Wholesome Grains: Insulin effects may explain healthful diet

Overweight people who eat whole grains rather than refined ones are better equipped to manage their blood-sugar concentrations, according to a new study. The finding, consistent with previous ones, could help explain the observation that diets rich in whole grains guard against type II diabetes and heart disease.

In the body, carbohydrates from grains and other food are converted into glucose, a sugar that circulates in the blood until the hormone insulin directs energy-hungry cells to absorb it. Some people develop insulin resistance, a condition in which response to the hormone is muted and the body must produce excess insulin to maintain healthy blood-glucose concentrations. This condition, also called low insulin sensitivity, increases a person’s risk of both type II diabetes and heart disease.

Previous research has suggested that diets high in refined grains, such as bleached flour and white rice, contribute to these diseases. Some data implicate insulin sensitivity as a middle link in the chain of causation.

To test whether intake of different types of grains affects insulin sensitivity, Mark A. Pereira of Children’s Hospital Boston and his colleagues had 11 overweight or obese adults eat only carefully prepared meals during two 6-week periods. In one period, each volunteer consumed a diet including 6 to 10 daily servings of whole grains, and in the other, a diet that was identical except for the substitution of refined grains for whole grains.

Average body weight and other factors known to affect insulin sensitivity and insulin concentrations in the blood were comparable during the two periods, the researchers report in the May American Journal of Clinical Nutrition. On the whole-grain diet, however, volunteers’ blood-insulin concentrations after overnight fasts were significantly lower, a condition that helps retain high sensitivity to insulin.

Moreover, the group on average had higher insulin sensitivity after 6 weeks on the whole-grain meals than after the refined-grain diet.

The data back up previous studies, says Gary Frost of Imperial College in London. He notes that the current study provided strict control of the volunteers’ diets. Another strength is that both groups ate foods that are converted into glucose at similar rates (SN: 4/8/00, p. 236: https://www.sciencenews.org/20000408/bob2.asp), meaning that the refinement of grain in the diet is the only important variable, he says.

Pauline M. Emmett, a research nutritionist at the University of Bristol in England, also lauds the study for providing “extra ammunition” that whole grains improve insulin sensitivity. She notes that 9 of the 11 volunteers preferred the whole-grain diet, so substituting it for a more typical diet may be feasible for most people.

However, Gerald M. Reaven, a metabolic endocrinologist at Stanford Medical Center, denigrates the study. He says the researchers used an unorthodox method to measure insulin sensitivity. Also, the published results of those tests omit important details, he says, such as the actual values for two volunteers who didn’t have higher insulin sensitivity on a whole-grain diet.

Refined grains, including much of the grain marketed as high in fiber, account for an estimated 95 percent of grains consumed by people worldwide, notes David J.A. Jenkins, a clinical nutritionist at the University of Toronto. Eating refined grains supplemented with wheat fiber probably doesn’t improve insulin sensitivity, he suggests.