Net carbs

Net carbs is the marketing convention of subtracting fibre, and often sugar alcohols, from total carbohydrate on the basis that those components are not digested and absorbed like other carbohydrate.

The logic behind subtracting fibre is sound. Fibre passes to the large intestine largely intact, contributes little absorbable energy, and does not raise blood glucose, so counting it as carbohydrate overstates the impact of a high fibre food.

The sugar alcohol half is where it becomes unreliable. Erythritol is largely unabsorbed and genuinely close to zero; maltitol is partially absorbed and does raise blood glucose noticeably; and subtracting all of them equally flatters products that do not deserve it.

There is no legal definition in the UK or EU, which is the practical problem. Nutrition labels here report total carbohydrate excluding fibre already, whereas American labels include fibre in the total, so net carbs is a calculation that means different things in different jurisdictions and is largely redundant on UK labels.

It exists mostly because low carbohydrate marketing needed a number that made processed products look compatible with a diet. A bar with fifteen grams of maltitol and a claim of two net carbs is doing something rather different from a plate of vegetables.

Sugar alcohols in quantity also cause gastrointestinal symptoms, which is the other cost of eating around a number.

In practice, ignore net carbs and read the actual label. If you are counting carbohydrate for a specific reason, count what the UK label already gives you and treat fibre as the thing you want more of rather than less.