Resistant starch could be decisive in determining the glycemic index of rice cultivars

Rice (Oryza sativa L.) is consumed by more than half of the world's population for whom it is the main source of nutrients and carbohydrates. Rice starch is hydrolyzed by enzymes in the digestive tract and converted into glucose which is the main energy source for metabolic functions. After meeting the energy requirement of the body, the extra calories from starch are stored as glycogen or fats for later use. Therefore, overeating rice with sedentary lifestyle potentially leads to some health problems, such as obesity, type-II diabetes, and colon diseases in long terms especially in Asian countries. Starch hydrolysis begins in the mouth with the action of salivary α-amylase and continues in the small intestine with involvement of other enzymes. However, the resistant starch (RS) which normally comprises < 3% of cooked rice escapes digestion and therefore, its calories are unavailable for use by cells. RSs are categorized into five types based on their mechanism of resistance to enzymatic digestion. Rice contains type 5 RS, wherein amylose forms complex with lipids making it more thermostable. The more the RS, the slower the digestion of rice and the lower is the glycemic index (GI), which is indicative of the ability of food to raise the blood sugar level. The GI of rice is known to be relatively high compared to other starchy foods. It was revealed that increased RS content in rice grain is mediated by soluble starch synthase (SSIIIa), which requires high level expression of granule bound starch synthase (gbssI). In this study, biochemical analysis was done to determine the GI, RS and amylose content (AC) in rice genotypes from different ecologies. Large variation in the value of GI (60.07-70.36), RS (0.35-2.57%) and AC (03.79-23.32%) was observed. Among the genotypes studied, Mahsuri showed lowest GI (60.07) and highest RS (2.57%). The highest value for GI (70.36) was found in Abhishek with relatively low RS (0.83%). O. brachyantha had the lowest RS content (0.35%) with relatively high GI (68.84). A significant negative correlation (R = −0.688) was also observed between GI and RS. Expression analysis of gbssI was carried out in developing grains of three rice genotypes (Mahsuri, Abhishek and Vandana) differing widely in GI, RS and AC. There was dramatic increase in the expression levels of the gene in the middle stage of grain development in all the three genotypes. Maximum expression of the gene was observed in Mahsuri at middle stage showing a positive correlation between RS content and gbssI expression in the rice cultivars studied. These findings emphasize upon the need to identify and develop rice genotypes with high RS, amylose and low GI which may be suitable for consumption by people suffering from diabetes, obesity and colon diseases.