White Rice Consumption and Risk of Type 2 Diabetes
White Rice Consumption and Risk of Type 2 Diabetes
In this meta-analysis of prospective cohort studies, we found that higher white rice consumption was associated with a significantly elevated risk of type 2 diabetes. This association seems to be stronger for Asians than for Western populations. A dose-response analysis showed that each serving per day of white rice consumption was associated with an 11% increase in risk of diabetes in the overall population.
Several caveats of this meta-analysis are worth discussing. Firstly, although the ethnicity stratified analysis did not show significant heterogeneity within each group, the limited number of studies may lead to diminished statistical power for detecting heterogeneity within each stratum. Secondly, although we included the results from only the fully adjusted models, because all individual studies were observational in nature the results of these studies may still be subject to residual confounding or other biases. Confounding by socioeconomic status is of particular concern because this is both a risk factor for type 2 diabetes and a predictor of rice consumption in Asian and Western populations. However, the US studies consisted of participants from the same professional background, so confounding by socioeconomic status was likely to be small. In addition, other studies controlled for indicators of socioeconomic status such as income and education. Nevertheless, residual confounding by socioeconomic status cannot be completely ruled out in these studies. Depending on the nature of uncontrolled or residual confounding, the associations seen in these individual studies and our meta-analysis could be biased in either direction. Large scale pooling projects, in which covariate adjustments and statistical analysis can be standardised, are needed to confirm the findings of this meta-analysis. Likewise, the dose-response relation could be more precisely modelled in such pooling projects.
Thirdly, all studies used food frequency questionnaires to assess levels of white rice intake. Although validation studies showed reasonable reproducibility and validity of self reported rice intake, measurement error is inevitable. Measurement error in assessment of exposure may lead to attenuation of true associations in a prospective study, especially when the exposure was assessed before disease assessment. Fourthly, although all studies excluded cases of self-reported diabetes at baseline, some undiagnosed cases may still be included in the analysis. However, the effect of such a bias is likely to be small. In the US studies, self reported diagnosis of diabetes was highly accurate; the Australian study further excluded any cases with a diagnosis date before baseline, even if they did not report diabetes at baseline interview; and in Asian studies, because rice is a staple food, substantial reduction of rice consumption after diagnosis of diabetes is unlikely. Lastly, we were unable to include brown rice in this meta-analysis or to evaluate the effects of substituting brown rice for white rice, because the association between brown rice and risk of diabetes was examined only in Sun et al’s study.
The strengths of this meta-analysis include the large sample size and long duration of follow-up of the included studies. In addition, most established risk factors for type 2 diabetes were adjusted for in the fully adjusted models in these studies. Moreover, inclusion of studies in both Asian and Western countries allowed us to investigate the dose-response relation on the basis of a wide spectrum of white rice intake levels.
Several potential mechanisms could explain the association between white rice consumption and risk of type 2 diabetes. Among Asian populations, which consume white rice as a staple food, white rice is the predominant contributor to dietary glycaemic load. For example, in women living in Shanghai, white rice accounted for 73.9% of dietary glycaemic load; in Japanese women, white rice explained 58.5% of dietary glycaemic load. In a meta-analysis that pooled data from cohort studies primarily done in Western populations, dietary glycaemic load was consistently associated with increased risk of developing type 2 diabetes. Similarly, recent investigations in Chinese and Japanese populations also support the hypothesis that high dietary glycaemic load is associated with increased risk of diabetes. The relatively weaker association for Western populations seen in this meta-analysis may be due to the fact that white rice intake was much lower than in Asians and, therefore, was only a minor contributor to dietary glycaemic load. In addition, the glycaemic index values of various white rice varieties depend on several factors including amylose content, other botanical structures, and processing methods. The contribution of white rice to dietary glycaemic load may vary substantially, especially when consumption levels are low. Nonetheless, high intake of white rice may also lead to increased risk of diabetes through mechanisms other than its contribution to dietary glycaemic load. Compared with minimally processed whole grains such as brown rice, white rice has a lower content of many nutrients including insoluble fibre, magnesium, vitamins, lignans, phytoestrogens, and phytic acid, which are lost during the refining process. Some of these nutrients, especially insoluble fibre and magnesium, have been associated with lower risk of type 2 diabetes in prospective cohort studies. Thus, a high consumption of white rice may lead to increased risk of diabetes because of the low intake of beneficial nutrients, in addition to its higher glycaemic load. Meanwhile, more data are needed to shed light on whether the interaction by ethnicity is due simply to substantially different white rice intake levels or to other mechanisms.
