CHANGES IN ENDOGENOUS INSULIN SECRETION DURING CHILDHOOD AS EXPRESSED BY PLASMA AND URINARY C‐PEPTIDE

Abstract

SUMMARY Basal fasting values of plasma C‐peptide (CP), plasma insulin and 24 h urine CP were determined in 224 normal non‐obese subjects of both sexes ranging in age from 1 to 20 years. Analysis of the results by age, pubertal rating, sex and bone age (BA) during childhood showed that mean±SD plasma CP levels in both sexes rose from 0.07 ± 0.08 pmol/ml at the age of 1–2 years to 0.21 ±0.11 pmol/ ml at 8–10 years. Mean ± SD plasma insulin levels in both sexes rose from 3.2 ± 4.3 μU/rnl at the age of 1–2 years to 5.9±4.5 μU/ml at 8–10 years. Mean ± SD urine CP levels rose from 6.5 ± 2.8 pmol/mg creatinine per 24 h at the age of 2.8 years to 7.7 ± 3.5 pmol/mg creatinine per 24 h at 8.11 years in both sexes. During puberty, plasma and urine CP and plasma insulin levels rose further to peak at pubertal stage P3, the values in females being higher (CP = 0.32 ± 0.06 pmol/ml) than those in males (CP = 0.22 ± 0.06 pmol/ml) ( P < 0.005). Plasma insulin levels in females were 13.2 ± 6.9 μU/ml and 6.4 ± 3.1 μU/ml in males (P<0.05). Urine CP levels were 14.5 ± 5.7 pmol/mg creatinine per 24 h and 10.8 ± 5.4 pmol/mg creatinine per 24 h in females and males respectively (P<0.05). With the rise in age, CP and insulin levels declined and towards adulthood were 0.23 ± 009 pmol/ml for plasma CP, 5.1 ± 4.2 μU/ml for plasma insulin and 8.7 ± 4.1 pmol/mg creatinine per 24 h for both sexes. There was a correlation between plasma CP and plasma insulin ( P =0.32, P< 0.001, n = 89) and in pubertal males and females plasma CP correlated with BA (r = 0.32, P <0.02). It is concluded that endogenous insulin secretion during childhood varies with age, pubertal stage and sex. The index of choice for the most accurate and discriminative estimation of insulin secretion would appear to be CP rather than insulin itself, due to the differences in metabolism of these substances.