Adding more water when cooking rice breaks down starch faster and pushes up the estimated glycemic index (eGI), according to a new study. Researchers cautioned, however, that the experiment used brown rice and relied on estimated figures, making it hard to apply the findings directly to firm or soft-cooked white rice.
A research team at Mahasarakham University in Thailand published the paper this year in the Italian Journal of Food Science, Vol. 38, No. 2. The team cooked Thai jasmine brown rice, which has low amylose content, at varying rice-to-water ratios and then analyzed starch gelatinization, digestibility and the estimated glycemic index. The researchers also compared differences by cooking method to determine how cooking conditions affect blood sugar responses.
Starch absorbs moisture and swells when exposed to water and heat, a process known as gelatinization. Once its structure loosens, digestive enzymes break it down more easily, speeding up glucose absorption. The study found that starch broke down more readily when more water was used than when less was used.
Weighing both texture and blood sugar indicators, the team proposed a rice-to-water ratio of 1 to 3 as the optimal condition. Steaming can also lower the potential for blood sugar increases, the researchers said. The paper noted that brown rice with high amylose content digests slowly and tends to have a lower glycemic index, but its firm texture limits consumer acceptance.
Findings do not point in the same direction across studies. A 2019 study on Thai instant rice published in the Journal of Food Science found that lower cooking temperatures and higher water ratios produced a lower estimated glycemic index. That study examined three Thai rice varieties with differing amylose content. The team there identified 82 degrees Celsius with 1.9 times the volume of water as the optimal condition. Processing conditions for instant rice differ from ordinary rice cooking, however, limiting direct comparison.
Beyond the amount of water used in cooking, portion size and grain type are also variables in blood sugar management. Even firm-cooked rice can sharply raise blood sugar if eaten in large quantities at once, as total carbohydrate intake rises. With soft-cooked rice, eating a moderate portion alongside fiber- and protein-rich foods such as vegetables and fish helps moderate blood sugar increases.
Another option is mixing white rice with grains such as brown rice, barley and oats. Brown rice is less milled than white rice and is rich in dietary fiber, B vitamins and minerals, helping slow the digestion and absorption of carbohydrates. Beta-glucan in oats is a viscous soluble fiber that helps regulate blood sugar responses.