Data on the association between brown rice intake and type 2 diabetes are limited. In Sun et al’s work in Western populations, brown rice intake was associated with a modestly decreased risk of type 2 diabetes, and the substitution of brown rice or other whole grains for white rice was associated with a significantly lower risk of diabetes. Because Asian populations consume white rice almost exclusively, no data on the relation between brown rice and risk of diabetes are available in these populations. Nevertheless, a 16 week clinical trial in 76 Korean men showed that isocaloric replacement of white rice with whole grains and legume powder (composed of 66.6% whole grains, 22.2% legumes, 5.6% seeds, and 5.6% vegetables) led to significant reductions in serum glucose and insulin concentrations, whereas body weight remained unchanged. However, a recent study in Shanghai found that substituting brown rice for white rice for 16 weeks did not substantially affect metabolic markers in middle aged men and women, although high density lipoprotein cholesterol and diastolic blood pressure were significantly improved among people with diabetes through the brown rice intervention. More studies with larger sample sizes and longer durations of follow-up are warranted to examine the effects of substituting brown rice for white rice on risk of diabetes.
In summary, this meta-analysis suggests that higher white rice intake is associated with a significantly elevated risk of type 2 diabetes, especially among Asian populations. The recent transition in nutrition characterised by dramatically decreased physical activity levels and much improved security and variety of food has led to increased prevalence of obesity and insulin resistance in Asian countries. Although rice has been a staple food in Asian populations for thousands of years, this transition may render Asian populations more susceptible to the adverse effects of high intakes of white rice, as well as other sources of refined carbohydrates such as pastries, white bread, and sugar sweetened beverages. In addition, the dose-response relations indicate that even for Western populations with typically low intake levels, relatively high white rice consumption may still modestly increase risk of diabetes.
Discussion
In this meta-analysis of prospective cohort studies, we found that higher white rice consumption was associated with a significantly elevated risk of type 2 diabetes. This association seems to be stronger for Asians than for Western populations. A dose-response analysis showed that each serving per day of white rice consumption was associated with an 11% increase in risk of diabetes in the overall population.
Strengths and Limitations
Several caveats of this meta-analysis are worth discussing. Firstly, although the ethnicity stratified analysis did not show significant heterogeneity within each group, the limited number of studies may lead to diminished statistical power for detecting heterogeneity within each stratum. Secondly, although we included the results from only the fully adjusted models, because all individual studies were observational in nature the results of these studies may still be subject to residual confounding or other biases. Confounding by socioeconomic status is of particular concern because this is both a risk factor for type 2 diabetes and a predictor of rice consumption in Asian and Western populations. However, the US studies consisted of participants from the same professional background, so confounding by socioeconomic status was likely to be small. In addition, other studies controlled for indicators of socioeconomic status such as income and education. Nevertheless, residual confounding by socioeconomic status cannot be completely ruled out in these studies. Depending on the nature of uncontrolled or residual confounding, the associations seen in these individual studies and our meta-analysis could be biased in either direction. Large scale pooling projects, in which covariate adjustments and statistical analysis can be standardised, are needed to confirm the findings of this meta-analysis. Likewise, the dose-response relation could be more precisely modelled in such pooling projects.
Thirdly, all studies used food frequency questionnaires to assess levels of white rice intake. Although validation studies showed reasonable reproducibility and validity of self reported rice intake, measurement error is inevitable. Measurement error in assessment of exposure may lead to attenuation of true associations in a prospective study, especially when the exposure was assessed before disease assessment. Fourthly, although all studies excluded cases of self-reported diabetes at baseline, some undiagnosed cases may still be included in the analysis. However, the effect of such a bias is likely to be small. In the US studies, self reported diagnosis of diabetes was highly accurate; the Australian study further excluded any cases with a diagnosis date before baseline, even if they did not report diabetes at baseline interview; and in Asian studies, because rice is a staple food, substantial reduction of rice consumption after diagnosis of diabetes is unlikely. Lastly, we were unable to include brown rice in this meta-analysis or to evaluate the effects of substituting brown rice for white rice, because the association between brown rice and risk of diabetes was examined only in Sun et al’s study.
