Dietary cation anion difference (CAD, Na + K - Cl, mEq kg -1 ) determines the pH and acid base status of a diet, consequently affecting the acid base balance in the body compartments of animals. After feeding, a low dietary CAD will contribute more acids to the animals than a high dietary CAD. An optimal dietary CAD will increase the acid buffer capacity of a diet and this will help animals to compensate for metabolic acidosis. It is hypothesized that with an optimal dietary CAD, less energy will be needed for acid base regulation, indirectly improving feed intake and growth of animals. In the present study the effect of dietary CAD on growth performance, energy metabolism, acid base balance in the blood and in the digestive system were investigated in African catfish and pigs. The study consisted of 3 parts. Part 1 dealt with the growth response to dietary CAD and dietary Na/K ratio. Part 2 dealt with the energetic response to dietary CAD. Parts 3 dealt with the acid base balance in the blood and in the digestive system in response to dietary CAD. A negative dietary CAD (-100 mEq kg -1 ) resulted in a low feed intake and growth in both African catfish and young pigs. In African catfish, increasing dietary CAD from -100 to 700 mEq kg -1 led to a linear increase in growth.
In pigs, the optimal dietary CAD was observed to be between 200 and 500 mEq kg -1 . The optimal dietary Na/K ratio in formulating dietary CAD was 1.5 to 2.5 (mol/mol) for African catfish. The lowest maintenance cost was observed at a dietary CAD level of 700 mEq kg -1 for African catfish. In pigs, dietary CAD of 200 mEq kg -1 tended to increase energy requirement for maintenance compared with dietary CAD of -100 mEq kg -1 , at restricted feeding. In pigs, a -100 mEq kg -1 CAD diet resulted in low blood pH, oxygen and HCO 3- content (mmol L -1 ) compared to a 200 mEq kg -1 CAD diet. During the postprandial period, however, pigs maintained a relative constant pH level in both portal and arterial blood within each CAD group. African catfish fed 700 mEq kg -1 CAD diet showed higher stomach digesta pH than fish fed -100 mEq kg -1 CAD diet both 0.5 and 3 h after feeding. However, no difference in pH of small intestine digesta was observed. In pigs, dietary CAD levels of -100 and 200 mEq kg -1 did not affect either stomach or small intestine digesta pH 2.5 h after feeding. The possible mechanisms of dietary CAD effect on feed intake and growth were discussed.