What is resistant starch?

Resistant starch is one of the more unusual forms of carbohydrate.

It is chemically a starch, but unlike most starch, it is not fully digested and absorbed in the small intestine. Instead, a proportion passes into the large intestine, where it can be fermented by gut microorganisms.

That gives resistant starch a nutritional profile that sits somewhere between starch and fibre.

It also explains why resistant starch is often discussed in relation to the gut microbiome, short-chain fatty acids and post-meal glucose response.

What makes a starch “resistant”?

Most dietary starch is broken down by digestive enzymes into glucose and absorbed in the small intestine.

Resistant starch escapes that process, at least in part.

The resistance can arise for different reasons. Sometimes the starch is physically trapped inside an intact plant structure. Sometimes the starch granule itself has a crystalline structure that digestive enzymes cannot easily access. Sometimes cooking and cooling causes starch molecules to reorganise into a more digestion-resistant form.

Those differences are why resistant starch is usually divided into several types.

The main types of resistant starch

RS1: physically inaccessible starch

RS1 is starch that digestive enzymes struggle to reach because it remains enclosed within an intact food structure.

Examples include some whole or partly milled grains, seeds and legumes.

The important feature is not the chemistry of the starch itself, but the physical barrier surrounding it.

RS2: naturally resistant starch granules

RS2 is found in certain uncooked starches whose granular structure resists digestion.

Common examples include green bananas, raw potato starch and high-amylose maize starch.

This is why green bananas are such a useful visual example of resistant starch. As a banana ripens, much of its starch is converted into sugars, so its resistant starch content falls substantially.

RS3: retrograded starch

RS3 can form after a starchy food is cooked and then cooled.

Cooking disrupts the original starch structure. During cooling, some starch molecules realign and recrystallise in a process called retrogradation. The resulting structure can be more resistant to digestive enzymes.

Foods in which this can occur include potatoes, rice and pasta.

This is the origin of the widely discussed “cook and cool” effect.

However, the amount of resistant starch formed is not fixed. It varies according to the food itself, its amylose content, the cooking process, cooling conditions, storage time and whether the food is reheated.

So it is more accurate to say that cooking and cooling can increase resistant starch, not that it transforms every bowl of rice or potato into a high-resistant-starch food.

RS4 and RS5

RS4 refers to starch that has been modified, commonly through food processing, to make it less digestible.

RS5 refers to starch that forms complexes with lipids. These starch-lipid structures can also be relatively resistant to digestion.

These forms are more relevant to food science and manufacturing than to most everyday discussions of whole foods.

Where do you find resistant starch in food?

Good dietary examples include:

• green bananas and plantains, particularly before they fully ripen;
• legumes, including lentils, chickpeas and beans;
• whole and minimally processed grains;
• high-amylose starches;
• cooked and cooled potatoes;
• cooked and cooled rice; and
• cooked and cooled pasta.

The amount can vary considerably between foods and even between different preparations of the same food.

That variability is one reason resistant starch is difficult to reduce to a simple food list or single daily target.

What happens when resistant starch reaches the colon?

Once resistant starch reaches the large intestine, some gut microorganisms can ferment it.

Fermentation produces metabolites including the short-chain fatty acids acetate, propionate and butyrate.

Butyrate is of particular interest because it is an important energy source for colon cells and has been extensively studied in relation to intestinal physiology.

A systematic review of 39 randomised controlled trials involving 2,263 adults examined resistant starch supplementation and short-chain fatty acid production.

Of the 23 studies that measured short-chain fatty acids, 16 reported an increase following resistant starch supplementation.

This does not mean every resistant starch produces the same fermentation profile. Different forms of resistant starch can favour different microbial populations and produce different responses between individuals.

Does resistant starch improve blood glucose?

This is an area where the details matter.

Because resistant starch is not fully digested into glucose in the small intestine, replacing some rapidly digestible starch with resistant starch can reduce the amount of immediately available carbohydrate in a meal.

Clinical studies have therefore examined effects on post-meal glucose, insulin sensitivity and longer-term glycaemic measures.

A 2023 systematic review and meta-analysis of 36 randomised controlled trials in people with type 2 diabetes or prediabetes found that some resistant starch types improved measures of post-meal glucose response.

However, the effects were not uniform across resistant starch types or outcomes.

That makes statements such as “resistant starch lowers blood sugar” too broad.

A more accurate summary is that some forms of resistant starch have demonstrated favourable effects on glycaemic response in certain populations, but the outcome depends on the type of resistant starch, dose, comparator and study population.

Is resistant starch a prebiotic?

Sometimes, but the terms are not synonymous.

Resistant starch is defined by its resistance to digestion in the small intestine.

A prebiotic, by contrast, is defined by a biological effect. The current scientific definition requires a substrate to be selectively used by host microorganisms and to confer a demonstrated health benefit.

Some resistant starches may meet that standard in particular contexts. Others may be fermentable without necessarily satisfying the full prebiotic definition.

This distinction is useful because “fermentable”, “resistant” and “prebiotic” describe different properties.

What is the difference between resistant starch and psyllium?

The contrast is useful because the two fibres have very different physical and fermentative properties.

Psyllium is highly viscous and gel-forming, but relatively poorly fermented.

Resistant starch is generally low in viscosity and much more fermentable.

Both are forms of dietary fibre, but their physical behaviour in the gastrointestinal tract is different.

Read: Psyllium, resistant starch and prebiotic fibre: what’s the difference? →

Does resistant starch replace vegetables, legumes or whole grains?

No.

A resistant starch ingredient can provide a specific fermentable substrate, but whole foods bring a much broader nutritional package.

Legumes, whole grains, vegetables, fruit, nuts and seeds provide combinations of different fibres, micronutrients, polyphenols and other plant compounds.

That is why the most useful way to think about resistant starch is as one component of fibre diversity, not as a complete substitute for it.

Why include resistant starch in a multi-source fibre blend?

Resistant starch contributes a different functional profile from highly viscous fibres such as psyllium.

Its principal distinction is that it can reach the colon relatively intact and provide a fermentable substrate for gut microorganisms.

That makes it complementary to fibres whose primary roles relate more to viscosity, water-holding or stool bulking.

This is one of the reasons MacroBlue combines several fibre sources rather than relying on a single fibre type.

Read: Why seven fibres? →

Sources

• Wang Y, et al. The molecular mechanisms and new classification of resistant starch: A review. 2025.
• Resistant starch and type 2 diabetes mellitus: Clinical perspective. 2024.
• Venkataraman A, et al. Tolerability and SCFA production after resistant starch supplementation in humans: a systematic review of randomized controlled studies. American Journal of Clinical Nutrition. 2022.
• A comparison of the effects of resistant starch types on glycemic response in individuals with type 2 diabetes or prediabetes: a systematic review and meta-analysis. 2023.
• Gibson GR, Hutkins R, Sanders ME, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the definition and scope of prebiotics. Nature Reviews Gastroenterology & Hepatology. 2017.