The strengths of this meta-analysis include the large sample size and long duration of follow-up of the included studies. In addition, most established risk factors for type 2 diabetes were adjusted for in the fully adjusted models in these studies. Moreover, inclusion of studies in both Asian and Western countries allowed us to investigate the dose-response relation on the basis of a wide spectrum of white rice intake levels.
Results in Relation to Other Studies
Several potential mechanisms could explain the association between white rice consumption and risk of type 2 diabetes. Among Asian populations, which consume white rice as a staple food, white rice is the predominant contributor to dietary glycaemic load. For example, in women living in Shanghai, white rice accounted for 73.9% of dietary glycaemic load; in Japanese women, white rice explained 58.5% of dietary glycaemic load. In a meta-analysis that pooled data from cohort studies primarily done in Western populations, dietary glycaemic load was consistently associated with increased risk of developing type 2 diabetes. Similarly, recent investigations in Chinese and Japanese populations also support the hypothesis that high dietary glycaemic load is associated with increased risk of diabetes. The relatively weaker association for Western populations seen in this meta-analysis may be due to the fact that white rice intake was much lower than in Asians and, therefore, was only a minor contributor to dietary glycaemic load. In addition, the glycaemic index values of various white rice varieties depend on several factors including amylose content, other botanical structures, and processing methods. The contribution of white rice to dietary glycaemic load may vary substantially, especially when consumption levels are low. Nonetheless, high intake of white rice may also lead to increased risk of diabetes through mechanisms other than its contribution to dietary glycaemic load. Compared with minimally processed whole grains such as brown rice, white rice has a lower content of many nutrients including insoluble fibre, magnesium, vitamins, lignans, phytoestrogens, and phytic acid, which are lost during the refining process. Some of these nutrients, especially insoluble fibre and magnesium, have been associated with lower risk of type 2 diabetes in prospective cohort studies. Thus, a high consumption of white rice may lead to increased risk of diabetes because of the low intake of beneficial nutrients, in addition to its higher glycaemic load. Meanwhile, more data are needed to shed light on whether the interaction by ethnicity is due simply to substantially different white rice intake levels or to other mechanisms.
Data on the association between brown rice intake and type 2 diabetes are limited. In Sun et al’s work in Western populations, brown rice intake was associated with a modestly decreased risk of type 2 diabetes, and the substitution of brown rice or other whole grains for white rice was associated with a significantly lower risk of diabetes. Because Asian populations consume white rice almost exclusively, no data on the relation between brown rice and risk of diabetes are available in these populations. Nevertheless, a 16 week clinical trial in 76 Korean men showed that isocaloric replacement of white rice with whole grains and legume powder (composed of 66.6% whole grains, 22.2% legumes, 5.6% seeds, and 5.6% vegetables) led to significant reductions in serum glucose and insulin concentrations, whereas body weight remained unchanged. However, a recent study in Shanghai found that substituting brown rice for white rice for 16 weeks did not substantially affect metabolic markers in middle aged men and women, although high density lipoprotein cholesterol and diastolic blood pressure were significantly improved among people with diabetes through the brown rice intervention. More studies with larger sample sizes and longer durations of follow-up are warranted to examine the effects of substituting brown rice for white rice on risk of diabetes.
Conclusions
In summary, this meta-analysis suggests that higher white rice intake is associated with a significantly elevated risk of type 2 diabetes, especially among Asian populations. The recent transition in nutrition characterised by dramatically decreased physical activity levels and much improved security and variety of food has led to increased prevalence of obesity and insulin resistance in Asian countries. Although rice has been a staple food in Asian populations for thousands of years, this transition may render Asian populations more susceptible to the adverse effects of high intakes of white rice, as well as other sources of refined carbohydrates such as pastries, white bread, and sugar sweetened beverages. In addition, the dose-response relations indicate that even for Western populations with typically low intake levels, relatively high white rice consumption may still modestly increase risk of diabetes.