GOOD GUT WEDNESDAY · 04
The Nourishment Edit
Cook a potato. Eat it hot. Then cook another one, cool it, and eat it later. Nutritionally, are they exactly the same?
Not quite.
Something interesting can happen to starch after certain starchy foods are cooked and cooled. Some of that starch can reorganise into a form that becomes more resistant to digestion in the small intestine. It’s called resistant starch. And what happens next takes us straight back to the extraordinary ecosystem living in our gut.
First: what exactly is resistant starch?
Most digestible starch is broken down in the small intestine into glucose that can then be absorbed. Resistant starch behaves differently. As its name suggests, it resists digestion in the small intestine — either completely or partially — allowing more of it to continue towards the large intestine.
There are different forms of resistant starch, and they arise for different reasons. Some starch is naturally protected within the physical structure of food. Some occurs naturally in foods such as raw potatoes and unripe bananas. But the type we’re interested in today can form when certain cooked starches are subsequently cooled. Scientists call this retrograded starch, commonly classified as resistant starch type 3.
What does cooling actually do?
When starch is cooked in the presence of water and heat, its structure changes in a process called gelatinisation. Starch granules absorb water and swell, and some of the organised molecular structure is disrupted. But cooling changes things again. As the cooked starch cools, some of its starch molecules — particularly amylose and parts of amylopectin — can begin to realign and form more ordered structures. This process is known as retrogradation.
Some of these newly organised structures become less accessible to our digestive enzymes. In other words: some of the starch becomes harder for us to digest. And that is where resistant starch enters the story.
So does cooling a potato really make a difference?
Yes — although the amount of difference depends on how the potato is prepared. Research analysing commonly consumed potatoes has found that resistant starch content can vary according to cooking method and serving temperature. In one study examining different potato varieties, baked potatoes contained more resistant starch than boiled potatoes, while chilled potatoes contained more resistant starch than potatoes served hot. Interestingly, the researchers found that preparation and temperature mattered more than the potato variety itself. That doesn’t mean every cold potato suddenly becomes a resistant-starch powerhouse. It means something more interesting:
the way we prepare and serve a food can influence the structure and digestibility of some of its starch.
And that’s where your gut microbes come in
If resistant starch escapes digestion in the small intestine, some of it can continue into the colon. There, members of the gut microbiota may ferment it. Microbial fermentation of resistant starch can produce short-chain fatty acids, including acetate, propionate and butyrate. If that sounds familiar, it should.
In Good Gut Wednesday · 03, we explored how gut microbes can ferment certain dietary fibres and produce these same microbial metabolites. Resistant starch gives us another fascinating example of the same broader idea: our microbes can metabolise components of food that our own digestive enzymes did not completely break down.
And resistant starch has attracted particular scientific interest because fermentation can favour the production of butyrate. Butyrate is an important energy source for cells lining the colon and is being extensively studied for its roles in intestinal physiology and host-microbe interactions. That does NOT mean that eating a cold potato guarantees a particular amount of butyrate. Microbial responses differ between people, and resistant starch content itself varies considerably depending on the food and how it is prepared. The biology is much more interesting than a simple food hack.
Potatoes aren’t the only example
This phenomenon isn’t exclusive to potatoes. Rice provides a particularly interesting example. A 2015 study compared freshly cooked white rice with rice subjected to different cooling conditions.
Freshly cooked rice contained 0.64 g of resistant starch per 100 g. Rice cooled at room temperature for 10 hours contained 1.30 g per 100 g. Rice cooled for 24 hours at 4°C and then reheated contained 1.65 g per 100 g.
In a small crossover study involving 15 healthy adults, the cooled-and-reheated rice also produced a lower post-meal glycaemic response than freshly cooked rice. That is an intriguing finding. But it should be interpreted carefully. One small study involving one preparation method and one type of rice does not establish a universal rule for every person, every rice variety or every cooking method. Other research has similarly shown that rice variety, cooking method, cooling and storage conditions can all influence resistant starch content.
