Buffering properties of fish aquafeeds: concepts, determinants and effects on physiology and growth

The present paper is a comprehensive review of the existing literature on aquafeed acid binding capacity (ABC), and its effects on the digestive physiology, metabolism, intestinal microbiota, and growth of farmed fishes. ABC can be defined as the quantity of acid (usually HCl) required to reduce diet pH to a specified endpoint, typically 3.0 (ABC-3) or 4.0 (ABC-4). The ABC of a diet may influence the secretion pattern and minimum gastric pH, as well as the total amount of HCl secreted during digestion. On the other hand, the acidity of the gastric lumen plays a crucial role in the digestive processes of many cultured fish species, facilitating the activation of gastric proteases, the initial hydrolysis of dietary proteins, and the solubilization of mineral nutrients. As the secretion of gastric HCl entails energy expenditure, it is plausible that the fraction of the specific dynamic action (SDA, defined as the energetic costs due to food digestion and assimilation), associated with gastric digestion in fish could be modulated through dietary formulations and their corresponding ABC. In fish, available information suggests that a decrease in diet ABC can lead to a reduction in total specific dynamic action (SDA), while the effects on growth rates remain ambiguous or transitory. Modifications to diet ABC through supplementation with calcium carbonate (CaCO₃) may negatively impact growth rates due to a concomitant decrease in phosphorus digestibility rather than an increase in HCl secretion. Furthermore, when the costs of HCl secretion were examined using proton-pump inhibitors, such as omeprazole, both SDA values and durations decreased similarly, being this reduction directly correlated with diminished fish growth rates. Regarding the influence of dietary buffering properties on fish gut microbiota, from the few available studies it can be deduced that the effects are not very significative, but this could be just a consequence of the immaturity of the research on diet ABC and fish microbiota. Further research is warranted to determine whether the manipulation of dietary ABC can lead to enhanced growth rates and improved intestinal health in cultured fish.