Glycaemic index, starch digestibility and resistant starch

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Glycaemic index, based on the relative increase in plasma glucose within 3 hours after ingestion of carbohydrate, with white bread or glucose as 100 percent, has been used as a guide for the diets of non-insulin-dependent diabetes mellitus (NIDDM). Waxy and low-amylose rices had higher glycaemic indices than intermediate- and high-amylose rices (Goddard, Young and Marcus, 1984; Juliano and Goddard, 1986; Jiraratsatit et al., 1987; Tanchoco et al., 1990; M.I. Prakoso, 1990, personal communication), (Table 34). Processing, such as parboiling and noodle-making, tends to reduce the glycaemic index of rice, particularly that of high- and intermediate-amylose rices (Panlasigui, 1989; Wolever et al.,1986). By contrast, Tsai et al. ( 1990) reported that waxy rice, rice gruel, steamed rice and rice noodles had similar glycaemic indices to that of white bread in NIDDM patients. Among high-amylose rices, the low-GT, hard-gel IR42 had a higher glycaemic index than the intermediate-GT, softer-gel IR36 and IR62 (Panlasigui, 1989). By contrast, Srinivasa Rao (1970) reported that the ingestion of hard-gel IR8 resulted in a lower peak plasma glucose level than ingestion of the softer-gel Hamsa; both have high amylose and low GT.

TABLE 31 - Effect of protein content on protein quality of raw milled rice based on nitrogen balance in growing rats

^a Based on 5.8 g Lysine/16 g N as 100%% (WHO, 1985).
^b Based on carcass N analysis (Murata, Kitagawa & Juliano, 1978).

Sources: Eggum & Juliano, 1973, 1975; Eggum, Alabata & Juliano, 1981 Eggum, Juliano & Maniñgat, 1982; Eggum et al., 1987 Murata, Kitagawa & Juliano, 1978; lRRI, 1976; Resurrección, Juliano & Eggum, 1978.

TABLE 32 - Nitrogen balance data of and average-protein milled rice diets in male preschool children

^aFirst casein diet: 76.8% apparent digestibility and 30.8% apparent retention (Roxas, Intengan & Juliano, 1979).
^b First casein diet: 86.1% apparent digestibility and 35.2% apparent retention (MacLean et al., 1978).

TABLE 33 - Replacement of average-protein rice by high-protein rice in various diets: effect on nitrogen balance

^a Clark, Howe & Lee, 1971.

^b Milled rice/surgeon fish fillet (Acantharus bleaker)). (100:17 by wt), (Roxas, Intengan & Juliano, 1975)- milled rice/dehulled mung bean (Vigna radiata [L.] Wilczek), (100:18.6 by wt), (Roxas, Intengan & Juliano 19;6).

TABLE 34 - Glycaemic index of cooked milled rice and rice products of varying amylose content in normal and non-insulin-dependent diabetes mellitus (NIDDM) subjects (%)

^a GIycaemic index (Gl) based on insulin response.
^b GI based on glucose response, with glucose drink as 100%.
^cThe two Gl values given are only relative values based on waxy rice as 100%.
^d GI based on white bread as 100%.
^e Letters denote Duncan's (1955) multiple range test. Values in the same column followed by the same letter are not significantly different at the 5% level.
^f Red rice
^g Intermediate gelatinization temperature.
^hLow gelatinization temperature.

It has been hypothesized that prolonged consumption of fibre-depleted milled rice is diabetogenic because of its low soluble fibre content (0.1 to 0.8 percent), particularly at minimum temperatures above 15°C (Trowel!, 1987). Enzyme-resistant starch is reported to be affected by processing, particularly autoclaving. It acts as soluble dietary fibre in the large intestine and may have a hypocholesterolaemic effect (Englyst, Anderson and Cummings, 1983). However, reported values for enzyme-resistant starch in rice are trace to 0.3 percent (Englyst, Anderson and Cummings, 1983; Holland, Unwin and Buss, 1988). In vitro resistant starch values are 0 percent for raw and cooked waxy rice and less than I percent in raw non-waxy rice and rice noodles, but 1.5 to 1.6 percent for cooked nonwaxy rice including parboiled rice. The low values may be related to the fact that rice is cooked as whole grains, which could prevent extensive starch association. A raw milled rice of amylose-extender IR36-based mutant rice had 1.8 percent in vitro resistant starch. Because of the importance of parboiled rice in South Asia, researchers at the National Institute of Animal Science, Foulum, Denmark are determining the enzyme-resistant starch of IR rices differing in amylose content using antibiotics to suppress hind-gut fermentation of the resistant starch (Björck et al., 1987). Resistant starch was higher in cooked intermediateGT rices than in low-GT rices and was increased by parboiling (B.O. Eggum, unpublished data). In vitro resistant starch obtained from cooked rices using pullulanase and ß-amylase was characterized to be essentially amylose (90 to 96 percent 13-amylolysis limits) with 55 to 65 glucose units (IRRI, 1991b), as earlier also reported for wheat and maize starch (Russell, Berry and Greenwell, 1989).

Microbial anaerobic fermentation of resistant starch in the large intestine produces lactate, short-chain fatty acids (acetate, propionate and butyrate), carbon dioxide and hydrogen. The fatty acids are absorbed from the intestinal lumen into the colonic epithelial cells and provide about 60 to 70 percent of the energy which would have been available had the carbohydrate been absorbed as glucose in the small intestine (Livesey, 1990). Thus, the complete digestion of cooked waxy and non-waxy rice starch in infants (De Vizia et al., 1975; MacLean et al., 1978) and of raw starch in growing rats (El-Harith, Dickerson and Walker, 1976; Eggum, Juliano and Maniñgat, 1982; Pedersen and Eggum, 1983) includes the resistant starch fermented in the large intestine or hind gut. Breath-hydrogen tests in Myanmar village children 1 to 59 months old showed a high prevalence of rice-carbohydrate malabsorption (66.5 percent), (Khin-Maung-U et al., 1990a). About half of the children were in a state of current underfeeding with past malnutrition, but there was no difference between children with or without rice-carbohydrate malabsorption (Khin-Maung-U et al., 1990b). Levitt et al. (1987) reported that rice was nearly completely absorbed by healthy adult patients and caused only a minimal increase in hydrogen excretion as compared to oats, whole wheat, maize, potatoes or baked beans.

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