Refined carbohydrates are associated with an increase in the risk of developing insulin resistance, metabolic syndrome and diabetes, when compared to whole grain equivalents. This is because the fibre content of whole grains is able to slow digestion and increase production of beneficial short-chain fatty acids through colonic fermentation. Products such as refined white rice can therefore increase the glycaemic load associated with a mixed meal and this may in time contribute to the development of insulin resistance. As well as whole grains, legumes are effective in lowering the glycaemic response to food, mainly due to the high protein and fibre content which slows gastric emptying. Regular consumption of canned and fresh beans has been shown to be effective at lowering total cholesterol and triglycerides, reducing body weight, decreasing systolic blood pressure and decreasing the risk of myocardial infarction.
Researchers1 have compared the effects of white rice and beans on the risk of developing metabolic syndrome in diabetic free Costa Rican adults. Food frequency questionnaires were used to assess the intakes of beans and white rice and the diagnosis of metabolic syndrome was based on the Adult Treatment III guidelines which stipulated the acquirement of 3 of the clinical symptoms of Low HDL cholesterol, high triglycerides, abdominal obesity, impaired fasting glucose or elevated blood pressure. Nearly 30% of the subjects met the criteria for the diagnosis of metabolic syndrome, and these subjects were more likely to be older women, but less likely to smoke and be physically fit. Interestingly those subjects with metabolic syndrome were more likely to consume fewer calories and use corn or sunflower oil for cooking rather than palm oil. Nearly 63% of subjects reported consuming white rice, but only 39% reported consuming beans.
Following multivariate analysis which controlled for possible confounding variables such as body mass index (BMI), a trend for higher systolic blood pressure, plasma triglycerides, fasting blood glucose, as well as lower HDL cholesterol was associated with increased consumption of white rice. With increased consumption of beans, only a decrease in diastolic blood pressure remained after adjustment for confounding variables including BMI. However, when the ratio of white rice to beans was analysed, an increasing ratio of beans to white rice was significantly associated with lower plasma triglycerides, systolic blood pressure and diastolic blood pressure as well as higher concentrations of plasma HDL cholesterol. The substitution of one serving of beans for one serving of white rice was associated with a 35% lower odds of having metabolic syndrome, a 33% reduction in the risk of having low HDL cholesterol, and a 45% lower odds of having elevated fasting glucose concentrations.
These results suggest that limiting white rice consumption by substitution of beans reduces the risk of developing metabolic syndrome. This supports data showing that introduction of refined carbohydrates to native populations eating traditional whole food diets, leads to an increase in the prevalence of Western style diseases. For white and Chinese women, an increase risk of diabetes has been shown to occur at intakes of refined carbohydrates of 300g/d, whereas in Japanese women an intake of 560g/d is required. In contrast, research has shown that the protective effects of beans may occur at intakes of between 86 to 150g/d, with protection from metabolic syndrome requiring roughly 100g/d. While some of these effects may be attributable to the protein and fibre content of beans, they are also a good source of certain phytonutrients that may reduce oxidative stress and regulate gene expression.
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