Resistant starch is a carbohydrate. It doesn't digest in the small intestine but ferments in the large to provide fibrous bulk and nourish bacteria. Here are the five types of resistant starch and their influence on human health.
Resistant starch bypasses the normal digestive process to become soluble or fermentable fiber in the large intestine. This causes feelings of fullness, and feeds beneficial bacteria. It's believed to help blood sugar control.
Resistant starch appears in five major forms, depending on its structure and processing.
1. RS1 – Physically Inaccessible Resistant Starch
RS1, found in whole, unrefined grains, has its starch trapped within fibrous cell walls. Common sources include seeds, legumes, barley, oats, and brown rice.
The fibrous structure of these foods prevents digestive enzymes from breaking down the starch. Bound tightly within the plant's cell walls, this type of resistant starch is difficult for digestive enzymes to access.
Whole legumes such as lentils or chickpeas keep a significant amount of this starch even after cooking. This type of resistant starch is linked to improved digestive health and steady energy.
2. RS2 – Resistant Starch Due to Starch Conformation
RS2 is starch undigestible because of its natural molecular structure. A tightly packed composition makes it inaccessible to digestive enzymes.
Foods considered rich in RS2 include green bananas, raw potatoes, and high-amylose corn starch. RS2 transforms into more digestible forms when heated.
Thus raw potatoes and green bananas retain their resistant starch content, but lose it when cooked. RS2 in its raw form has been linked to blood sugar control by slowing release of glucose to the bloodstream.
Like all resistant starches, RS2 supports the digestive microbiota as a prebiotic.
3. RS3 – Resistant Starch Formed Through Retrogradation
RS3 is a type of resistant starch created when certain starchy foods are cooked and then cooled. The gelatinized starch molecules dissolved during cooking reform in a less digestible way.
Common examples of RS3 include cooled rice, potatoes, and pasta. A freshly cooked bowl of rice has more digestible starch, but refrigerated for several hours the starch retrogrades, forming more resistant starch.
When the food is reheated after cooling, the resistant starch content remains. RS3 is considered to lower glycemic response, so blood sugar levels rise more gradually after eating.
RS3 also feeds beneficial bacteria. Clostridium spp. produce short-chain fatty acids like butyrate, important to colon health.
4. RS4 – Chemically Modified Resistant Starch
RS4 are starches chemically altered to resist digestion. These modifications are usually done during food processing to enhance specific functional attributes, such as texture or shelf life.
Modified starches are frequently used in processed foods like baked goods or snack bars, in sauces and dressings. They also provide some health benefits. RS4 is generally recognized as safe for consumption.
This type of resistant starch behaves similarly to natural forms. It passes through the digestive system largely intact. It fuels GI bacteria and is believed to promote overall digestive health.
RS4 is of interest in industrial and functional food applications because it expands the potential of resistant starch. It can add fiber to foods in which it doesn't naturally occur.
5. RS5 – Starch-Lipid Complexes
RS5 is formed when resistant starch interacts with lipids or fats. This combination resists digestion.
RS5 has potential applications in food technology, with different melting behaviors and textures than other starches. Frying potatoes in oil or adding fats to a starchy dish can form RS5.
The starch-lipid complexes may improve the food’s glycemic response as well as fermentation benefits in the large intestine. This type of resistant starch is an emerging area of research.
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