The increase in obesity and its consequences for health continually drives new research and one ongoing avenue of exploration is resistant starch. In this context, researchers in Sri Lanka have recently been looking at starch in rice and how cooking can increase the amount of resistant starch in this commonly eaten food.
Starch in food can be digestible or indigestible. Unlike digestible types of starch, indigestible or resistant starch is not broken down in the small intestine where carbohydrates are normally metabolised to glucose and other simple sugars and absorbed into the bloodstream. Glucose in the blood is converted into glycogen and stored in the liver and muscles for future use, but any excess glucose that does not get converted to glycogen ends up being stored as fat which can contribute to weight gain. Thus, the researchers reasoned that if they could transform digestible starch into resistant starch, then that could lower the number of usable calories in rice.
The team experimented with 38 kinds of rice from Sri Lanka, developing a new way of cooking rice that increased the resistant starch content. In this method, they added a teaspoon of coconut oil to boiling water. Then, they added a half a cup of rice. They simmered this for 40 minutes, but boiling for 20-25 minutes would also suffice. Then, they refrigerated the rice for 12 hours. This procedure increased the resistant starch by 10 times compared with rice prepared in the usual manner. Reheating the rice for consumption does not affect the resistant starch levels.
Resistant starch has many characteristics that could contribute to weight management including reducing excessive insulin secretion after meals, increasing release of gut satiety peptides, increasing fat oxidation, lowering fat storage in the body’s fat cells, and preservation of lean body mass. Retention of lean body mass during weight loss or maintenance would prevent the decrease in basal metabolic rate and, therefore, the decrease in total energy expenditure, that occurs with weight loss. Resistant starch also has fibre-like properties that may increase the thermic effect of food thereby increasing total energy expenditure. Due its ability to increase fat oxidation and reduce fat storage, resistant starch has been promoted in the popular press as a “weight loss wonder food” but resistant starch’s weight loss attributes are not yet proven.
The next step for the Sri Lankan team will be to complete studies with human beings to learn which varieties of rice might be best suited to increasing the content of resistant starch. The team also will check out whether other oils besides coconut have this effect. Cooking and cooling potatoes also increases the level of resistant starch in potato. The significance of the cooking and cooling process is that amylose, the digestible part of the starch, leaches from the rice granules into the cooking liquid solution as a random coil polymer. Upon cooling, the random coil polymers begin to re-associate as double helices stabilised by hydrogen bonds that converts the digestible amylose into resistant starch. Reheating the rice or potato for consumption does not affect the levels of resistant starch, but care should be taken to reheat any food properly because of the risk of microbial growth.