Effects of coffee ingestion on oral glucose tolerance curves in normal human subjects

Oral glucose tolerance tests were performed on 23 normal subjects and then repeated one week later. On one occasion, the test meal consisted of glucose dissolved in water and flavored with lemon juice; on the other occasion, 5 Gm. of instant coffee were also added to the meal. The order of administration of the respective meals was randomized. Serial blood samples were obtained and analyzed for blood glucose concentration, serum free fatty acid levels and the serum immunoreactive insulin values. Paired comparisons of the data were made and the following results were obtained: (1) The subjects ingesting coffee plus glucose had significantly lower blood glucose levels 30 and 60 minutes postprandium than those consuming the glucose solution without coffee. (2) Three hours after ingestion of the test meal, the free fatty acid levels of the subjects receiving coffee with glucose were significantly higher than those receiving glucose without coffee. (3) No statistically significant differences between the two groups were found at any time period for the serum immunoreactive insulin levels. It is possible that coffee ingestion reduced the peak postprandial blood glucose levels by mobilizing a hormone from the gastrointestinal tract such as secretin, pancreozymin, or the newly discovered substance with glucagon-like immunoreactivity described by Unger et al 37.