Background: Glycemic index (GI) is thought to affect glucose homeostasis. Recently, the OmniCarb Trial reported that GI did not improve insulin sensitivity. We performed an ancillary study in OmniCarb to determine the effects of GI on glucose homeostasis and inflammation.
Methods: The OmniCarb Trial was a randomized crossover feeding study conducted in overweight or obese adults without diabetes (N=163). Participants were fed each of four diets for 5 weeks, separated by 2-week washout periods. Weight was held constant. The four diets were: high GI (GI ≥65) with high carbohydrate (58% kcal), low GI (GI ≤45) with low carbohydrate (40% kcal), low GI with high carbohydrate; and high GI with low carbohydrate. We measured and compared the following biomarkers at the beginning and end of each dietary period: glycated albumin (GA), fructosamine, fasting glucose, and high sensitivity C-reactive protein (CRP). GA and fructosamine reflect glycemia over a 2-4 week period. Generalized estimating equations were used to compare within-person, end-of-period differences in biomarkers between diets.
Results: Mean age was 53 years, mean BMI was 32 kg/m2; study population was 52% female, and 50% black. Reducing GI had no effect on GA or fructosamine (Figure), but increased fasting glucose in the setting of a high carbohydrate diet (+2.2 mg/dl; P = 0.02). Reducing carbohydrate content decreased GA in the setting of a high GI diet (-0.2%; P = 0.03) and decreased fructosamine in the setting of a low GI diet (-4 μmol/L; P = 0.003). Reducing carbohydrate while simultaneously increasing GI significantly reduced both GA (-0.2%; P = 0.04) and fructosamine (-4 μmol/L; P = 0.009). Neither reducing GI nor amount of carbohydrate affected CRP.
Conclusions: During this 5 week trial, reducing the amount of carbohydrate, regardless of high or low GI, decreased GA and fructosamine. This suggests that reducing the carbohydrate content of diets, rather than lowering GI, is a better strategy for lowering glycemia in adults at risk for diabetes.