S. S. Gul1, R. A. Hamilton1, T. Phupitakphol1, L. Wei1, S. K. Hyoju1, D. Hu1, W. Zhang1, M. H. Gharedaghi1, H. Huo1, K. P. Econompolous1, S. A. Morrison1, S. Hamarneh1, R. Hodin1 1Massachusetts General Hospital,Department Of Surgery,Boston, MA, USA
Introduction: Diet sodas, despite being calorie-free, cause minimal weight loss. Aspartame (ASP) is a non-nutritive sweetener, used in sugar-free food with the objective of reducing obesity rates. In the gut, it is metabolized into aspartic acid, phenylalanine (PHE) and methanol. PHE inhibits intestinal alkaline phosphatase (IAP), a gut enzyme shown to prevent metabolic syndrome in mice. This led us to hypothesize that the aspartame in diet sodas could be the reason why there is minimal weight loss associated with consumption of sugar-free drinks. This study aimed to assess whether consumption of aspartame could promote the development of metabolic syndrome in mice fed a high fat diet.
Methods: IAP was added to solutions of diet soda and controls of regular soda and water to study the effects of ASP on IAP in vitro. pH settings were adjusted to imitate the different acidic environments along the intestinal canal. IAP activity was measured through pNPP assays after 1-12 hours. For the acute in vivo model, 6 week old C57BL/6 male mice were anesthetized, and a 6 cm closed loop section of the small intestine was created with 2,0 nylons. ASP, PHE or water (controls) was injected in the loops before closing the abdomen. After 3 hours, mice were sacrificed and luminal contents from the loop were harvested and IAP activity was assessed using pNPP assays. The chronic in vivo model used 6 week old C57BL/6 male mice that were fed either a chow or a high fat diet (HFD), along with tap water (controls) or water + ASP (150mg/kg/day) for 18 weeks. Mice were monitored for weight gain, food and water intake. GTT was conducted before sacrifice.
Results: In the in vitro study, IAP activity was significantly lower (p<0.05) in solutions containing ASP compared to controls without ASP at pH 3-6. In the bowel loop model, IAP activity was significantly (p<0.05) reduced in mice that received ASP compared to controls. At 18 weeks of the chronic study, mice in the HFD+ASP group had gained more weight compared to HFD+water group (42.38±3.12 vs 48.05±1.61; p=0.0001). Glucose levels were significantly higher in the HFD+ASP group after 16 hours of fasting (p=0.02), and in both ASP groups after 6 hours of fasting (p=0.02 and p=0.006 respectively). Although no significance was reached, a trend was seen indicating that mice receiving ASP had lower luminal IAP activity compared to the controls (p=0.07). Serum TNF-alpha was increased in both ASP groups (p=0.005), and was highest in HFD+ASP (p=0.01), indicating higher level of inflammation in groups receiving ASP.
Conclusion: IAP has been proven to halt the inflammatory changes leading to the metabolic syndrome caused by the increase in endotoxin levels (LPS) associated with a high fat diet. This protective effect by IAP could be inhibited by phenylalanine – one of aspartame’s metabolites – perhaps explaining the observation that diet drinks are not associated with the weight loss that people expect.