Relationship between the acid-binding capacity (ABC)-4 value of the complete diet and stomach pH. (A) shows the correlation between the ABC-4 value of the complete diet and stomach pH. The data represent the ABC-4 value of 31 experimental diets containing protein-rich feedstuffs (6–8 piglets/experimental treatment) (between 2017 and 2019; Schothorst Feed Research B.V., Lelystad, The Netherlands). The ABC-4 value was measured in meq/kg using the methodology described by Lawlor et al. [151]. Each dot represents the average stomach pH per dietary treatment. (B) shows the effect of acidifying the complete diet on individual stomach pH. The experimental diets contained one of three different protein-rich feedstuffs that are known for their high (i.e., fish meal), intermediate (i.e., Hi-Pro Soybean Meal) and low ABC-4 value (i.e., maize gluten). The values displayed in (B) represent the stomach pH from all protein-rich feedstuffs together. The low ABC-4 diets were acidified by replacing limestone with calcium formate (either completely or partially) and adding citric acid. The unaffected diets did not contain organic acids. The average ABC-4 values of the unaffected diet were 308 meq/kg (SD = 26), with a value of 243 meq/kg (SD = 95) for the low ABC-4 diets. The average stomach pH of the unaffected diet was 4.22 (SD = 0.997), with a value of 3.57 (SD = 1.075) for the low ABC-4 diet.