**2. As dietary supplements in animal nutrition**

In animal husbandry, natural and synthetic zeolites have been mostly used to improve productive performance. The proposed mechanisms involved in achieving the increase in productive performance in animals are: ammonia binding, reducing toxic effects of ammonia produced by intestinal microbial activity; low passage rate of digesta through the intestines and more efficient use of nutrients; enhanced pancreatic enzymes activity-favorable effect on feed components hydrolysis over a wider range of pH, improved energy and protein retention; elimination of mycotoxin growth inhibitory effects [32].

Due to the beneficial effects of the gradual release of ammonia ions on microbial synthesis in the rumen, zeolites are used especially in high non-protein nitrogen feed ratio. *In vitro* and *in vivo* experimental studies have shown that 15% of ruminal NH4 + can be adsorbed by zeolites, thus reducing the toxic effects of urea (increased rumen pH and ammonia concentration in rumen and blood). Thus clinoptilolite (6%) in the feed of dairy cows receiving urea significantly reduced the concentration of NH4 + in the rumen [32]. Also, a decrease in ruminal pH in diets with 1% clinoptiloliten is reported [38].

Milk fever and ketosis are the most common metabolic diseases that occur in cows with high milk production. Cows that received zeolite (1 kg zeolite/day for 4 weeks before calving) did not experience subclinical hypocalcemia [39]. Also, the administration of zeolite A (sodium aluminosilicate) to pregnant cows during the dry period (1.4 kg zeolite A/day in the last 2 weeks of gestation) reduced the incidence of milk fever. The mode of action of synthetic zeolite A is to reduce the bioavailability of fodder calcium at the gastrointestinal level (calcium binding capacity of zeolite is 110 mg/g Ca at pH 11), stimulating Ca-homeostatic mechanisms before calving. At calving, the plasma level of calcium was significantly higher in the experimental group (*p* < 0.0001); with a slight drop of inorganic magnesium and phosphorus, that set up a week postpartum [40].

When clinoptilolite was administered (2.5%) in the last month of gestation, the incidence of milk fever was 5.9%, compared to 38.9% in the control group. Also, clinoptilolite (2.5%) administered during the dry period reduced the incidence of ketosis (5.9%) by improving the energy metabolism through increased production of propionate in the rumen and by better recovery of feed [32]. Katsoulos

et al. revealed that long-term clinoptilolite administration (from 4 weeks before calving to the next dry period) at different doses (1.25 and 2.5%) did not have adverse effects on the liver and serum glucose concentrations, ketone bodies, total protein and urea did not change, with a higher milk production and a lower ketosis incidence [41]. Moreover, other important minerals such as: Cu, Zn, Fe were not influenced by the long-term administration of clinoptilolite (1.25 and 2.5%), which highlights the safety of this natural compound [19].

Also, in combination with yeast, clinoptilolite (Rumencure: yeast 60% and clinoptilolite 40%) given to cows for a long period (30 days) had no apparent adverse effects on their liver function and on some biochemical parameters (glucose, ketone bodies, blood urea nitrogen and total proteins) [42].
