Dilution of the 75-g oral glucose tolerance test increases postprandial glycemia: implications for diagnostic criteria

Background: Dilution has been noticed to increase the glycemic response to various sugars, including glucose. This effect may contribute to the poor r eproducibility of the oral glucose tolerance test (OGTT). To test this hypo thesis we assessed the effect of diluting a 75-g OGTT on 2-hour postprandia l blood glucose based diagnostic outcomes, incremental glycemia and area un der the glucose curve. Methods: On 3 different occasions, 10 subjects (mean age 40 [and standard e rror of the mean (SEM) 3.2] years; mean body mass index 27.2 [and SEM 1.2] kg/m(2)) without previously diagnosed dysglycemia were given a 300-mL, 600- mL or 900-mL 75-g OGTT in random order. The protocol followed the American Diabetes Association's guidelines. Finger-prick capillary blood samples wer e obtained at fasting and then 15, 30, 45, 60, 90 and 120 minutes after the start of the test. Results: At 30, 45 and 60 minutes, incremental glycemic concentrations were significantly higher with the 900-mL meal (means [and SEMs]: 4.9 [0.4] mmo l/L, 5.1 [0.6] mmol/L and 4.6 [0.8] mmol/L, respectively) than with the 600 -mL (means [and SEMs]: 4.0 [0.3] mmol/L, 4.2 [0.6] mmol/L and 3.6 [0.7] mmo l/L, respectively) and the 300-mL meals (means and [SEMs]: 3.8 [0.5] mmol/L , 4.0 [0.5] mmol/L and 3.2 [0.6] mmol/L, respectively) (p < 0.05). The same was true for peak incremental blood glucose, regardless of time (p < 0.05) . The area under the curve for the 900-mL meal (mean [and SEM] 404 [57] min .mmol/L) was significantly higher than for the 600-mL (mean [and SEM] 331 [ 51] min.mmol/L) and 300-mL meals (mean [and SEM] 280 [48] min.mmol/L) (p < 0.05). No other significant differences were observed. Interpretation: Dilution of the 75-g OGTT will likely not affect current sc reening practices that use 2-h postprandial glucose levels as the basis for diagnosis. It may, however, bias the interpretation of older criteria that rely on intermediate time points because these midpoints appear to be sens itive to alterations in the total volume of the meal ingested.