Aims/hypothesis: The aim of this study was to determine the impact of the common food additive carrageenan (E407) on glucose tolerance, insulin sensitivity and insulin signalling in a mouse model and human hepatic cells, since carrageenan is known to cause inflammation through interaction with toll-like receptor (TLR)4, which is associated with inflammation in diabetes.
Methods: Male C57BL/6J mice were given carrageenan (10 mg/l) in their drinking water, and underwent a glucose tolerance test (GTT), an insulin tolerance test (ITT) and an ante-mortem intraperitoneal insulin injection. HepG2 cells were exposed to carrageenan (1 mg/l × 24 h) and insulin. Levels of phospho(Ser473)-protein kinase B (Akt), phospho(Ser307)-IRS1, phosphoinositide 3-kinase (PI3K) activity and phospho(Ser32)-inhibitor of κB (IκBα) were determined by western blotting and ELISA.
Results: Glucose tolerance was significantly impaired in carrageenan-treated 12-week-old mice compared with untreated controls at all time points (n = 12; p < 0.0001). Baseline insulin and insulin levels at 30 min after taking glucose during the GTT were significantly higher following carrageenan treatment. During the ITT, glucose levels declined by more than 80% in controls, but not in carrageenan-treated mice. Carrageenan exposure completely inhibited insulin-induced increases in phospho-(Ser473)-Akt and PI3K activity in vivo in mouse liver and in human HepG2 cells. Carrageenan increased phospho(Ser307)-IRS1 levels, and this was blocked when carrageenan-induced inflammation was inhibited.
Conclusion: This is the first report of the impact of carrageenan on glucose tolerance and indicates that carrageenan impairs glucose tolerance, increases insulin resistance and inhibits insulin signalling in vivo in mouse liver and human HepG2 cells. These effects may result from carrageenan-induced inflammation. The results demonstrate extra-colonic manifestations of ingested carrageenan and suggest that carrageenan in the human diet may contribute to the development of diabetes.