Two new studies in The BMJ provided more objective and detailed evidence that consuming fruits, vegetables, and whole grains lowers risk for type 2 diabetes mellitus (T2DM).
The first study looked at two biomarkers of fruit consumption, vitamin C and carotenoids, rather than relying on self-reports from questionnaires. “Previous research studies have typically used dietary food frequency questionnaires to assess fruit and vegetable intake, which are subject to measurement error and recall bias,” Nita Forouhi, PhD, of the University of Cambridge School of Clinical Medicine in England, and colleagues wrote online.
“Circulating plasma vitamin C and carotenoids have been proposed as objective biomarkers of fruit and vegetable intake, with evidence for their validity from observational and experimental studies,” Forouhi’s group said. They analyzed blood samples from more than 22,000 individuals and reported that higher plasma vitamin C was associated with a lower risk of developing T2DM (hazard ratio per standard deviation 0.82, 95% CI 0.76-0.89). They found a similar inverse association for total carotenoids (HR per standard deviation 0.75, 95% CI 0.68-0.82).
The researchers also reported a dose-response relationship. They categorized individuals into five groups based on a composite score that included both vitamin C and carotenoids. Compared with the lowest-scoring group, risk for diabetes was reduced by 50% for the highest scoring group and by 41% to 23% for the other groups.
Given that few people eat the recommended five servings of fruit and vegetables per day, the dose-response relationship has important implications, Forouhi and colleagues said. “Our findings suggest that higher fruit and vegetable consumption is inversely associated with the incidence of type 2 diabetes, regardless of whether this increase in consumption is from a level below or above the recommended five a day threshold. The public health implication of this observation is that the consumption of even a moderately increased amount of fruit and vegetables among populations who typically consume low levels could help to prevent type 2 diabetes,” they said.
Forouhi’s group analyzed blood samples from 22,833 participants in the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort. The samples were taken and stored at baseline. During an average of 9.7 years of follow up, 9,754 participants developed T2DM. The analyses adjusted for factors including age, sex, smoking status, and education. Limits of the study included that biomarkers are often influenced by individual genetic and metabolic variations, which were not accounted for, the authors noted.
However, use of biomarkers was also the study’s main strength, Lauri Wright, PhD, director of the clinical nutrition program at the University of North Florida in Jacksonville, said in an email to MedPage Today. “The use of biomarkers is a strength of the research because it is a more objective and accurate assessment of fruit and vegetable intake than a food frequency questionnaire,” Wright said. “Food recalls have notoriously been subject to recall bias and measurement error which has resulted in some inconsistency in the research on the link between fruit and vegetable intake and diabetes.”
Examining Whole Grains
The second study took a closer look at whole grain consumption. It relied on self-reports on dietary questionnaires from participants of the Nurse’s Health Study and similar cohorts, but in addition to considering whole grain consumption overall it also examined specific foods. These included whole grain breakfast cereal, oatmeal, dark bread, brown rice, added bran, wheat germ, and popcorn.
“Despite a similar proportion of bran and germ…individual whole grain foods usually contain various amounts of dietary fiber, antioxidants, magnesium, and phytochemicals which might result in differential effects of different types of whole grain foods on cardiometabolic health,” Qi Sun, MD, of the Harvard T.H. Chan School of Public Health in Boston, and colleagues explained in their BMJ report.
Sun’s group reported that, compared with the lowest intake group, the highest consumption levels of several commonly eaten whole grain foods were significantly associated with a lower risk of T2DM:
- Breakfast cereal: HR 0.81 (95% CI 0.77-0.86)
- Dark bread: HR 0.79 (0.75-0.83)
- Oatmeal: HR 0.79 (0.75-0.83)
- Brown rice: HR 0.88 (0.82-0.94)
- Added bran: HR 0.85 (0.80-0.90)
- Wheat germ: HR 0.88 (0.78-0.98)
The odd food out was popcorn (HR 1.08, 95% CI 1.00-1.17), for which the study found a J-shaped association in which diabetes risk was not significantly raised until individuals ate more than one serving per day.
The study also found that, overall, participants in the highest category of whole grain consumption had a 29% lower rate of T2DM (95% CI 26%-33%) compared with those in the lowest category. “These findings provide further support for the current recommendations that promote increased consumption of whole grain as part of a healthy diet for the prevention of type 2 diabetes,” Sun and colleagues wrote.
The study included more than 158,000 women and 36,000 men from the Nurse’s Health Study, the Nurse’s Health Study II, and the Health Professionals Follow-Up Study. None of these participants had diabetes, cardiovascular disease, or cancer at baseline. During 4.6 million person-years of follow up, 18,629 participants developed T2DM. An important limitation of the study was that it included primarily white healthcare professionals and so might not be generalizable to other populations, the researchers said.
Forouhi and colleagues were supported by the EU FP6 Programme, the InterAct project, the EPIC-CVD project, and the MRC Cambridge Initiative. No authors reported relationships with industry.
Sun’s group was supported by the National Institutes of Health. No authors reported relevant conflicts of interest.
Wright reported no conflicts of interest.