บทความงานวิจัย

March 9, 2021 Professor Dr Apichart Vanavichit, at the Rice Science Center, reveals the hard solution to develop low glycemic rice for diabetes, starting with comment on soft-texture white rice as a health risk factor

Rice is a primary energy food that provides approximately 50% of the daily energy needs for Asians. However, the popularity of refined, soft-white rice contributing to high caloric intake is a significant risk factor that accounts for people being overweight.

It could signal the initiation of metabolic syndrome and result in non-communicable diseases (NCDs) such as obesity (OB), type 2 diabetes (T2B) and cardiovascular diseases (CVD). The prevalence of T2D in Thailand has increased rapidly from 6.5% in 2015 to 10.7% in 2030, with the largest increase among the elderly population (72%). In developing countries, NCD has become an immense burden among the poor elderly whose life depends on their children.

There are strong links between OB and T2B with consuming high glycemic index foods, sedentary lifestyle with OB and T2B. Recently, 415 million people, or one in every 11 persons, are identified as T2D. The disease prevalence in Thailand was one in every ten persons or 3.2 million people aged from 35 years old. Therefore, T2D has become an increasing threat to younger generations as well. The causes of T2D are complex, involving personalised nutrigenomics, sedentary habits, and glycemic indexes of foods. As the primary source of daily calorie intake for Asians, the glycemic index of rice is the main genetic improvement target. The glycemic index (GI) concerns the structural properties of carbohydrate in inducing post-prandial blood glucose spike.

Low glycemic rice: A hard lesson

Breeding low GI rice requires an understanding of carbohydrate structure, converting carbohydrate to glucose (digestibility), and insulinotropic response. Carbohydrate is a large complex of starch and fibre. Starch is composed of amylose (AM) and amylopectin (AP) polymers. The hydrolysis of AM and AP into glucose units has a direct impact on GI. Glycemic index compares how fast carbohydrate-rich foods and pure glucose affect post-prandial blood glucose in a target population. Rice with high amylose content tends to have a lower GI than rice with lower amylose content. Therefore, soft-tender rice tends to have a higher GI than hard-cooked rice.

Well-known rice cultivars with low GI (GI<55) were, for example, Doongara rice from Australia and parboiled-polished Basmati rice from India. In Thailand, low GI rice varieties were successfully developed for both white and whole-grain rice. For white rice, we successfully developed high yield, low GI PinK+4 combined with good agronomic characters and multiple resistances to Flash flooding (Sub1A), Bacterial leaf blight (xa5, Xa21), Leaf blast (QBL), and Brown planthopper (Bph, Tps) (FBBB) (Figure 1). The GI analysis of five PinK+4 lines on healthy human subjects revealed GI ranging from 48-79 (Nounmusing et al., 2018). The PinK+4 lines were improved further to develop Climate-ready, Low GI Rice by stacking more genes controlling tolerance to reproductive heat stresses and increasing water-use efficiency. However, so far, all low GI rice varieties, including the Australian Doongara, are hard-texture cooked rice.

Asian rice consumers love soft-texture white rice, particularly countries in East Asia, including China, Japan, Taiwan and Korea, prefer very soft-sticky Japonica rice. The rest of the Asians consume variable rice ranging from highly sticky rice (Laos and Thailand) to soft-fluffy cooked rice (Thailand, Myanmar, India, Sri Lanka, Bangladesh). Those hard–texture low GI rice varieties are expected to be less popular in Thailand and Asia. A strong health-conscious programme to promote using low GI rice is necessary. The best marketplace for hardy, low GI rice is for food industries to develop low GI rice snacks, pastries, noodles and retorted rice, etc.

Soft solutions of low GI rice: whole grain rice

How to develop low GI soft-texture rice for Asians? Because of its rapid digestibility, soft and sticky rice generates such a high GI as, for example, GI=91 for the soft-texture white Thai Jasmine (as glucose GI=100). If low GI comes with high amylose hard rice, what other GI-reducing factors are available? The current hypothesis suggests dietary fibre, resistance starch, polyphenol and amylase inhibitors are potential modifying factors. As bran fraction is a 30-50% higher crude fibre, rich in lipid, polyphenol and amylase inhibitors, improving the soft-cooking properties of whole grain may not significantly affect GI. Two such varieties, known as Sinlek (brown) and Riceberry (purple), are popular soft-tender whole grain rice in the Thai market.

In the short-term GI study with T2D and high cholesterol patients, Sinlek and Riceberry rice varieties showed intermediate GI equal to 58 and 62, respectively. By consuming two meal plans with Sinlek rice per day for four months, T2D and CVD patients showed significant improvement in reducing fasting blood glucose, glucose level, and triglyceride but no changes were observed in LDL-C, cholesterol and HbA1. These results are quite encouraging to advance genetic improvement for low GI soft rice.

The soft-texture, intermediate GI Riceberry and Sinlek rice varieties were improved further for resistance to diseases and insect, improved heat tolerance, water-used efficiency, and enriched polyphenol. These soft-tender whole grain rice varieties provide low to intermediate GI in a good balance of caloric content with ample dietary fibre, nutrients and micronutrients. High caloric foods rich in dietary fibre can control your appetite, resulting in fullness feeling, and prolonged hunger during intermittent fasting (IF).

Reduce insulin insensitivity is a critical outcome of consuming whole-grain intermediate GI rice. Also, consuming soft-texture intermediate GI rice provides prebiotic conditions useful for gut microbiota enrichment. Reduced gut microbiota is the cause of OB and T2D among antibiotic-resistant patients. Whole-grain rice or partially polished rice has more protein and four times more vitamin B1, and two times more calcium, magnesium and iron than polished rice. Whole grain pigmented rice is the most important source of anti­oxidants, providing well-balanced nutrient content for Low GI rice. The grand challenge to develop soft-texture rice to reduce double malnutrition is not too far from our reach.

Reference

Nounmusing et al. 2018. The effect of low and high glycemic index-based rice varieties in test meals on post-prandial blood glucose, insulin and incretin hormones response in prediabetic subjects. IFRJ 25(2):835-841.

Open Access Government – April 2021 edition.
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