Rice consumption in the United States has increased in recent decades from 27% of adults reporting consumption of >1/4 oz of rice per day (NHANES 1999–2004) [1
] to 84% of adults (NHANES 2005–2010) [2
]; however, daily intake is still relatively low, with 59% of adults consuming 0.25 to 0.5 oz equivalents of rice per day (NHANES 2005–2010). Globally, rice intake varies widely per capita, from 5.2 kg/year in Europe to 77.2 kg/year in Asia [3
]. With trends of increasing rice consumption, it is necessary to understand the differences in carbohydrate availability among the different cultivars of rice.
Rice is typically considered a highly digestible source of carbohydrate, but rate of digestion and the resulting glycemic response varies among rice cultivars and preparation techniques [4
]. Amylose content of rice affects glycemic response due to the tendency of amylose to retain its crystalline structure after cooking. This reduces enzyme accessibility, and results in greater proportions of slowly-digestible starch and resistant starch (RS). A study of sixteen rice cultivars grown in the United States reported wide ranges in amylose content (0.3%–31.3% dry weight basis (dwb)), slowly digestible starch (13.6%–23.3% dwb) and resistant starch (2.0%–6.7% dwb), despite the total starch content being highly uniform (82.2%–86.6% dwb) among the cultivars [6
]. This study previously reported that the rapidly digestible, slowly digestible starch and resistant starch content of Dixiebelle and Rondo to be 61.7%, 23.2%, 4.7% and 60.9%, 16.8%, 6.3% dwb, respectively. When compared to low-amylose rice, the starch profiles were not consistently, significantly different. The variations in starch characteristics translated to differences in in vitro
digestibility among the cultivars.
Resistant starch, a type of dietary fiber, is defined as all types of starch and starch degradation products that resist digestion and absorption in the small intestine when consumed, and enter the large intestine [7
]. Resistant starch is associated with many health benefits such as improved glycemic control, improved insulin sensitivity, improved digestive health and weight management (as reviewed by [9
]). Rice with high RS content has the potential to exhibit the beneficial physiological effects associated with RS intake. This is of particular interest, as rates of obesity, diabetes, and cardiovascular disease are increasing worldwide. Further research is necessary to understand the variations in health outcome within a single type of food, for example, rice.
The objective of this randomized, single-blind, crossover clinical trial study was to investigate the effects of white (milled) rice of two cultivars having high RS on postprandial blood glucose, insulin, and appetite in healthy adults, compared to a control short-grain conventional white rice. Palatability of the rice cultivars was also investigated. It was hypothesized that high RS rice consumption would result in lower blood glucose and insulin concentrations and would be more satiating, compared to the control rice.
Solid foods containing RS have shown therapeutic effects on postprandial blood glucose response and insulin in healthy individuals. Slowly digestible starch content may also reduce postprandial glucose response. The high RS rice samples, when matched for volume, significantly attenuated postprandial blood glucose response with an RS dose amount of 2.25 g (Dixiebelle) and 2.10 g (Rondo), compared to the short grain rice (1.4 g). In a previous study, the RS dose per serving of rice that significantly improved postprandial blood glucose response in healthy adults (Chinese, nine males, seven females aged 23–26 years, BMI of 18–24 kg/m2
) was 8.05 g RS, about 6 g more than the present study [12
]. Therefore, an 8.05 g RS dose in a serving of rice may be unnecessary. However, Chiu et al.
(2013) found that a 4.4 g RS dose in a serving of rice had no effects on postprandial blood glucose response in healthy adults (12 males, nine females, mean age 29, mean BMI of 22.9, ethnicity not stated) [13
]. The effective RS dose in rice on postprandial blood glucose response needs to be further investigated, but it should be noted that RS dose may not be the only factor. High amylose rice also contains slowly digestible starch, which would contribute to blood glucose attenuation [6
The present study also found that insulin response was significantly lower at 60 min with an RS dose of 2.25 g (Dixiebelle) and 2.10 g (Rondo), and at 120 min with the RS dose of 2.10 g (Dixiebelle), compared to the short grain rice. In the Li et al.
(2010) study, the RS dose value of 8.05 g in a serving of rice significantly decreased insulin concentrations after intake starting at 45 min, then at 60, 90 and 120 min [12
]. Because insulin concentration was not measured at 45 min in the present study, it is unknown whether Dixiebelle or Rondo affected insulin response at 45 min. In addition, the subjects in the present study were habitual rice eaters, determined at the initial screening for eligibility. A recent study investigated the effects of chronic intake of rice with added RS (derived from corn starch) on glycemic response in those with or at risk for type 2 diabetes [14
]. The addition of 6.51 g RS added to short grain rice, daily, for four weeks significantly reduced postprandial glucose and insulin AUC, and fasting insulin. This study [14
] suggests that the addition of RS has beneficial effects.
