title: Preserved circadian rhythm of serum insulin concentration at low plasma glucose during fasting in lean and overweight humans.
abstract: Circadian rhythms in glucose metabolism are well documented. Most studies, however, evaluated such variations under conditions of continuous glucose supply, either via food intake or glucose infusion. Here we assessed in 30 subjects circadian variations in concentrations of plasma glucose, serum insulin, and C-peptide during a 72-hour fasting period to evaluate rhythms independent from glucose supply. Furthermore we assessed differences in these parameters between normal-weight (n = 20) and overweight (n = 10) subjects. Blood was sampled every 4 hours. During fasting, plasma glucose, serum insulin, and C-peptide levels gradually decreased (all P < .001). While there was no circadian variation in plasma glucose levels after the first day of fasting, serum levels of insulin were constantly higher in the morning (8.00 h) than at night (0.00 h) (P < .001), although the extent of this morning-associated rise in insulin levels decreased with the time spent fasting (P = .001). Also, morning C-peptide concentrations were higher compared to the preceding night (P < .001). The C-peptide/insulin ratio (CIR) decreased during prolonged fasting (P = .030), suggesting a decrease in hepatic insulin clearance. Moreover, CIR was significantly lower in the morning than at the night of day 1 and day 2 of fasting (P = .010 and P = .004, respectively). Compared to normal-weight subjects, overweight subjects had higher plasma glucose, as well as serum insulin and C-peptide levels (all P < .03). Data indicate preserved circadian rhythms in insulin concentrations in the presence of substantially decreased glucose levels in normal-weight and overweight subjects. This finding suggests a central nervous system contribution to the regulation of insulin secretion independent of plasma glucose levels.
authors.affiliation: Department of Internal Medicine I, University of Luebeck, 23538 Luebeck, Germany.
authors.forename: Volker
authors.initials: V
authors.lastname: Merl
authors.forename: Achim
authors.initials: A
authors.lastname: Peters
authors.forename: Kerstin M
authors.initials: KM
authors.lastname: Oltmanns
authors.forename: Werner
authors.initials: W
authors.lastname: Kern
authors.forename: Christian
authors.initials: C
authors.lastname: Hubold
authors.forename: Manfred
authors.initials: M
authors.lastname: Hallschmid
authors.forename: Jan
authors.initials: J
authors.lastname: Born
authors.forename: Horst L
authors.initials: HL
authors.lastname: Fehm
authors.forename: Bernd
authors.initials: B
authors.lastname: Schultes
mesh_terms: D000328:Adult; D001786:Blood Glucose / Q000378:metabolism*; D001835:Body Weight; D002096:C-Peptide / Q000097:blood; D002940:Circadian Rhythm; D005215:Fasting; D005260:Female; D006801:Humans; D007328:Insulin / Q000097:blood*; D008297:Male; D008875:Middle Aged; D009765:Obesity / Q000097:blood*; D012737:Sex Factors; D013997:Time Factors
chemical_list: D001786:Blood Glucose; D002096:C-Peptide; D007328:Insulin