Effect of Fat Content of the Evening Meal on Postprandial Glycaemia Observed in a Tolerance Test Performed on the next Morning

Oral glucose tolerance test (GTT) has been the common feature of several of our previous studies (Sahi et al. 1985; Siddhu et al. 1986; Sud et al. 1988 a, b), frequently the same volunteer has participated in more than one study. We observed considerable intra-individual variation in the postprandial glycemic response. On analysing part of our data, we observed a mean coefficient of variation of 16.2 (SD 1.6) % in 0.5 h values, and 16.8 (SD 4.7) in areas under the 2-h glucose curves. Poor reproducibility of GTT has been reported also by West et al. (1964), Mc Donald et al. (1965) and Harding et al. (1973). All these studies have also reported a variation of about 20% in the postprandial glucose levels. Further, there is no consistent relationship between any obvious characteristic of an individual, and the degree of variability shown by him or her. McDonald et al. (1965) did observe some correlation (r=0.28) between blood glucose values and variability, but found that prediction on this basis was not reliable. The cause of the intra-individual variation thus remains unknown. It has been shown recently that perfusion of the ileum with fats slows the rate of gastric emptying (Read et al. 1984). It is possible that if the subject takes a high fat meal on the evening before GTT, some of the dietary fat may be present in the ileum on the morning of the test. If this fat delays gastric emptying in the same manner as ileal fat infusion in the experiments of Read et al. (1984), it could slow down the rate of delivery of glucose during the GTT, thereby reducing postprandial glycemia. Gastric emptying has, in fact, been reported to be a major determinant of glycemia observed during GTT (Thompson et al. 1982). Since the fat content of the meal consumed by the subject on the evening before the GTT is generally not rigidly controlled, it may be at least partly responsible for the poor reproducibility of GTT. The present study was designed to test this hypothesis on a set of volunteers by doing GTT twice on each subject one preceded by a usual dinner on the evening before the test (control), and once preceded by an identical dinner with 50g additional butter (experimental). Since the gastrointestinal transit of the glucose solution administered during GTT may be regulated differently as compared to that of a solid meal, additional experiments involving a bread meal tolerance test were also included in the present study.