The Oral Glucose Tolerance Test (OGTT): Effect of Rate of Ingestion of Carbohydrate and Different Carbohydrate Preparations

The glucose load of the oral glucose tolerance test (OGTT) is well standardized. However, recommendations on rate of ingestion and nature of the load are vague. In this study the effect on blood glucose, serum insulin, C-peptide, and plasma gastric inhibitory polypeptide (GIP) of giving 75 g glucose in 300 ml over 1 and 10 min (Gl and G10) was investigated in six subjects. In five an isocaloric amount of partially hydrolyzed starch (Hycal) was also used (HI and H10). The fast glucose intake, compared with the slow ingestion, resulted in an earlier rise in blood glucose levels, accompanied by a faster serum insulin and C-peptide response. Between 90 and 135 min blood glucose concentrations were significantly higher after the 10-min glucose intake. At 120 min blood glucose levels were 5.5 ± 0.5 and 4–7 ± 0.5 mmol/L, respectively, for G10 and Gl (P < 0.05). In the first half hour after slow and fast Hycal intake no differences were seen in blood glucose, serum insulin, and C-peptide levels. Between 45 and 120 min blood glucose levels were significantly higher after the 10-min Hycal intake. At 120 min blood glucose levels were 5.3 ±0.2 and 4–4 ± 0.1 mmol/L, respectively, for H10 and HI (P < 0.01). Except for a faster rise in glucose and insulin levels after glucose loading in 1 min, no further differences were found, when compared with Hycal. No significant differences were seen in the GIP responses. Thus differences in rates of ingestion can cause significant differences in later results. A standard time for glucose ingestion should be specified.