Resistant starch

Resistant starch is starch that isn't digested in the small intestine – it travels on to the large intestine, where it behaves like fibre. It occurs naturally in pulses, oats and unripe bananas. The most interesting form for cooking, though, develops as food cools: when cooked potatoes, rice or pasta sit in the fridge overnight, the starch molecules rearrange themselves (retrogradation) and become partly resistant to our digestive enzymes.

What it does: gut bacteria ferment resistant starch into short-chain fatty acids such as butyrate – the substance the cells of the gut lining live on. Blood sugar rises more gently after a cooled or reheated portion than after a freshly cooked one. In a small trial at the University of Surrey (2014), the rise in blood sugar after cooled and reheated pasta was about half as high as after freshly cooked pasta. And because resistant starch provides only about 2 instead of 4 kilocalories per gram, the side dish brings slightly less energy with it.

The numbers remain modest: freshly cooked potatoes contain about 1 gram of resistant starch per 100 grams; once cooled, it's about 3 grams. For a 200-gram portion, that makes around 4 grams more fibre – a nice contribution to the 30 grams a day recommended by the German Nutrition Society (DGE), but not a weight-loss trick. The calories drop by only a few per cent.

In practice, this means: potato salad, rice salad, pasta salad and reheated leftovers are a good thing. Reheating doesn't destroy the resistant starch. Combined with vegetables and protein, yesterday's leftovers become a plate of slow-release energy.

If you don't usually eat much fibre, increase the amount gradually – bloating is a sign that your gut bacteria are still adjusting. And with rice: let cooked rice cool quickly – ideally in shallow containers – and then put it in the fridge, as the German Federal Institute for Risk Assessment (BfR) advises. Try to eat it within a day. The reason is the bacterium Bacillus cereus: its spores survive cooking, germinate in lukewarm rice, multiply and produce a toxin that even reheating doesn't destroy.