Blood Glucose, Acetate, Propionate, and Beta- Hydroxybutyrate (BHB) Responses to Ruminally Degradable Starch and Fiber.

In ruminants, starch is acknowledged as glucogenic, and fiber as acetogenic. Although these impacts of dietary nutrient concentrations are well characterized, effects of ruminal nutrient degradability on blood metabolites are poorly understood. The focus of this study was to evaluate the effects of degradable fiber and starch supplies on blood volatile fatty acid (VFA), glucose, and beta-hydroxybutyrate (BHB) concentrations. Six Holstein heifers (BW 725 ± 25 lb) were used in a partially replicated Latin Square experiment with 4 treatments (low or high ruminally degradable fiber and starch arranged factorially). During each period, animals were adapted to treatment diet for 17 days, and on day 18 blood samples were collected by coccygeal venipuncture. Blood samples were centrifuged for 15 minutes immediately after collection to isolate plasma, and were preserved in 4oC. Blood glucose and BHB were quantified using colorimetric assay kits, and blood VFA concentrations were determined via GCMS. Data were analyzed using a mixed model with ruminally degradable fiber and starch as fixed effects and cow and period as random effects. The interaction between degradable fiber and starch concentrations were also tested. Blood acetate (P>0.10), propionate (P>0.10), and glucose (P>0.10) were not influenced by starch, fiber, or the starch by fiber interaction (P>0.10). The lack of a significant change in blood acetate and propionate was unexpected given that shifts in fermentation patterns, and subsequently, shifts in blood VFA concentrations were anticipated. Although BHB was not affected by starch or the starch by fiber interaction (P>0.10), dietary supply of degradable fiber (P=0.002) increased blood BHB concentrations. The significant relationship between dietary degradable fiber supplies and blood BHB concentrations suggests enhanced fermentation of fiber in the rumen contributes to blood BHB concentrations. Blood BHB is influenced by the breakdown of free fatty acids in the liver, or by increased ruminal butyrate production. Given that these diets were in excess of the animal's energy requirements and had similar fatty acid content, it is unlikely that the elevated BHB is indicative of liver ketogenesis, rather, we hypothesize that the elevated BHB was caused by increased butyrate absorption from the rumen. Additional work is needed to understand how and why soluble fiber contributes to butyrate production and absorption rates. [ABSTRACT FROM AUTHOR]