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The 5 types of Resistant Starch

Nuances of Carbohydrate Metabolism

Dr. Jason Fung
10 min readDec 7, 2023


What is Starch?

Starches are polysaccharides (long chains of sugar molecules) arranged into two main structures — amylopectin and amylose. Most starches in the human diet (wheat, rice, oats etc.) are about 70% amylopectin and 30% amylose and are digested at varying rates, with differing rates of blood glucose increase and insulin release, with all its weight loss implications.

Rapidly digested starches are quickly absorbed by the intestines and causes a large increase in blood glucose and insulin. This effect can be measured by the glycemic index (GI). The hormone insulin tells our body to store energy, in the form of glucose or body fat, which is bad if you have too much stored glucose (type 2 diabetes) or too much stored body fat (obesity.

Starches can be classified as

1. Rapidly Digestible Starch (RDS) — releases glucose within 20 minutes of enzymatic hydrolysis (breaking down into glucose) — Bad

2. Slowly Digestible Starch (SDS) — releases glucose within 20–120 minutes — Better

3. Resistant Starch (RS) — cannot be digested — Best

Previously, we believed the length of the glucose chain mostly determined how quickly blood glucose rose. Longer chains, so-called ‘complex carbohydrates’ were supposed to release glucose more slowly. Shorter chains, so-called ‘simple carbohydrates’ were supposed to release their glucose quickly into the blood. This view is completely wrong.

Chain length plays virtually no role in determining how quickly or slowly a carbohydrate is absorbed. This complex vs simple classification is as useful as a wet newspaper. Instead how this long chain of glucose is arranged (its molecular structure) plays an important role. Starches are arranged into two major structures — amylopectin and amylose.

Amylopectin versus Amylose

Amylopectin is highly branched with 2000–2 million glucose molecules with branches every 24–30 glucose units. This structure…



Dr. Jason Fung

Nephrologist. New York Times best selling author. Interest in type 2 diabetes reversal and intermittent fasting. Founder