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Knowledge about the impact of substituting high GI starchy foods by low GI starchy foods on metabolic parameters in obese subjects

The glycaemic index has been developed in order to classify food according to the glycaemic response after carbohydrates ingestion. A 5-weeks nutritional, interventional trial was conducted using 2 types of dietary regimens. Overweight healthy subjects of both sexes were given dietary advice to replace their usual starchy foods ad libitum by either Low Glycaemic Index (LGI) or High Glycaemic Index (HGI) starchy products. Foods were considered as having a low GI whenever GI<50% and a high GI whenever GI>70%. Moreover, between Day 1 and Day 36, the subjects from the LGI group have consumed daily a breakfast containing plain biscuits (LGI biscuits) and the subjects from the HGI group have consumed daily a breakfast containing extruded cereals (HGI).

The biscuits and the extruded cereals resulted from different technological process and exhibited differences in concentrations of Rapid Available Glucose (RAG) and Slow available Glucose (SAG). Plain biscuits contained 39% of SAG and 61% of RAG, the extruded cereals contained less than 2% of SAG. These breakfasts represented around 20% of daily energy intake. They were isocaloric (around 431 kcal) and contained the same amount of proteins (12%), lipids (26%) and carbohydrates (62%). The only variable parameter was the glycaemic index (45 ± 6 % and 70 ± 12 % for biscuits and cereals, respectively). The 38 subjects were men and women (LGI diet: n= 19; HGI diet: n= 19) [BMI: 27.3 +/- 1.5 kg/m2] aged 20-60 y. The study was a parallel, randomized, intervention trial with 2 matched groups.
On the first day (d1) and on the last day (d36) of the nutritional intervention, glycaemic, insulinic, lipid blood profile, body composition and metabolism were measured after ingestion of the same cereals products than usual but whom carbohydrates were labeled with stable isotope 13C.

Results: The diet groups did not differ in total energy or macronutrient intakes. Dietary goals were reached in both groups with a significant difference in GI between groups (p<0.0001). Mean body weight decrease was significant in the LGI diet group (-1.1 ± 0.3 kg, p=0.004) and this decrease was significantly greater than in the HGI group (-0.3 +/- 0.2 kg, p=0.04 between groups). No significant differences in body fat mass were observed. However, neither diet altered fasting insulin, insulin resistance indexes (QUICKI, HOMA-IR), lipid profile and substrate oxidations. The LGI diet decreased total cholesterol by 9.6% (p<0.001), HDL cholesterol by 2.5% (p=0.04), LDL cholesterol by 8.6% (p=0.01) and both LDL to HDL ratio (10.1%, p=0.003) and total to HDL cholesterol ratio (8.5%, p=0.001). No significant changes in lipid parameters were observed in the HGI group. At d1, before 5-weeks diet, after ingestion of the plain biscuits or of the extruded cereals, the area under curve of glycemia was lower for LGI than HGI. By using 13C labeled starch, the exogenous glucose appearance at d1 has been measured and was 68.7% of 13C glucose ingested for LGI vs 96.3% for HGI in 270 min (40.2 +/- 1.9 vs 56.4 +/- 2.5 g respectively, p<0.02). Total glucose appearance at d1 trended to be lower after LGI than after HGI breakfasts (63.9 +/- 2.0 vs 69.0 +/- 1.8 g, NS). On the same day, the endogenous glucose production was significantly less inhibited during LGI breakfast despite a similar level of insulinemia (23.7 +/- 2.1 vs 12.6 +/-2.0 g respectively, p=0.001). Comparing d36 and d1, differences between acute and chronically ingestion have been found: the 5-weeks dietary intervention did not alter neither exogenous glucose appearance, total glucose appearance nor endogenous glucose production.
Conclusion: The cholesterol profile has been improved in the LGI group after the 5-weeks intervention, and in this group, loss of 1 kg of body weight has been registered. But no effect on the glucose turnover has been observed.
GI is useful to classify starches but could not resume all the metabolic properties of the food. The metabolic results could be modulated by the other components of the meal. More studies are needed to conclude on a positive effect of the GI of the diet on the metabolic parameters.

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