Does rice affect diabetes risk?

“Replacing white rice with brown rice and wholemeal bread could cut the risk of diabetes by a third,” the BBC reported.

This news story is based on a study that found that people who ate white rice more than five times a week had a 17% higher risk of type 2 diabetes than those who ate it less than once a month. The researchers estimated that a person’s diabetes risk was reduced by 16% if a portion of white rice was replaced with brown rice and 36% if it was swapped with wholegrains.

This well-conducted study has many strengths, but also has several limitations. As a cohort study, it cannot prove causation, but only draw associations. It is possible that other factors are responsible for the difference in risk, although researchers did adjust their findings to account for some of these. Also, the higher risk associated with white rice was based on a high intake (more than five servings a week).

These findings support current advice that most carbohydrate intake should come from whole rather than refined grains. Brown rice is recommended over white because wholegrains have more nutrients and are better for health.

Where did the story come from?

The study was carried out by researchers from the Harvard School of Public Health, Brigham and Women’s Hospital and Harvard Medical School, all in Boston, Massachusetts. It was funded by the US National Institutes of Health and published in the peer-reviewed journal Archives of Internal Medicine.

Overall, the BBC’s coverage was accurate, but reporting that the risk of diabetes could be cut “by a third” by replacing white rice with brown rice and wholemeal bread is possibly misleading. The researchers did estimate that switching from white rice to wholegrains could reduce the risk of diabetes by 36%, but this is what is known as a ‘relative’ risk reduction. As such, it only indicates the likelihood of diabetes developing in people who eat white rice compared with those who eat wholegrains. Although relative risk reduction is often used by researchers in their results, it gives no indication of what the risk of developing the disease is to begin with. In this case, that risk was about 5%, or five people in every hundred, developing diabetes.

The BBC correctly pointed out that the study was based on self-reported questionnaires, which could make its results more prone to error.

What kind of research was this?

The researchers point out that rice consumption is increasing rapidly in Western diets. Because of the differences in processing and nutrient content, they argue that brown and white rice may have different effects on the risk of type 2 diabetes. White rice causes immediate rises in blood sugar levels, as measured by the glycaemic index (GI), whereas brown rice, like other wholegrains, releases sugars and energy more slowly. A higher dietary GI has been consistently associated with a higher risk of type 2 diabetes. Research in Asian populations, where rice is often the main source of carbohydrates, has found a higher risk of diabetes associated with high intake, but little is known about white rice intake and diabetes risk in people following Western diets.

To examine the relationship between type of rice consumption and diabetes risk, the researchers used data from three large prospective cohort studies of nurses and other health professionals in the US, all of which included information about diet. A prospective cohort study follows groups of people over a period of time. By recording details such as diet and lifestyle, this type of research is useful in looking at the risk factors that might be associated with the development of certain conditions. However, on its own, a prospective cohort study cannot prove cause and effect.

The studies used here were the Health Professionals’ Follow-up Study and the Nurses’ Health Study (which has two separate parts). Overall, the current study examined the diet, lifestyle practices and health status of nearly 40,000 men and about 157,000 women.

What did the research involve?

All three studies used similar Food Frequency Questionnaires (FFQs). The questionnaires were distributed to participants at the beginning of each study and then every four years between 1984 and 2003. Participants were asked how often on average they consumed a standard portion size of each food (including rice).

For the current study, researchers then divided participants into five categories of white rice intake, ranging from less than one serving a month to more than five servings a week; and into three categories of brown rice intake, from less than one serving a month to more than two servings a week. They also looked at people’s intakes of wholegrains generally, including, for example, bran, barley and wholewheat.

The researchers then looked at the number of people in all the studies who went on to develop type 2 diabetes between the first questionnaire and 2006. People who reported this diagnosis were sent another questionnaire to confirm it, using established criteria for self-reported diagnosis. Standardised statistical methods were then used to analyse any association between type of rice intake, wholegrain intake and the development of diabetes.

The results were adjusted for age and also to take into account things that might influence the risk of type 2 diabetes. This included established risk factors, such as ethnicity, body mass index (BMI), smoking, alcohol intake, multivitamin use, lack of physical activity and family history of diabetes. The nurses studies were also adjusted for oral contraceptive use, postmenopausal status and use of HRT. Researchers carried out further adjustments to take into account other dietary factors that may influence risk, such as total energy intake, and intake of red meat, fruits and vegetables, coffee and wholegrains.

What were the basic results?

Of the 197,228 people who took part in all three studies, 10,507 people developed diabetes during 14-22 years of follow-up. This equates to an absolute risk of just over 5%. These were the basic results, after researchers adjusted for other risk factors:

• People who ate more than five servings of white rice a week had a 17% greater risk of diabetes than those who ate less than one serving a month (pooled relative risk 95% confidence interval [CI]), 1.17 (1.02-1.36).
• People who ate more than two servings a week of brown rice had an 11% lower risk than those who ate less than one serving a month (pooled relative risk, 0.89 [95% CI, 0.81-0.97]).
• The researchers estimate that replacing 50 grams a day (about a third of a serving) of white rice with the same amount of brown rice would result in a 16% lower risk of type 2 diabetes (95% CI, 9%-21%).
• Replacing the same amount with wholegrains generally was associated with a 36% (30-42%) lower diabetes risk.

How did the researchers interpret the results?

The researchers comment that regular consumption of white rice was associated with a higher risk of diabetes, whereas brown rice was associated with a lower risk, independent of other risk factors.

They suggest that public health authorities should recommend that people swap refined grains, such as white rice, with wholegrains, with the aim of reducing type 2 diabetes.

Conclusion

The study appears to be the first to evaluate white and brown rice intake in relation to diabetes risk among a Western population. Its strengths include its large sample size, high rates of follow-up and that it carried out repeat assessments of the participants’ diets. The fact that all three cohort studies had similar findings means they are unlikely to be due to chance. Also, researchers took into account many established risk factors.

However, despite the quality of the study, the results do not prove that eating white or brown rice directly raises or lowers the risk of type 2 diabetes. The study has some limitations, which are noted by the researchers:

• This was a cohort study and so cannot prove causation, but only draw associations.
• The study populations were primarily health professionals, of European ancestry, so the results may not automatically apply to other groups.
• Although the researchers took into account many factors in their analysis, it is possible that other confounding factors are responsible for these findings.
• The participants reported their diets themselves. This potentially introduces bias, as people who develop diseases may be more inclined to remember lifestyle habits that are thought to contribute to the development of these diseases. The researchers point out that potential error was minimised by stopping any updating of dietary intakes after participants had reported a disease, such as diabetes.
• Diabetes diagnoses were not confirmed by glucose tolerance tests. However, the researchers say that the extra questionnaire confirming diagnosis has proved to be highly reliable at confirming the diagnosis in previous studies.

It should also be highlighted that the only statistically significant increase in risk among people eating white rice was the 17% increase in people who ate it five times or more per week compared with those who ate it less than once a month. Any increased risk for people in between, such as those who ate one serving a week, was not significant and so these findings are more likely to be due to chance. Also, the reduced risk for people eating more brown rice was only “moderate”, according to the researchers.

The researchers’ conclusions are in line with general recommendations that people should include more wholegrains in their diet, rather than refined carbohydrates, because they are thought to have a number of health benefits. It is possible that a lower risk of diabetes could be one of these benefits. Keeping active, and eating a balanced diet that is low in saturated fat, salt and sugar, with plenty of fruit and vegetables, are all recommended to reduce the risk of diabetes or heart disease.