Fiber, polyols and available carbohydrates

Fiber does not raise blood glucose, because your digestive enzymes cannot break it down. Fiber is a carbohydrate, but it passes through the small intestine almost unchanged [1]. The bacteria in your colon ferment part of it and produce short-chain fatty acids. No glucose comes out of that process. The energy you gain is small and does not change your insulin needs [2].

Fiber does change the way blood glucose rises after a meal. Soluble fiber absorbs water and forms a gel, which slows down gastric emptying and the passage of glucose into the blood. The glucose peak then comes later and is lower [3]. Insoluble fiber is what keeps your bowel moving properly. The amount of carbohydrate you count stays the same, no matter how much fiber the food you measured contains. If you are in the United States or in Canada, though, fiber is included in the carbohydrate figure on the label. In that case it has to be subtracted [4].

No, not if you are reading a European Union label. There the "Carbohydrate" line already means available carbohydrate, and fiber appears separately, underneath it [1]. If you subtract it once more, you end up with a smaller, incorrect number, which can later translate into an insulin dose that is too low. The rule is simple: take the "Carbohydrate" value exactly as it stands [5].

The American rule "subtract fiber when it exceeds 5 g" comes from the way the United States label is written. There the "Total Carbohydrate" line includes fiber, so subtracting it does matter [4]. You will meet this rule in American books, apps and recipes. Always check which kind of label you have in front of you. On the American one, fiber appears below, but inside the carbohydrate line, as the figure below shows.

You take the "Carbohydrate" figure from the label directly and subtract nothing. Their fiber, of which there is plenty, has already been set aside [1]. Cooked beans, lentils and chickpeas contain roughly 12–20 g of carbohydrate per 100 g. Lentils sit toward the lower end and chickpeas toward the upper one. Cooked broad beans come close to lentils, and green peas are slightly lower.

Weigh pulses cooked, not dry, because they double or even triple their weight once boiled. Drained canned pulses are similar to what you cook at home, but any sweet tomato sauce added to them contains sugar. A meal with beans raises blood glucose more slowly [6]. Hummus and lentil pasta are digested faster. At the start it is worth checking your blood glucose two to three hours after the meal.

They are the part of food that is actually converted into glucose: starch and sugars. The name sounds technical, but the idea behind it is simple. They are digested in the small intestine and pass into the blood, while fiber stays in the gut. Available carbohydrates are the ones you count when you work out your insulin dose [7].

On the EU label, the "Carbohydrate" line shows exactly this value, so you use it directly, with no further adjustment. In American composition tables, however, you find the total, from which you have to subtract the fiber [4]. In an American table, cooked chickpeas have around 27 g of carbohydrate per 100 g and about 8 g of fiber. That leaves roughly 20 g of available carbohydrate. The final figure for available carbohydrate is the same, whichever route you took to reach it.

Polyols are sweeteners used instead of sugar in "sugar-free" products such as chewing gum, chocolate, candy or lozenges. You recognize them by name: sorbitol, maltitol, xylitol, isomalt, erythritol. Despite what they are called, they are neither sugar nor alcohol. Your body absorbs them only partly, so they raise blood glucose less than sugar does. They also provide less energy [8].

They do not all behave the same way, though. Erythritol is absorbed and then excreted unchanged in the urine, so in practice it does not affect blood glucose [9]. Maltitol sits at the opposite end and raises blood glucose the most of them all. Xylitol, sorbitol and isomalt sit somewhere in the middle. On the European label they are included in "Carbohydrate" and may be broken out underneath, on the "of which polyols" line.

You start from the "Carbohydrate" figure, which already includes the polyols. If the label also shows an "of which polyols" line and the amount exceeds 5 g per serving, you subtract half of the polyols from the total carbohydrate. The result is the amount of available carbohydrate, which you then use to work out your insulin dose [10]. If the only polyol declared is erythritol, you subtract it in full, because it has no effect on blood glucose.

For example: 29 g of total carbohydrate, of which 18 g are polyols, means 29 − 9 = 20 g of available carbohydrate. Watch out for the trap on the front of the pack. "Sugar-free" is a legal claim about sugars, not about carbohydrate. A product can be sugar-free and rich in starch. Sugar-free cookies are in fact still made of flour [11]. So read the table on the back, not the advertising on the front. Then check your blood glucose, because the same product behaves differently in every person.

They do not raise blood glucose and they provide no carbohydrate. Saccharin, aspartame, sucralose, acesulfame K and stevia are hundreds of times sweeter than sugar. They are therefore used in very small amounts. A sweetener tablet or a zero-type drink does not change your insulin dose [12]. In practice, there is nothing here for you to count.

The trap lies in the sweetener powders sold for cooking or baking. So that they can be measured by the spoonful, they contain a dextrose or maltodextrin carrier, which is real carbohydrate. Read the "Carbohydrate" value per 100 g of product, especially if you use several teaspoons. The cake you put the sweetener into is counted anyway [5].

The glucose peak becomes lower and comes later. The gel formed by soluble fiber slows down gastric emptying, and glucose reaches the intestinal wall gradually [3]. The same amount of carbohydrate therefore produces a smoother glucose curve. The number of grams of carbohydrate stays unchanged, though. This effect is clearly seen with pulses, oats, vegetables and whole fruit [13].

The insulin dose stays the same, but its timing may change. If you inject too early, you risk too large a drop in blood glucose at the beginning, followed by a rise later on. If you use an insulin pump, an extended bolus covers a meal like this better [14]. Fat in the same meal pushes the rise even later. Your doctor will work out with you the interval that suits you best.

Yes, and this is their main drawback. The part not absorbed in the small intestine reaches the colon, where it draws water through the intestinal wall and is fermented by bacteria. The result is gas, bloating, cramps and loose stools. Problems usually appear above 10–20 g of polyols a day, especially with maltitol and sorbitol [15]. Digestive tolerance varies widely from person to person.

The label warns you when a product contains more than 10% added polyols, through the statement about a laxative effect [16]. In practice, a whole bar of "sugar-free" chocolate can go beyond that daily range. So increase the amount of your favorite polyol product gradually, in order to find your own personal limit. A faster gut transit can change the absorption of the whole meal.