Produced by plants, resistant starch is a form of plant-based dietary fibre in our food that’s good for our health.
Where resistant starch comes from
Resistant starch, as the name suggests, is a type of starch.
Starch is a form of carbohydrate polymer produced by plants to store their energy. It is made of glucose molecules joined together that form granules inside the plant.
We eat parts of plants like tubers and grains, containing starch, to give us energy.
The glucose molecules in starch can either be packed in straight, linear chains or highly branched chains. The straight, linear chains of glucose are called amylose and the branched chains are called amylopectin.
A microscope image of wildtype starch granules. Image by Brendan Fahy
A form of dietary fibre
Some starch is digested in our small intestine. But not all starch is digested here.
Resistant starch can’t be broken down by the enzymes in our small intestine. So resistant starch continues along the digestive tract to the large intestine where it is broken down by our gut microbes.
Resistant starch is a form of dietary fibre due to its ability to resist digestion in the small intestine.
A microscope image of starch granules high in amylose and other changes in starch structure. Image by Brendan Fahy
Types of resistant starch
There are five types of resistant starch, which each resist digestion in the small intestine in different ways. Some of these occur naturally while others are the result of cooking or processing.
- Resistant starch type 1 (RS1) – This type of starch is resistant because the starch is physically stuck inside the structure of the food surrounding it. Structures like cell walls and protein matrices block enzymes from breaking down the starch.
- Resistant starch type 2 (RS2) – The second type of starch is resistant because of its native structure and composition. Amylose tends to be less digestible than amylopectin. So, starch granules higher in amylose are likely to escape digestion in the small intestine.
- Resistant starch type 3 (RS3) – Another way starch can gain resistance to digestion in the small intestine is through cooking and then cooling. This is called retrograded starch. During heating in water the starch polymers leach out of granules, which when cooled, they form complexes that cannot be broken down by digestive enzymes.
- Resistant starch type 4 (RS4) – Chemically modified starch is the fourth form of resistant starch which is formed through chemical reactions where bonds are formed between the glucose polymers.
- Resistant starch type 5 (RS5) – There are two forms of resistant starch type 5. The first is where amylose in bonded to lipid molecules, though either cooking or processing. The second is where maltodextrin, a form of processed starch, is rearranged through processing in a way which means its structure becomes resistant to digestion in the small intestine.
PulseON®, an example of RS1. Image credit Cathrina Edwards
Why resistant starch is good for us
Resistant starch has several health benefits due to its ability to avoid digestion in the small intestine.
Blood glucose levels
Firstly, foods with resistant starch release sugar more slowly than regular starch, due to its slower digestion. This means it’s less likely to cause blood sugar spikes, so foods high in resistant starch can be helpful for preventing and managing diabetes.
Help you feel fuller for longer
The slower digestion of resistant starch may also make us feel fuller for longer. This is known as increased satiety.
Good for our gut microbes
Resistant starch is broken down in the large intestine by good gut microbes, who use it as fuel for fermentation. It’s therefore thought that resistant starch can help to encourage a healthy microbiome by promoting growth of good gut microbes.
The gut microbes break down the resistant starch into compounds called short-chain fatty acids which have health impacts across the body.
How can we increase the amount of resistant starch in our diet
Currently there are no official recommendations of exactly how much resistant starch we should be eating. But we are recommended to eat 30g of fibre a day.
Foods naturally high in resistant starch include less ripe bananas, pulses and potatoes.
Because some forms of resistant starch are formed through cooking and cooling, leftovers like cold pasta and potatoes are high in resistant starch too.
At the Quadram Institute we’re working to develop foods with more resistant starch and study how they are digested.
Dr Brittany Hazard’s group are developing new breeds of wheat with higher levels of resistant starch to improve the health impact of wheat.
Meanwhile, Dr Cathrina Edwards’s team have created a new ingredient from chickpeas called PulseON®. PulseON uses specially developed milling and drying processes that preserve the cellular structure containing the starch, making it more resistant to digestion.
In Dr Fred Warren’s group, researchers study the structural factors that control the breakdown of starch throughout the digestive tract. Together with Professor Pete Wilde and a wider collaboration with the John Innes Centre, Imperial College London, and the University of Glasgow, they studied wrinkled peas, naturally occurring variants of peas that produce a greater amount of resistant starch.
By combining knowledge of food structure with experiments and clinical trials, we are learning more about why resistant starch is good for us and how to increase our intake in our diet.
Banner image: Starch granules from wheat with more amylose and other changes in starch structure. Image by Brendan Fahy