In ruminants, propionic acid followed by AA and triglycerides represent the main sources of glucose (Van Soest, 1994). Animals fed a 15% glycerin diet had elevated blood glucose possibly derived from ruminal propionate or from the glycerol directly absorbed through the rumen wall. When fed 30% glycerin, however, animal blood glucose returned to control levels. This effect may result from changes to the complex glucose-regulating endocrine system or from the putative
threshold effect previously mentioned. The lower blood concentrations of total protein and albumin may be due to increased ruminal proteolytic activity. The lack of effect of glycerin on BUN was also reported by Boyd et al. (2013) and Chung et al. (2007), whose results also disagree with Shin et al. (2012), which observed linear decrease in BUN concentration and attributed this effect to an improved use of dietary N by ruminal microorganisms. Dietary crude glycerin did not affect plasma minerals despite its high NaCl (6%) concentration. An increase in blood gamma-glutamyl transferase enzyme in animals fed 30% crude glycerin was observed, but no other liver enzymes suffered alterations. From this data liver damage cannot be inferred, because no changes were observed in activity of liver enzymes alanine aminotransferase
and aspartate aminotransferase; however, long-term effects of high-glycerin-content diets should be further evaluated.