When you cook rice, pasta or potatoes and then cool them, part of the starch turns into resistant starch. We explain the real effect of this change on calories, blood sugar and gut health, whether reheating alters the result and the safety rules to follow when keeping cooked rice.
There is a piece of advice you often come across on social media: “Cook your rice, leave it in the fridge overnight and eat it the next day. The calories are halved and it won’t raise your blood sugar.” It sounds too good. So is there real knowledge behind it, or is this one more nutrition myth?
The answer is both. The phenomenon the advice rests on is real, and its name is resistant starch. But its effect is not as great as it is made out to be. In this article we explain what actually happens in cooled rice and pasta, how this is reflected in calories and blood sugar, and the safety rule you really must know when keeping cooked rice.
What is resistant starch?
Starch is a carbohydrate made up of linked glucose units, found in grains, potatoes and pulses. Normally it is broken down by enzymes in the small intestine and passes into the blood as glucose. Resistant starch, as the name suggests, is the part that resists this digestion: it passes through the small intestine largely unchanged and reaches the large intestine.
There it behaves like a fibre. Gut bacteria ferment it, and in the process short-chain fatty acids are formed. One of these, butyrate, is the main source of energy for the cells of the large intestine. In other words, resistant starch is a carbohydrate that feeds your gut bacteria rather than you.
What happens when rice and pasta cool?
The starch in raw rice has a tight, orderly structure. When it is cooked in water, the starch granules take up water, swell and open out; digestive enzymes can now reach it easily. This is why hot, freshly cooked rice raises blood sugar quickly.
When a cooked starchy food is cooled, a change in the opposite direction begins. Some of the opened-out starch chains draw close together again and form a tight, crystal-like structure. This new structure stands up to enzymes. As the hours in the fridge increase, the amount of resistant starch formed also increases somewhat. The same thing happens in pasta, potatoes, bulgur and bread.
Does cooled rice really have fewer calories?
It does, but only very slightly. Because resistant starch is not digested in the small intestine, it gives less energy than ordinary starch. But the part that turns into resistant starch on cooling is a small fraction of the total starch on the plate. In practice this amounts to a difference in energy of a few mouthfuls at most in a portion of rice.
Claims that the calories are halved are not based on sound published data. Treating cooled rice as a “free” food and making the portion bigger more than cancels out the small gain. If you have put three ladles on your plate instead of two, the fact that the rice was cooked yesterday will not save the situation.
What is the effect on blood sugar?
Here the picture is a little more favourable. Some studies have shown that rice, pasta and potatoes that have been cooked, cooled and then reheated raise blood sugar after a meal less than when freshly cooked. The reason is that part of the starch is not turned into glucose and the rest is digested more slowly.
But two points must not be forgotten. First, the effect is moderate and varies from person to person. Second, there are much stronger factors determining the blood sugar response: the size of the portion, what is eaten with the rice and the order within the meal. A small portion of fresh rice served with yoghurt, meat or beans and a plate of salad produces a more balanced response than a large plate of cooled rice eaten on its own. We cover this in detail in the article on meal sequence and blood sugar.
Does resistant starch disappear on reheating?
Most of it does not. The resistant structure formed on cooling stands up well to reheating. So you do not have to eat rice or pasta cold; when you heat it up after keeping it in the fridge, a considerable part of this structure is retained. In some small studies, the blood sugar response to pasta that had been cooled and reheated was reported to be even lower than to the cold version; but these findings need to be confirmed by larger studies.
Which foods contain resistant starch?
- Starchy foods that have been cooked and cooled: Rice, pasta, bulgur and potatoes. Potato salad, cold pasta salad and kısır, the Turkish bulgur salad, are good examples for this reason.
- Pulses: Chickpeas, lentils, haricot beans and borlotti beans naturally contain high amounts of resistant starch; no cooling is needed.
- Bananas that are not fully ripe: In a banana whose skin is still tinged with green, a considerable part of the starch is resistant; as the banana ripens it turns into sugar.
- Oats: Especially when eaten uncooked, soaked in milk or yoghurt.
- Whole grains and seeds: Grains whose outer layer has not been removed are more resistant to digestion.
As you can see, you do not have to go to trouble with rice in order to get resistant starch. Eating pulses several times a week is a far more reliable source.
Its contribution to gut health
Perhaps the most valuable side of resistant starch is not the calorie count but its effect on the gut. It acts as a prebiotic; it is food for beneficial bacteria. The short-chain fatty acids formed during fermentation nourish the gut wall and help to keep the environment of the gut healthy. Like fibre, it may also support the feeling of fullness.
On the other hand, fermentation means gas. In people with a sensitive gut, increasing resistant starch all at once can lead to bloating; the amount needs to be raised gradually.
Take care when keeping rice: food safety
This is the least talked-about but most important part of the subject. Cooked rice left at room temperature provides a favourable environment for the growth of a bacterium that can cause food poisoning. The spores of this bacterium survive cooking, and the toxin it produces is not destroyed by reheating. So thoroughly heating rice that has stood on the worktop for hours does not make it safe.
- Do not leave cooked rice in the pan on the hob for hours.
- Spread it in a shallow dish to cool and put it in the fridge within one to two hours at the latest.
- Eat rice kept in the fridge the next day if possible, and within a few days at the latest.
- When reheating, make sure it is steaming hot all the way through.
- Do not cool and reheat rice that has already been reheated once.
The same rules apply to pasta, bulgur and potatoes.
How to apply this in daily life
It is better to think of this information not as a diet trick but as a practical way of organising the kitchen. Cooking bulgur, rice or pasta in one go at the start of the week, dividing it into portions and putting it in the fridge both saves time and allows resistant starch to form. A cold bulgur salad taken for lunch, a salad of boiled and cooled potatoes dressed with olive oil, or a pasta salad with chickpeas are tasty examples of this routine.
The rule that does not change is this: the portion stays the same, and the rest of the plate is completed with protein and vegetables.
In brief
- When cooked starchy foods are cooled, part of the starch turns into resistant starch.
- Calories fall, but only slightly; the claim that they are “halved” is not realistic.
- The blood sugar response after a meal may fall somewhat; portion size and what is on the plate still matter more.
- Reheating does not destroy most of the resistant starch.
- Cooked rice is not left at room temperature; it is cooled quickly and kept in the fridge.
- Pulses are among the richest sources of resistant starch and need no cooling.
Frequently asked questions
Can I eat as much cooled rice as I like?
No. Cooling does not make rice calorie-free. The portion size is the same as for fresh rice.
Does freezing bread have the same effect?
A similar change in structure takes place in bread. In one small study, bread that had been frozen, thawed and toasted was reported to raise blood sugar slightly less than fresh bread. The effect is limited; the type and amount of bread matter far more.
I have diabetes or insulin resistance. Will this method help me?
It may make a small contribution, but it does not take the place of treatment or of a nutrition plan. Your carbohydrate amount and meal pattern should be decided together with your doctor and dietitian. For details, see the nutrition for insulin resistance page.
