usual in your country. If your insulin-to-carbohydrate ratio is expressed in grams, you count in grams and have nothing to convert. If your doctor has set one unit of insulin per portion, counting in portions is then quicker. Internationally, the vast majority of pumps and bolus calculators work in grams [8].

In practice, patients often combine the two, and sometimes get muddled between them. A common situation is a quick count in portions at a restaurant or when visiting, followed by conversion into grams for your own journal or bolus calculator. Portions help you see the meal as a whole, and grams help you estimate the insulin dose more precisely. You do not have to choose once and for all. You can switch if a system does not suit you. It is best, though, to pick one and stay with it in the long run [9].

Accurate enough to get the dose right, and no more. Rounding to 5 g of carbohydrate is usually enough for ordinary meals. If you work in portions, estimating to the nearest half portion is probably enough. Accuracy to the gram is an illusion, because the food is not that exact, and often neither is the label. Labeling rules allow a tolerance of up to ±20% between the declared value and the real one [10] [11].

What matters is how big the error is compared with how big the meal is, not the error in absolute grams. At 80 g of carbohydrate, an error of 5 g changes the dose very little. In a 15 g snack, the same error means a third of the total. Watch out too for foods with many grams of carbohydrate in a small volume, such as bread, rice, sweets and sweetened drinks [12].

Yes, by about as much as one unit of insulin, and that is not a small amount. In an average adult, one unit of rapid-acting insulin generally covers around 10 g of carbohydrate, but it may cover only 3-5 g. So 10 g of carbohydrate left uncounted means, roughly, one missing unit, or even more. Blood glucose can rise unexpectedly, by several tens of mg/dL (a few mmol/L), depending on your insulin sensitivity [5] [10].

These figures are only a guide, and you set the ratio and your correction factor together with your doctor. In children and in very sensitive people, the same 10 g of carbohydrate has a very large impact. In someone with high insulin needs, it probably matters less. So it is worth checking with your doctor what 10 g of carbohydrate means for you.

For every meal you note down five things:

• the time of the meal, so that you can compare one day with another;
• the grams of carbohydrate, as you estimated them;
• the insulin dose you gave yourself;
• the blood glucose before the meal, measured or read from your sensor;
• the blood glucose two hours after the meal, which shows how the dose worked [13].

With these five pieces of information, anyone can redo the calculation and consider possible changes to treatment. Without them, a high blood glucose at the next meal can look like a mystery. Write them down as soon as you get the chance, not in the evening, from memory [2].

Add anything out of the ordinary too: a very fatty meal, exercise, alcohol, an illness or an injection given later than it should have been. All these things can help explain variable blood glucose readings for the same number of grams of carbohydrate. If you use portions, write the grams alongside, so that you can compare meals later on. A journal kept properly for a week is worth more than a month of memories [14].

It is the amount in a small meal or in an ordinary side dish. You find it in two slices of bread weighing 30 g each. A small cup of cooked pasta or a medium potato comes to the same. All three look very different. Volume tells you nothing about the grams of carbohydrate [1].

You can reach the same figure from other foods. Two medium pieces of fruit, a small 300 ml bottle of juice or a glass of milk with a slice of bread come to about the same. The exercise is worth doing at home, with the kitchen scale, so that your eye learns what 30 g of carbohydrate looks like. After a few weeks you may be able to recognize this amount without weighing the food [7] [15].