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.