Does eating more fibre help against high blood sugar?

Water-soluble fibre, such as that found in legumes, vegetables and fruit, lowers blood sugar after a meal and also improves blood sugar regulation over the longer term. The likely mechanism is that it slows digestion, so that glucose enters the bloodstream more gradually. Insoluble fibre, such as that found in wholegrain bread, appears to have much less of this effect.

In women with gestational diabetes, a daily additional intake of 9.5 grams of fibre over 8 weeks led to a measurable reduction in post-meal blood sugar. In addition, after giving birth they less frequently had impaired glucose tolerance. In people with type 2 diabetes, multiple studies show that a high-fibre diet significantly improves blood sugar regulation. A high fibre content also neutralises the negative effect that a large amount of carbohydrates would otherwise have on blood sugar.

Even so, 'eating more fibre' is no guarantee of a lower blood sugar peak. An early classic study found no relationship at all between the fibre content of food and how sharply blood sugar rose after a meal. Fat and protein content turned out to play a larger role in that study. This shows that the overall composition of a meal matters, not fibre content alone.

High or prolonged fibre intake can cause side effects: flatulence, diarrhoea and reduced absorption of minerals. Caution is warranted in pregnant women, children and people with nutritional deficiencies. Fibre supplements have also been studied, but the studies are small and partly funded by commercial parties; their findings are therefore harder to translate into practice.