Prospective studies have noted a correlation between rice intake and increased risk of type 2 diabetes in multiple cohorts (as reviewed by [15
]). Although this meta-analysis and systematic review reported a relative risk in the total population of 1.11 (1.08 to 1.14 95% CI, p
for linear trend < 0.001), the range of intakes differed tremendously between Asian cohorts and Western cohorts. For example, the upper threshold for lowest quintile or quartile in the Chinese and Japanese cohorts reviewed by Hu et al.
exceeded the lower threshold for the highest quintile or quartile in the Western cohorts [15
]. Prospective studies included in Hu et al.
’s review collected dietary intake using a food frequency questionnaire that did not differentiate the starch digestibility of the white rice samples used. As shown in the present study, white rice yields differing physiological effects, depending on the carbohydrate identity.
For other solid foods (muffins, bread, a nutrition bar, and corn cakes), a range from 6.5–15.6 g RS dose per serving significantly lowered postprandial blood glucose response and insulin response from 30 min to 2 h in healthy adults after intake [16
]. However, Hallstrom et al.
(2011) found a serving of bread (7.7 g RS) had no significant effects on insulin response in healthy adults (seven males, seven females, aged 20–35 years, mean BMI of 22.2 kg/m2
, ethnicity not stated) [20
]. Studies have also investigated the effects of intake of RS in solid foods on individuals with T2DM. Individuals with T2DM are afflicted with abnormally high fasting blood glucose concentrations, so postprandial blood glucose attenuation from RS intake may help with self-management. A treatment of rice with 7.8 g RS had no significant effects on postprandial blood glucose and insulin response in T2DM patients (seven males, five females, mean age 58, mean BMI of 30, ethnicity not stated, T2DM duration mean of four years) [21
]. However, when subjects with untreated borderline T2DM (nine males, 11 females, mean age 50.5 years, fasting blood glucose 100–140 mg/dL, BMI not stated, Japanese) consumed bread with 6 g RS, postprandial blood glucose and insulin responses were significantly reduced [22
]. Individuals with borderline T2DM may benefit from a lower dose of RS to see effects, while diabetic individuals may need a higher RS dose. Also, ethnicity may influence glycemic response. In the Yamada et al.
(2005) study, subjects were Japanese, but ethnicity is not stated for the subjects in the Larsen et al.
(1996) study [21
]. However, in another study, subjects at risk for developing T2DM with insulin resistance (eight males, seven females, mean age 36, mean BMI of 37, African American) consumed bread with 12 g RS daily for 14 weeks, but no significant differences in blood glucose or insulin response were found [23
]. The role of ethnicity in glycemic response maybe related to predisposition for diabetes, but further research is needed.
The RS dose in rice of the present study is much lower than the RS dose in the studies previously discussed. The inconsistency across food forms suggests that RS dose, alone, may not be relevant, and that food source and/or the proportions of digestible starch, slowly digestible starch, and RS may be more important.
Studies on RS in solid foods and its effects on appetite are limited. Willis et al.
(2009) found that among muffins with different fiber types (RS, low fiber, corn bran, beta-glucan and oat fiber, polydextrose), the RS muffin (8 g RS) was the most satiating in healthy adults (seven males, 13 females, aged 18–65 years, mean BMI of 23, ethnicity not stated) [24
]. However, in another study, there were no significant differences in appetite ratings after consumption of different fiber-type bars (RS (10 g), inulin, oligofructose, corn fiber) in healthy women (22 females, mean age 25, mean BMI of 23, ethnicity not stated) [25
Few studies have investigated the effect of rice, without any added ingredients or meal items, on appetite. Chiu et al.
(2013) found no significant differences in appetite between short grain rice and the rice containing 4.4 g RS in healthy adults [13
]. In the present study, the only significant difference found was at 30 min, where subjects were more hungry after Dixiebelle treatment than after the Rondo treatment. Subjects preferred the short grain rice as compared to either of the long grain varieties Dixiebelle or Rondo for taste, texture, and pleasantness. All other ratings (fullness, satisfaction, how much can you eat) had no significant differences. However, subjects were significantly more satisfied before Rondo intake than before short grain rice intake, which may have influenced their appetite after treatment intake as well.
The high RS rice cultivars were chosen for this study based on potential for expanded commercial production. Patindol et al.
(2010) compared the starch characteristics of 16 rice cultivars grown in the southern United States, at five different growing locations [6
]. The RS amount in Dixiebelle and Rondo cultivars was stable across growing locations in Arkansas, Louisiana, Missouri, Mississippi, and Texas, while other cultivars were not stable in RS amounts. Additionally, a pilot sensory evaluation indicated that habitual rice consumers preferred Dixiebelle and Rondo cultivars over other high RS cultivars (unpublished data).
There are some limitations to this study. First, only three measurements of venous blood glucose and insulin concentrations were taken, thus AUC for venous blood glucose and insulin was not calculated. The study visit protocol was modified to have only three venous blood draws (at 0, 60, and 120 min) for subject comfort and study visit ease. Also, the study subjects were healthy individuals, with no issues in carbohydrate metabolism. The influence of slowly digestible starch content on the observed glycemic response is unknown, and while the present finding suggest a beneficial role in mediating blood glucose concentrations, further work is necessary to confirm this effect in individuals with pre-diabetes or diabetes.