Starches are one of the main forms of dietary carbohydrates which composed of a number of monosaccharides or sugar (glucose) molecules linked together with α 1-4 and/or α 1-6 linkages. Starch is composed of two biochemical constituents namely amylose and amylopectin. Amylose is a linear polymer of glucose units, whereas, amylopectin is the branched polymer containing glucose subunits joined through α 1-4 as well as α 1-6 linkage.


According to the rate and extent of digestion, starch can be classified into rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS), in relation to its physiological effects after consumption. RDS is rapidly digested and absorbed in the duodenum and proximal regions of the small intestine, which leads to fast elevation of blood glucose and consequently high insulin levels. This sharp increase in glucose concentration is an underlying problem for a series of health complications such as obesity, diabetes and cardiovascular diseases. Resistant starch is not digested in the upper gastrointestinal tract, but its microbial fermentation in the colon produces short chain fatty acids (SCFA) that are beneficial for colonic health. Slowly digestible starch, an intermediate starch fraction between RDS and RS, is digested slowly throughout the entire small intestine to provide sustained glucose release with low initial glycaemia and a subsequent slow and prolonged release of glucose.


Resistant starch is further classified into four major groups; RS1, RS2, RS3, and RS4. RS1 is physically inaccessible starch, such as that located in plant tissue structures. RS2 is condensed and partially crystalline native (uncooked) starch granules. RS3 consists mainly of retrograded or recrystallized amylose, which forms in cooked products after cooling, e.g., in breads, corn flakes, or potatoes. RS4 can be produced by chemical modifications, such as conversion, substitution, or cross-linking. Such modifications prevent digestion of RS4 by blocking access to enzymes and by forming a typical linkages, e.g.,1 → 2, 1 → 3, 1 → 4, and 1 → 6.


RS has received much attention for both its potential health benefits and functional properties which are given in the following table:

Table

Health properties of resistant starches

Potential physiological effects Conditions where there may be a protective effect
Control of glycaemic and insulinaemic responses Diabetes, impaired glucose and insulin responses, the metabolic syndrome
Improved bowel health Colorectal cancer, ulcerative colitis, inflammatory bowel disease, diverticulitis, constipation
Improved blood lipid profile Cardiovascular disease, lipid metabolism, the metabolic syndrome
Prebiotic and culture protagonist Colonic health
Increased satiety and reduced energy intake Obesity
Increased micronutrient absorption Enhanced mineral absorption, osteoporosis
Adjunct to oral rehydration therapies Treatment of cholera, chronic diarrhoea
Synergistic interactions with other dietary components, e.g. dietary fibres, proteins, lipids Improved metabolic control and enhanced bowel health


Thus, Resistant starch, the portion of starch and starch products that resist digestion, appears to confer several health properties of RS.

References:

1. Sajilata M.G., Singhal R.S. and Kulkarni P.R. (2006) Resistant starch - A review. Comprehensive Reviews in Food Science and Food Safety., 5: 1-17.


About Author / Additional Info:
I am a first year Ph.D research scholar in division of biochemistry, IARI, New Delhi