Spotlight on Resistant Starch

Boil a potato, chill it in the fridge for a day, and the insulin response it triggers drops by about 28%. Nothing about the potato’s calories or carbs changed on the label, yet your body handles it differently.

The reason is resistant starch, a kind of starch that behaves like fiber. Here’s what it is and what the research says about it, from the fridge to the fruit bowl.

What is resistant starch?

Most starch in bread, rice and potatoes breaks down into glucose in your small intestine and enters your bloodstream. Resistant starch is the portion that escapes that digestion and travels on to the large intestine.

Once it arrives in the colon, gut bacteria ferment it much like they ferment fiber. That’s why nutrition scientists often group resistant starch with dietary fiber, even though chemically it’s still starch.

A 2013 review in Advances in Nutrition by Diane Birt and colleagues lays out why this matters. Starch that skips digestion in the small intestine doesn’t add to the blood sugar rise after a meal, and in the colon it becomes food for microbes that produce short chain fatty acids.

Resistant starch also adds fewer usable calories than ordinary starch, since your own enzymes don’t break it down for energy in the small intestine.

The amount of resistant starch in a food isn’t fixed. It changes with ripeness, cooking, cooling and processing, which is what makes this fiber so interesting to work with in your own kitchen.

The five types of resistant starch

Researchers sort resistant starch into five types based on why it resists digestion:

• RS1, physically trapped starch: starch locked inside intact cell walls, as in whole or coarsely milled grains, seeds and beans.
• RS2, raw starch granules: tightly packed granules that enzymes struggle to break into, found in raw potatoes, green bananas and high-amylose corn.
• RS3, retrograded starch: starch that has been cooked and then cooled, letting its chains realign into a tighter, harder-to-digest structure.
• RS4, chemically modified starch: starch altered in processing to resist digestion, used mostly as a food ingredient.
• RS5, starch bound to fats: starch that forms complexes with fats during cooking, which slows its breakdown.

For everyday eating, RS2 and RS3 matter most. Green bananas and green banana flour supply RS2, while cooked and cooled potatoes, rice and pasta supply RS3.

Heat tends to break RS2 down, since cooking swells and opens the starch granules. Cooling then rebuilds some resistance as RS3, which is the step people can control at home.

Cooked and cooled potatoes: what chilling really changes

The clearest test of the cooling idea comes from Lund University in Sweden. In a 2005 study in the European Journal of Clinical Nutrition, 13 healthy volunteers ate breakfasts of boiled potatoes prepared in different ways, each serving holding the same 50 g of available carbohydrate.

One version was freshly boiled. Another was boiled and then stored at 8°C for 24 hours, and a third was cold-stored and dressed with a vinaigrette of olive oil and white vinegar.

Chilling raised resistant starch by more than half and cut the insulin response by about 28%. Adding vinegar pulled the glycemic index down by 43% compared with hot boiled potatoes.

This was a small, single-meal study, so it shows what happens after one breakfast rather than over months. The direction is still useful, and potato salad made the night before is an easy way to use it.

Our recipe eBooks include potato, rice and bean dishes that work well made ahead and served cold.

Myth vs fact: chilling turns potatoes into a low-carb food

Myth: Once a potato has been cooked and cooled, most of its starch becomes resistant and it no longer counts as a carb-heavy food.

Fact: In the Lund study, resistant starch rose from 3.3% to only 5.2% of total starch. Cooling softens the insulin response, but the great majority of the starch is still digested as usual.

Green banana flour and green banana biomass

Unripe bananas are rich in RS2, and that starch gradually turns to sugar as the fruit ripens. Green banana flour and a cooked green banana paste, sometimes called green banana biomass, are ways of capturing it before that happens.

A 2019 trial in the British Journal of Nutrition tested this in Brazil. Researchers randomized 113 adults with prediabetes or type 2 diabetes to nutrition counseling alone or counseling plus 40 g a day of green banana biomass, supplying about 4.5 g of resistant starch, for six months.

HbA1c, a measure of average blood sugar over several months, fell in both groups, so the counseling itself helped. Fasting blood sugar and body weight dropped significantly only in the green banana group, while insulin levels and insulin resistance didn’t change in either.

