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Can whole grains improve insulin resistance in obese adults?

What impact does replacing refined grains with whole grains in a macronutrient-matched diet have on weight loss and glucose regulation? This study aims to find out.

Study under review: A whole-grain diet reduces peripheral insulin resistance and improves glucose kinetics in obese adults: A randomized-controlled trial.

Introduction

Insulin resistance is recognized as one of the fundamental underlying causes of type 2 diabetes. A meta-analysis[1] of six large observational studies comprising nearly three million person-years of follow-up found that eating more whole grains was associated with a significantly lower risk of developing type 2 diabetes. This same meta-analysis also reported on 21 randomized control trials and found that eating more whole grains modestly reduced fasting glucose and insulin levels. These findings raise the question: do whole grains affect insulin resistance? This question is worth asking considering that only 5% of U.S. adults[2] meet the recommended intake level of the U.S. dietary guidelines for whole grain consumption, despite 56% of adults meeting total grain intake recommendations. More info on U.S. grain consumption is shown in Figure 1.

While there have been clear associations between whole grain intake and glucose metabolism and insulin sensitivity in observational/epidemiological trials, there has been little evidence to suggest that whole grains themselves actually improve insulin sensitivity. One small study[3] (n=11) provided initial evidence that consuming whole grains for six weeks lowered fasting insulin and increased peripheral insulin sensitivity in people with established hyperinsulinemia. However, another study[4] in otherwise healthy overweight and obese adults reported that eating whole grains for six weeks had no effect on insulin sensitivity.

Both studies used the gold-standard hyperinsulinemic-euglycemic clamp to measure insulin sensitivity, but the former provided all food to the participants while the latter simply had participants add whole grains into their habitual diet. The latter study also involved an otherwise healthy cohort compared to the use of individuals with hyperinsulinemia in the first study.

In order to build upon the limited data investigating the link between whole grain consumption and insulin sensitivity, the present study examined the effect of whole grains, compared to refined grains, on body composition, insulin signaling, and glucose metabolism in obese adults with normoglycemia and normal insulin signaling at baseline.

Previously, consumption of whole grains has been associated with higher levels of insulin sensitivity and better glucose control than consumption of non-whole grain foods. While these associations are robust and interesting, there is still insufficient experimental data to infer a causal role. The present study was a randomized control trial examining the effects of whole grain consumption on insulin sensitivity in obese adults.

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