Whole-wheat bread, brown rice and oatmeal all count as whole grains. But does it matter whether those grains stay largely intact or are ground into flour?
A new study suggests it might. In a 12-week clinical trial, adults with type 2 diabetes who ate less processed whole grains had slightly better blood sugar control than those eating more finely milled versions. The difference was small, but it raises an interesting question about whether all whole-grain foods affect the body in quite the same way.
Published in The American Journal of Clinical Nutrition, the study involved 160 adults in New Zealand. Researchers received funding from several sources, including the Baking Industry Research Trust of New Zealand, and reported that the funders had no role in the research.
Participants were randomly assigned to receive regular deliveries of whole-grain foods to replace the grain products they normally ate.
One group received foods that retained more of the grain's original structure, including brown rice, unrolled oats, bulgur wheat and bread made with pieces of intact or coarsely cracked wheat. The other received brown rice pasta, oat flour, whole-grain couscous and bread made with finely milled whole-wheat flour.
Both groups were eating whole grains, not refined white-flour products. And despite the differences in processing, their overall intake of whole grains and nutrients was similar.
After 12 weeks, the less processed group had an advantage of about 0.2 percentage points in HbA1c, a blood test that shows average blood sugar levels over the previous two to three months.
That's a modest difference, but one that researchers found was statistically significant. Average HbA1c decreased slightly in the less processed group and increased somewhat in the more finely milled group.
The researchers found no significant differences in body weight, fasting blood sugar or cholesterol. People eating less processed grains also reported somewhat better bowel habits, although that was a secondary finding.
So why might two whole-grain foods affect blood sugar differently?
Part of the explanation may come down to how much the grain has been broken apart. When grains are ground into flour, their starch may become easier for the body to digest and absorb. Grains that remain more intact may be digested more slowly, potentially leading to smaller changes in blood sugar.
Earlier studies have found differences in blood sugar levels after individual meals containing intact versus finely milled grains. Other research has also suggested that food processing may affect how the body responds to meals, even when their nutritional content is similar. This trial took the question a step further by looking at blood sugar control over several months rather than just the hours after eating.
Still, the findings come with limitations. Of the 160 participants, 143 provided follow-up blood samples. The study lasted only 12 weeks, and researchers couldn't determine which individual grain foods contributed most to the results. Participants also continued their usual diabetes treatments throughout the trial.
The study didn't examine whether choosing less processed grains would reduce diabetes complications over time or whether the same effects would occur in people without diabetes.
And the findings don't mean whole-wheat bread, oat flour or whole-grain pasta are unhealthy. These foods still provide valuable nutrients, and the researchers weren't comparing them with refined grains.
Instead, the results suggest there may be another distinction worth considering when choosing whole grains: how much of the grain's original structure remains.
For people with type 2 diabetes, foods such as brown rice, bulgur and less finely processed oats may offer a small advantage for blood sugar control. But the study doesn't suggest that people need to give up other whole-grain foods, which can still be part of a healthy eating pattern.
The study received funding from a Lottery Health Grant, the Baking Industry Research Trust of New Zealand and the Riddet Institute. Harraway & Sons supplied the oats used in the study.