A Meta-Analysis on Quantitative Sodium, Potassium and Chloride Metabolism in Horses and Ponies

The goal of this meta-analysis was to (i) identify any potential differences in the apparent and true digestibility, renal excretion, and retention between ponies and horses and (ii) examine the impact of work on these parameters. Additionally, the study aimed to (iii) evaluate the effects of water deficiency. This meta-analysis used data from 33 studies and plotted them in diagrams similar to the Lucas test against mineral intake. Three studies involved ponies that were later diagnosed with pituitary pars intermedia dysfunction (PPID). These were compared with other data to identify quantitative differences, as they may have clinical significance. If any significant difference was observed, the data were not used for the evaluation of the aforementioned goals. Data were compared within certain intake ranges using t-tests and ANOVA, followed by Holm-Sidak post hoc tests. Working equines showed significantly higher apparent and true Na digestibility and lower endogenous faecal Na losses compared to non-working ponies and horses, suggesting a rather important role of the gastrointestinal tract in the regulation of Na metabolism in equines. The true K digestibility was also significantly higher in working animals than in non-working ones, but the differences were quantitatively smaller than for Na. Retention plus sweat losses for Na, K and Cl were higher in working animals compared to equines in maintenance. Horses showed higher Na and K retention plus sweat losses compared to ponies. These effects are likely attributable to greater sweat losses in working equines, particularly horses. The apparent and true Cl digestibility was significantly higher in ponies than in horses. A clinical relevance of this observation is rather unlikely, as the digestibility remained very high in both. Water deprivation influenced serum Cl levels and increased renal K excretion, which subsequently led to reduced K retention. Compared to other animal species, no effects on blood K levels or Na metabolism were observed. The ponies diagnosed with PPID exhibited a significantly lower apparent K digestibility compared to healthy animals, which could be an important factor to consider when feeding soaked hay, due to potential electrolyte losses during soaking.