The important lesson isn’t: “Always eat cold rice.”
It’s: food structure can change after cooking.
What about pasta?
Pasta is another interesting area of research. Studies have examined whether cooking, cooling and reheating pasta can alter resistant starch and subsequent glucose responses. For example, research on chickpea pasta found that cooling for 24 hours at 4°C followed by reheating increased measured resistant starch compared with freshly cooked pasta.
More recent research has also investigated cooled and reheated conventional pasta. But again, the magnitude of the effect depends on the food, its composition and how it is prepared. So we shouldn’t turn this into a universal rule that all leftover pasta automatically contains a clinically meaningful amount of resistant starch.
Does reheating destroy resistant starch?
Not necessarily. Some retrograded starch structures can persist after reheating, depending on the food and the temperature used. Studies using cooked, cooled and reheated rice, for example, have still measured increased resistant starch compared with freshly prepared rice under particular experimental conditions. But once again, there isn’t one number that applies to every kitchen. The outcome can depend on:
- the food itself
- its starch composition
- cooking method
- cooking time
- cooling temperature
- cooling duration
- storage
- reheating method
- reheating temperature
Which is exactly why nutrition science is more useful than nutrition hacks.
This isn’t permission to fear freshly cooked carbohydrates
There is another trap worth avoiding. Once people hear that cooling can increase resistant starch, it is easy to jump to the conclusion that freshly cooked potatoes, rice or pasta are somehow “bad”. That isn’t what the research says. Foods are more than their resistant starch content. A potato is not simply starch. Rice is not simply glucose. And the nutritional quality of a meal cannot be reduced to whether it was eaten hot or cold.
Resistant starch is simply one fascinating example of how food processing and preparation can influence food structure and digestion. That’s a much more useful idea than categorising food as good or bad.
Yesterday’s potato may meet your gut differently today
Perhaps the most interesting part of resistant starch isn’t the possibility of another nutrition rule. It’s the reminder that food is dynamic. Cooking changes it. Cooling can change it again. Reheating may change aspects of it again. And those structural changes can influence what our own digestive enzymes can access — and consequently what may travel onwards to our gut microbes. So the potato sitting in your refrigerator isn’t necessarily identical, from a starch-structure perspective, to the one you ate steaming hot yesterday.
Yesterday’s potato may meet your gut differently today. And that is exactly the kind of detail that makes the relationship between food and our microbiome so fascinating.
Scientific Evidence
Birt DF, Boylston T, Hendrich S, et al. (2013).
Resistant Starch: Promise for Improving Human Health.
Advances in Nutrition, 4(6), 587–601.
DOI: 10.3945/an.113.004325
Raigond P, Ezekiel R, Raigond B. (2015).
Resistant starch in food: a review.
Journal of the Science of Food and Agriculture, 95(10), 1968–1978.
DOI: 10.1002/jsfa.6966
Sonia S, Witjaksono F, Ridwan R. (2015).
Effect of cooling of cooked white rice on resistant starch content and glycemic response.
Asia Pacific Journal of Clinical Nutrition, 24(4), 620–625.
DOI: 10.6133/apjcn.2015.24.4.13
Maatouk GR, Humphreys MR, et al. (2016).
Resistant starch analysis of commonly consumed potatoes: Content varies by cooking method and service temperature but not by variety.
Food Chemistry, 208, 297–300.
Bojarczuk A, Kęszycka P, Marszałek K, Gajewska D. (2024).
The Effect of Cooking and Cooling Chickpea Pasta on Resistant Starch Content, Glycemic Response, and Glycemic Index in Healthy Adults.
Metabolites, 14(11), 585.
DOI: 10.3390/metabo14110585
The Nourishment Edit is Lynn’s exploration of food, nourishment and the science behind how we eat. Our articles are for educational purposes and are not intended as medical advice.
Researched and edited by
Lynn Glenda Pinto
Founder, Lynn’s
