19  It should be noted that the total net flux of N as part of amino acids is not zero in any of the studies measuring net fluxes. The estimates for studies that included glutamine and at least 10 other amino acids are + 26 nmol g−1 min−1 [588]; − 17.3 nmol g−1 min−1 [589] and − 30 nmol g−1 min−1 [590]. Eriksson et al. [588] noted that their data could not be reconciled with N balance. The net effluxes found by Grill et al. [589] and Strauss et al. [590] were dominated by net release of glutamine, which conceivably could be balanced by small net influxes for many amino acids, each below the limit of detection, or a net influx of NH4+ though there is no evidence for this [359]. How N balance is achieved remains to be clarified.