Measurement of intestinal protein synthesis by continuous arterial tracer application: Intrinsically difficult problems arising from different protein pools and study techniques

Background & aims: During continuous tracer infusion and steady state conditions, it is unclear whether different study designs (single vs. multiple biopsies) will yield identical rates of intestinal protein synthesis. Methods: We reviewed the available literature and used hypothetical pool models to derive protein synthetic rates for different study techniques. Results: There is strong evidence that intestinal mucosa consists of different protein pools with different turnover rates. In the smaller rapid turnover pool, isotopic enrichment during continuous tracer infusion and steady state conditions may reach a plateau after a few hours. There is mathematical evidence, that metabolic effects in the slow turnover pool might be missed, if only a single measurement of mixed mucosal enrichment is performed after a metabolic manipulation, and if this metabolic manipulation and continuous tracer infusion is started simultaneously. Conclusion: True values for protein synthesis in the dominating slow turnover pool will only be obtained, if sequential mucosa enrichments are measured after a plateau has been reached in the rapid turnover pool, and if metabolic manipulations are only started beyond this point. However, this approach precludes the simultaneous registration of metabolic effects in the rapid turnover pool.