In these three prospective cohort studies of U.S. men and women, we found that regular consumption of white rice was associated with higher risk of T2D, whereas brown rice intake was associated with lower risk. In addition, our data suggest that replacing white rice intake with the same amount of brown rice or whole grains was associated with a lower risk. These associations were independent of lifestyle and dietary risk factors for T2D, as well as ethnicity.
In Asian populations in whom rice is a staple food, higher white rice consumption has been associated with elevated risk of diabetes or metabolic syndrome.6, 29, 30
For example, white rice consumption was prospectively associated with developing type 2 diabetes in Chinese women living in Shanghai.6
In addition, in Asian Indians and Japanese, higher intake of refined grain including white rice was associated with metabolic risks in cross-sectional analyses.29, 30
In comparison to Asian populations, white rice intake in Western populations was much lower. White rice consumption contributed, on average, less than 2% of total energy intake in our study populations. In contrast, in the aforementioned Chinese female population, white rice consumption accounted for 53.7% of total energy intake (Xiao-Ou Shu, personal communication).6
Likewise, according to the Japanese National Nutrition Survey, white rice accounted for 29% of daily total energy intake in Japanese.31
Consumption of rice or food groups consisting of rice in relation to risk of type 2 diabetes was also evaluated in Western populations, but mixed results were observed.32–34
However, brown rice was not separated from white rice or other refined grains in these studies 32–34
. To our knowledge, the current studies are the first prospective investigations conducted among Western populations that have specifically evaluated white rice and brown rice intake in relation to T2D risk. In our cohorts only 0.9% (NHS I) to 2.2% (NHS II) of total participants reported having five or more servings per week (≥107 grams/day) of white rice, which was within the lowest reference level (<200 grams/day) in the prospective study of Chinese women.6
However, by pooling data from the three studies, we detected a significant association for white rice intake. Our data are consistent with the Chinese study, in which white rice intake of 300 grams/day or more (equivalent to 2 servings/day in our analysis) was associated with a 78% increased risk of T2D in comparison to intake levels of less than 200 grams/day.6
We observed a moderate, inverse association for brown rice intake. Because brown consumption levels were rather low in our participants, we could not determine whether brown rice intake at much higher levels is associated with a further reduction of diabetes risk. Nonetheless, we found that substitution of brown rice for white rice was associated with a significantly lower risk of developing diabetes. Consistent with our previous analyses,9, 10
we found a significant inverse association between whole grain consumption and diabetes risk. Substitution of whole grains for white rice was more strongly associated with diabetes risk than the substitution of brown rice. This observation may result from the more reliable estimates of the association with diabetes for whole grains than those for brown rice, because of the low overall consumption of brown rice. In addition, whole grains included multiple grains with various nutrient compositions and, thus, possibly various effects on glucose response. For example, whole wheat and barley generate lower glucose response than brown rice: the GI values were 41 ± 3 for whole wheat, 25 ± 1 for barley, and 55 ± 5 for brown rice.2
As a consequence, in comparison to whole wheat and barley, the same amount of brown rice likely bears a higher glycemic load, which is an established risk factor for T2D.35
Depending on the botanical structure, amylase contents, and processing methods, both white rice and brown rice demonstrated a wide variety of GI values,2, 36–38
which made it difficult to directly compare white rice with brown rice for effects on postprandial glucose response.2
Despite of this inconsistency inherent to rice GI values, in general, white rice consumption generates a relatively stronger postprandial glucose response than the same amount of brown rice2
This notion was corroborated by the observation that isocaloric replacement of white rice with whole grains (66.6%; primarily composed of brown rice and barley) and legume powder (22.2%) significantly decreased postprandial glucose and insulin levels in a randomized clinical trial.39
The high GI of white rice consumption is likely the consequence of disrupting the physical and botanical structure of rice grains during the refining process, in which almost all of the bran and some of the germ are removed.40
The other consequence of the refining process includes loss of fiber, vitamins, magnesium and other minerals, lignans, phytoestrogens, and phytic acid,7
many of which may be protective factors for diabetes risk. Intact rice grains contain nearly exclusively insoluble fiber.7
In both observational and experimental studies, insoluble fiber intake was consistently associated with improved insulin sensitivity and decreased risk of developing T2D.4, 5, 41, 42
In addition, higher magnesium intake has been consistently associated with reduced risk of T2D in cohort studies or improved glucose metabolism in clinical trials.43–45
The combination of these mechanisms may explain the beneficial effects of replacing white rice with brown rice or other whole grains.
The strengths of the current study include a large sample size, high rates of follow-up, and repeated assessments of dietary and lifestyle information. The consistency of the results across all three cohorts indicates that our findings are unlikely due to chance. The current study was subject to a few limitations as well. First, our study populations primarily consisted of working health professionals with European ancestry. Although the homogeneity of socioeconomic status helps reduce confounding, the generalizability of the observed associations may be limited to similar populations. However, the biological mechanisms underlying the positive associations observed in both our study populations and the Chinese study6
are likely to be the same in other populations. Second, because diet was assessed by FFQs, some measurement error of rice intake assessment is inevitable. However, the FFQs used in these studies were validated against multiple diet records, and reasonable correlation coefficients between these assessments of rice intake were observed.14
Since we employed a prospective study design, any measurement errors of rice intake are independent of study outcome ascertainment, and, therefore, are likely to attenuate the associations towards the null. Moreover, we calculated cumulative averages of rice intake to minimize the random measurement errors caused by within-person variation. To minimize the possibility of systemic measurement error incurred by recall bias, we not only excluded participants with a history of major chronic diseases at baseline but also stopped updating dietary intake after participants reported having diagnoses of diseases that might influence their subsequent report of diet. Third, we did not perform oral glucose tolerance tests to confirm diabetes diagnoses because this is infeasible in large cohort studies. However, the supplementary questionnaire that we used for the confirmation of self-reported diabetes diagnoses has been demonstrated to be highly accurate.20, 21
Lastly, although we adjusted for established and potential risk factors for T2D, residual confounding is still possible.
Our data suggest that regular consumption of white rice is associated with an increased risk of T2D, whereas replacement of white rice by brown rice or other whole grains is associated with a lower risk. The current Dietary Guidelines for Americans identifies grains, including rice, as one of primary sources for carbohydrate intake and recommends at least half of carbohydrate intake should come from whole grains.46
Rice consumption in the U.S. population is increasing.8
However, most rice consumption is refined white rice,8
as seen in our studies. From a public health point of view, replacing refined grains such as white rice by whole grains, including brown rice, should be recommended to facilitate the prevention of type 2 diabetes.