Whole Grains Grind Down Diabetes Risk

NUTHETAL, Germany, May 14 -- Increased fiber intake from grain, but not from fruit or vegetables, may help prevent diabetes, German researchers confirmed.

In both a meta-analysis and a large prospective study, greater cereal fiber intake reduced the risk of type 2 diabetes onset by about 30%, found Matthias B. Schulze, Dr.P.H., of the German Institute of Human Nutrition Potsdam-Rehbruecke here, and colleagues.

Magnesium, a trace mineral in grain products, had a similar effect in the meta-analysis but not in the study, they reported in the May 14 issue of the Archives of Internal Medicine.

Most previous studies have also found lower diabetes risk with higher whole grain and cereal fiber consumption. The American Diabetes Association prevention guidelines suggest a total dietary fiber intake goal, but different types of fiber are thought to have differing effects.

Findings have been unclear for fruit and vegetable fiber and likewise inconsistent for magnesium.

To help pin those questions down, the researchers analyzed outcomes for the Potsdam, Germany, cohort of the larger European Prospective Investigation Into Cancer and Nutrition (EPIC) study.

The study included 9,702 men and 15,365 women (aged mainly 40 to 65 and 35 to 65 years, respectively) from the general population who did not have diabetes at baseline.

Participants reported food intake frequency and sociodemographic and lifestyle characteristics on a questionnaire on enrollment in the study. They filled out follow-up questionnaires every two to three years afterwards to monitor for development of diabetes. Potential cases were confirmed with the diagnosing physician.

Over the mean of seven years of follow-up, there were 844 incident cases of type 2 diabetes.

After adjustment for diet and other potentially confounding factors, the only factor associated with significantly lowered diabetes risk was fiber intake from grain. The relative risk for participants who ate the most cereal grain (average 17 g/day) was 27% lower compared with those who averaged only 7 g/day (RR 0.73, 95% confidence interval 0.57 to 0.94, P=0.02).

The benefit was essentially unchanged by further adjustment for other fiber intake.

Comparing highest with lowest intake quintiles, the researchers reported:

• Total fiber intake was not associated with diabetes risk (relative risk 0.86, 95% confidence interval 0.65 to 1.14, P=0.19).
• Soluble fiber did not lower diabetes risk (RR 0.78, 95% CI 0.60 to 1.01, P=0.09) and this was further attenuated by adjustment for insoluble fiber intake (RR 0.83, P=0.45).
• Insoluble fiber was not associated with lower diabetes risk (RR 0.82, 95% CI 0.61 to 1.08, P=0.10) and this was further attenuated by adjustment for soluble fiber intake (RR 0.93, P=0.62).
• Fiber intake from fruit had no significant impact on diabetes risk (RR 0.89, 95% CI 0.71 to 1.13, P=0.36).
• Fiber intake from vegetables had no impact on diabetes risk (RR 0.93, 95% CI 0.75 to 1.17, P=0.64).
• Magnesium was not linked to lower diabetes risk (RR 0.90, 95% CI 0.72 to 1.12, P=0.44).

The researchers also conducted separate meta-analyses of prospective cohort studies of type 2 diabetics for effects of fiber and magnesium.

They pooled eight studies of grain, vegetable, or fruit fiber intake and five of magnesium intake. Again comparing highest to lowest intakes, the findings were:

• Reduced diabetes risk with higher cereal fiber intake (RR 0.67, 95% CI 0.62 to 0.72).
• No effect on diabetes risk from fruit fiber (RR 0.96, 95% CI 0.88 to 1.04).
• No effect on diabetes risk from vegetable fiber (RR 1.04, 95% CI 0.94 to 1.15).
• Reduced diabetes risk with greater magnesium intake (RR 0.77, 95% CI 0.72 to 0.84).

Cereal fiber consumed in the United States consists mainly of whole-grain and bran products made from wheat and corn. The German cohort also had rye bread as an important source.

Since "it is likely that cereal fiber is a marker for whole-grain foods," the researchers concluded, "Whole-grain foods are therefore important in diabetes prevention."

While part of the benefit may stem from trace magnesium content, cereal fiber was associated with diabetes risk independent of magnesium in the analysis of the EPIC cohort, they noted.

The researchers said the study was limited by the single assessment of diet, which may have underestimated the association with diabetes if participants changed their diet after the baseline measurement.