Effect Of Dietary Carbohydrate (Amount And Type) On Risk Factors For Cardiovascular Disease And Diabetes: The Omnicarb Trial

Background: Foods that have similar carbohydrate content can differ in the amount they raise blood glucose. The effects of this property, called the glycemic index (GI), on risk factors for cardiovascular disease and diabetes, are not well understood, especially in the context of a healthful dietary pattern that itself improves risk factors. Aims: To study the effects of the amount of dietary carbohydrate (40% vs 58% energy) and GI (40% vs 65% on the glucose scale) on insulin sensitivity (determined by glucose and insulin levels during a 2 hour oral glucose tolerance test), systolic blood pressure (SBP), and fasting lipids (LDL-C, HDL-C, and TG) in non-diabetic, overweight adults with SBP 120 - 159 mmHg. Methods: We randomized 163 adults, mean age 53 y, in a 4 period crossover feeding study. The 4 diets, each studied for 5 weeks, were (1) higher-carbohydrate, higher-GI; (2) higher-carbohydrate, lower-GI; (3) lower-carbohydrate, higher-GI; and (4) lower-carbohydrate, lower-GI. All diets were based on a healthful DASH-type dietary pattern. At the end of each feeding period in a subsample of participants (N=51-57) we obtained hourly blood samples for glucose, insulin, and TG over 12 hours while participants ate the day’s meals in the research facility. Results: The four study diets did not significantly differentially affect insulin sensitivity, LDL-C, or SBP. The lower-carbohydrate or lower-GI diets increased HDL-C by about 4% (with between diet P-values <0.016) and lowered TG by 20% (P<0.031, compared to the higher-carbohydrate, higher-GI diet. Most of these effects were accounted for by lower amount of carbohydrate rather than lower GI. Similarly, the lower carbohydrate or lower GI diets lowered 12-hour blood glucose by an average of 20% (P<0.031) compared to the higher carbohydrate, higher GI diet. The lower-carbohydrate, lower-GI diet reduced 12-hour serum insulin levels the most by 17% (P=0.032). Compared to levels measured when the participants were eating their own diets, all study diets lowered SBP by an average of 9 mm (7%) and LDL-C by 13 mg/dL (9%). Conclusions: In the context of healthful dietary patterns, varying the amount of carbohydrate or its GI in non-diabetic overweight adults does not affect fasting insulin sensitivity, SBP or LDL-C over 5 weeks. HDL-C and triglycerides improved on the lower carbohydrate diets but were not affected by GI. However, postprandial glycemia is reduced either by lowering GI or amount of carbohydrate, and hyperinsulinemia is reduced by the combination of low GI and low carbohydrate. We speculate that consistent associations in populations of reduced CVD event rates with lower intakes of carbohydrate and GI, if causal, result more from reduced postprandial glycemia and hyperinsulinemia than from lower BP and improved lipid risk factors.