Food constituents as a cause of variation of C-peptide excretion in the urine.

Since C-peptide immunoreactivity (CPR) is excreted at a much higher rate than insulin in the urine, the urinary CPR (U-CPR) level could be a good measure of pancreatic B-cell function. In 10 normal subjects and 17 patients with non insulin-dependent diabetes mellitus (NIDDM), the 24-hour U-CPR level was 49.6 +/- 4.5 (mean +/- SE) micrograms, and 59.1 +/- 7.9 micrograms, respectively. When measured repeatedly during 4-37 consecutive days, the mean levels of coefficient of variation (c.v.) of 24-hour U-CPRs in each individual in normal and diabetic patients were 23.4 +/- 3.2%, and 39.1 +/- 1.2%, respectively. Thus, the daily fluctuation of U-CPR was considerably large not only in NIDDM but also in normal healthy subjects. In order to investigate factors responsible for these U-CPR variations, we analyzed the effect of food constituents on U-CPR excretion in this paper. In 8 healthy subjects 5-hour U-CPR excretions were measured after ingesting 5 kinds of isocaloric 300 kcal test meals, i.e. glucose, starch, protein, fat, and mixed meal which consisted of equal kcal of starch, protein and fat. Five hour U-CPR excretion after glucose, starch and protein meal ingestion was 9.5 +/- 1.3 micrograms, 13.7 +/- 1.9 micrograms, and 7.4 +/- 0.9 micrograms, respectively. Fat meal induced no increase in U-CPR excretion. After the mixed meal ingestion, 5-hour U-CPR was 8.2 +/- 0.6 micrograms, which was approximately the mathematical average for the U-CPR after 3 meals. We conclude that the cause of variations in the U-CPR excretion may be ascribed not only to the ingested total calories, but also to the nutritional components of the diet. Therefore, care must be taken in reading a daily U-CPR measurement in assessing pancreatic B cell function.