Although most of them have deleterious effects on fitness, natural selection increases the representation of those having beneficial effects, which can become fixed in a population. This combined action of mutation and selection promotes the adaptation to environmental changes, and in the long term leads to the evolution of populations. In the framework of the fitness landscape described by Wright, populations placed near the top of a fitness peak will experience less beneficial mutations than populations placed far from the adaptive optimum. This dependence of the mutation effects on the degree of adaptation of populations led to the theoretical prediction that mutation rates would be reduced in constant environments, in which the population has had enough time to adapt. Once the optimum has been attained, a homogeneous population of individuals with the optimal phenotype is the best adaptive solution, so the generation of further diversity is not necessary. Nevertheless, the actual situation is that environments never remain static. They continuously undergo changes that alter the fitness landscapes, displacing populations towards suboptimal fitness regions, where the amount of mutations with positive effects increases. These LEE011 poorly-adapted populations could benefit from having higher than standard mutation rates. Finally, it may seem paradoxical to observe that a rise in calprotectin is observed in parallel with increased volume of enteral feeding, but a rise is also observed when there is a need to interrupt enteral feeding, i.e., in cases of poor digestive tolerance. To further address this issue we divided samples according to the terciles of enteral feeding volume received, and, within each tercile, we separated samples depending on the need to interrupt enteral feeding or not. As shown in Fig 3, on one hand, calprotectin tends to be higher for the upper tercile of enteral feeding volume: this is consistent with the fact that calprotectin increases with enteral volume. On the other hand, for any given tercile of enteral volume administered, calprotectin was higher when feeding intolerance occurred. Should we have very large numbers of feeding interruptions within each tercile, we might be able to define ‘safe’ levels of calprotectin for a given volume of feeding. Interestingly, all instances when feeding had to be interrupted were associated with a calprotectin level.205 mg/g. This is consistent with the good sensitivity of calprotectin, yet this cutoff level has a poor specificity as well, as discussed with the ROC curve. In conclusion the current study demonstrates for the first time that calprotectin excretion can be linked to the gut bacterial establishment. We speculate that the enhanced expression of this protein possessing many potential functions including antimicrobial properties may participate in the innate immune system.