Gut Peptide Responses Following Bariatric Surgery- Long Term Follow Up and Impact on Diabetic Status
Research For Life
Reports of research work funded by grants prior to 2015
Department of Endocrinology
As of the August 2015 we are in the process of refining our methodology prior to commencing the funded study. The proposed gut peptide study involves the use of a liquid meal test and a GLP-1 multiplex assay. One of the chief issues with using standard volume liquid meal tests to assess gut peptide responses is that, in our experience, patients who try to consume 200ml in a ten minute time frame frequently experience nausea or vomiting, thus limiting the use of the test.
In order to adapt the meal test to reduce limiting nausea and vomiting, a new 50ml low volume meal was developed. We have recently studied this new meal test looking in particular at glucose, insulin, GLP-1, GIP, C-Peptide, PYY, and Leptin concentrations over three hours from ingestion. We compared this to the usual 200ml liquid meal in order to evaluate performance of the low volume meal test in particular in relation to the magnitude of glucose change, GLP-1 change and GIP change over time. The aim was to see if the low volume meal test in its current form can be used to study gut peptide responses to a liquid meal.
In the last month we have completed analysis of that data and have identified two significant issues: the efficacy of the liquid meal test and the specificity of the GLP-1 assay we have been using.
While our low volume liquid meal test did produce a detectable rise in serum glucose concentrations, it did not produce a consistent statistically significant change in GLP-1 from baseline compared to a standard (200ml) volume meal test. We believe the reasons for this relates to the lower caloric content and volume of the liquid meal test which has a direct effect on degree of GLP-1 response provoked. In addition levels of serum active GLP-1 in the fasting state typically occur at the lower limit of detection for many assays.
Given that some of the clinical effects of GLP-1 are believed to be mediated before secreted GLP-1 is processed in the enterohepatic circulation, in future we plan to assess Total GLP-1 as well as active GLP-1 in studies of GLP-1. We will also further evaluate the average maximum tolerated volume in patients following RYGB using a standardised questionnaire. Finally, we are in the process of selecting a better assay with greater specificity for use in future studies on GLP-1.
The mean serum concentration of GLP-1 in the fasting study over 20 samples, using a Merck Millipore Multiplex Assay for gut peptides, was several fold higher than fasting GLP-1 samples in comparable populations reported in the literature using the same assay. Unfortunately the supplier has not been able to provide sufficient specificity data for the assay (particularly cross reactivity for glucagon, GLP-2, GLP-1 9-36). Further reading into potential reasons for this raises the possibility that the magnetised beads principle, upon which the current assay is based, suffers from issues with low specificity generally.
This information has proven invaluable in informing or approach to the measurement of GLP-1 in our planned study. The very purpose of this pilot study was to evaluate the robustness of our techniques. We have now identified at least two ways in which we can improve our methodology for the upcoming study. In order to make maximum use of this information we will carry out an additional study of a low volume meal, but refine it to maximise caloric content and volume, as well as changing to a hormone assay with greater specificity. Once this study is complete we will use these techniques in a study of participants who have previously undergone RYGB.