The trial wasn’t blinded and some participant numbers differed between sections of the paper, so the results are best read as encouraging rather than definitive. Even so, it’s one of the longer real-food trials of resistant starch.

In the kitchen, green banana flour can replace part of the regular flour in pancakes or muffins. Baking heats it, though, so some of its RS2 is likely lost along the way.

Resistant starch and butyrate

Butyrate is a short chain fatty acid that colon cells use as their main fuel, and it’s one of the main reasons researchers care about resistant starch.

A 2019 study in mBio gave 174 young adults one of several fermentable fiber supplements for two weeks. Potato resistant starch raised butyrate the most, while resistant starch from high-amylose maize and inulin didn’t raise it significantly.

Responses varied widely from person to person. People whose butyrate-producing microbes increased tended to be the ones whose butyrate rose, which suggests your existing gut bacteria shape what you get from resistant starch.

That means two resistant starch sources aren’t automatically equal. The plant it comes from and the microbes waiting for it both affect the result.

Resistant starch and blood sugar: what the trials add up to

The potato and green banana studies point in the same direction. Resistant starch tends to soften the blood sugar or insulin rise after a meal and may help modestly over months.

The size of the effect depends on the dose and the person. A single cooled potato meal and six months of green banana paste are very different exposures, and neither study can tell you exactly what your own numbers would do.

The Lund study also hints that what you eat alongside the starch matters. The biggest drop in glycemic index came once the vinegar dressing was added, a reminder that resistant starch works as one part of a meal rather than in isolation.

What’s consistent is the mechanism. Starch that escapes the small intestine can’t add glucose there, so swapping some quickly digested starch for resistant starch gives your blood sugar less to deal with at that meal.

Resistant starch foods to work into your week

According to the Birt review, the richest everyday sources are legumes, whole and coarsely milled grains, slightly green bananas and starchy foods that have been cooked and cooled.

That gives you plenty of practical options. Lentil or bean salads, overnight oats, sushi-style rice, pasta salad and potato salad all fit the pattern.

The cooling step is what builds resistance, so the habit to build is simple: cook ahead and chill. Store cooked rice and potatoes in the fridge promptly, as you would any leftovers.

Cold dishes such as potato salad are the simplest way to use it. Whether rewarming keeps all of the gain hasn’t been tested in the studies covered here.

To see how much total fiber suits your own day as you add these foods, our fiber calculator gives you a personal figure.

Gut health and getting used to resistant starch

Because gut bacteria ferment resistant starch, it can cause gas or bloating, especially from concentrated powders like raw potato starch or green banana flour.

Food sources tend to be gentler because the amounts are smaller and spread across meals. A spoonful of green banana flour stirred into a smoothie is a far bigger dose of fermentable starch than a side of potato salad. Starting with one cooled-starch meal a day lets your gut bacteria adapt.

If you have IBS, note that some sources, such as beans, carry other fermentable carbs too. Cooled rice and potatoes are often easier starting points.

Our gut health guides walk through how to build up fermentable foods at a pace your gut can handle.

One thing to try this week

Cook a double batch of potatoes or rice at dinner one night and chill half for the next day. Turn it into a potato salad with a vinegar-based dressing or a cold rice bowl with beans and vegetables.

It costs no extra money and only a little planning, and it puts both the cooling and the vinegar findings from the Lund study on your plate.

Sources

• Birt DF, Boylston T, Hendrich S, et al. “Resistant Starch: Promise for Improving Human Health.” Advances in Nutrition. 2013. doi.org
• Leeman M, Östman E, Björck I. “Vinegar Dressing and Cold Storage of Potatoes Lowers Postprandial Glycaemic and Insulinaemic Responses in Healthy Subjects.” European Journal of Clinical Nutrition. 2005. nature.com
• Costa ES, França CN, Fonseca FAH, et al. “Beneficial Effects of Green Banana Biomass Consumption in Patients with Pre-Diabetes and Type 2 Diabetes: A Randomised Controlled Trial.” British Journal of Nutrition. 2019. cambridge.org
• Baxter NT, Schmidt AW, Venkataraman A, et al. “Dynamics of Human Gut Microbiota and Short-Chain Fatty Acids in Response to Dietary Interventions with Three Fermentable Fibers.” mBio. 2019. doi.org