Predictors of incretin concentrations in subjects with normal, impaired, and diabetic glucose tolerance.
OBJECTIVE
Defects in glucagon-like peptide 1 (GLP-1) secretion have been reported in some patients with type 2 diabetes after meal ingestion. We addressed the following questions: 1) Is the quantitative impairment in GLP-1 levels different after mixed meal or isolated glucose ingestion? 2) Which endogenous factors are associated with the concentrations of GLP-1? In particular, do elevated fasting glucose or glucagon levels diminish GLP-1 responses?


METHODS
Seventeen patients with mild type 2 diabetes, 17 subjects with impaired glucose tolerance, and 14 matched control subjects participated in an oral glucose tolerance test (75 g) and a mixed meal challenge (820 kcal), both carried out over 240 min on separate occasions. Plasma levels of glucose, insulin, C-peptide, glucagon, triglycerides, free fatty acids (FFAs), gastric inhibitory polypeptide (GIP), and GLP-1 were determined.


RESULTS
GIP and GLP-1 levels increased significantly in both experiments (P < 0.0001). In patients with type 2 diabetes, the initial GIP response was exaggerated compared with control subjects after mixed meal (P < 0.001) but not after oral glucose ingestion (P = 0.98). GLP-1 levels were similar in all three groups in both experiments. GIP responses were 186 +/- 17% higher after mixed meal ingestion than after the oral glucose load (P < 0.0001), whereas GLP-1 levels were similar in both experiments. There was a strong negative association between fasting glucagon and integrated FFA levels and subsequent GLP-1 concentrations. In contrast, fasting FFA and integrated glucagon levels after glucose or meal ingestion and female sex were positively related to GLP-1 concentrations. Incretin levels were unrelated to measures of glucose control or insulin secretion.


CONCLUSIONS
Deteriorations in glucose homeostasis can develop in the absence of any impairment in GIP or GLP-1 levels. This suggests that the defects in GLP-1 concentrations previously described in patients with long-standing type 2 diabetes are likely secondary to other hormonal and metabolic alterations, such as hyperglucagonemia. GIP and GLP-1 concentrations appear to be regulated by different factors and are independent of each other.
Department of Medicine I, St. Josef-Hospital, Ruhr-University Bochum, Gudrunstr. 56, 44791 Bochum, Germany.
Kirsten
K
Vollmer
Jens J
JJ
Holst
Birgit
B
Baller
Mark
M
Ellrichmann
Michael A
MA
Nauck
Wolfgang E
WE
Schmidt
Juris J
JJ
Meier
D000368:Aged; D001786:Blood Glucose / Q000378:metabolism; D003924:Diabetes Mellitus, Type 2 / Q000097:blood*; D005230:Fatty Acids, Nonesterified / Q000097:blood / Q000378:metabolism; D005260:Female; D005749:Gastric Inhibitory Polypeptide / Q000378:metabolism; D005934:Glucagon / Q000097:blood / Q000378:metabolism; D052216:Glucagon-Like Peptide 1 / Q000378:metabolism; D005947:Glucose / Q000378:metabolism; D018149:Glucose Intolerance / Q000097:blood*; D005951:Glucose Tolerance Test; D006801:Humans; D054795:Incretins / Q000378:metabolism*; D008297:Male; D008875:Middle Aged; D014280:Triglycerides / Q000097:blood / Q000378:metabolism
D001786:Blood Glucose; D005230:Fatty Acids, Nonesterified; D054795:Incretins; D014280:Triglycerides; D005749:Gastric Inhibitory Polypeptide; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D005947:Glucose