Effects of long-term postgastric infusion of casein or glutamic acid on small intestinal starch digestion, energy balance, body composition, and metabolic fates of luminally assimilated glucose carbon in cattle
Authors:   , Acharya Subash 
Published: February 2023

Small intestinal starch digestion (SISD) in ruminants is limited, but SISD can be improved by postruminal supply of casein or glutamic acid. There are no reports on the effects of increases in SISD in response to greater postgastric nutrient flows from casein or glutamic acid together with cornstarch on energy balance, measures of body composition and fates of luminally assimilated glucose carbon in cattle. Twenty-four steers (BW = 179 [plus or minus] 4 kg) were duodenally infused with raw cornstarch (1.46 [plus or minus] 0.04 kg/d) and either 413 [plus or minus] 7.0 g casein/d, 121 [plus or minus] 3.6 g glutamic acid/d or water (control). Measures of small intestinal starch digestion and nutrient excretion were collected across 4 d after 42 d of infusion. Measures of energy balance via indirect calorimetry were collected across 2 days after 48 d of infusion, and measures of responses in glucose use for lipid synthesis and splanchnic release of absorbed glucose were determined by continuously infusing stable isotopomers on day 58. Subsequently, cattle were slaughtered and changes in total body nutrient composition were determined by measures of carcass specific gravity. There was tendency of increasing SISD (P = 0.09) when steers were infused with casein treatment and the increase was 26.6 percent more than that of control. Ileal flow of ethanol-soluble starch (small-chain [plus or minus]-glycosides) increased (P [less than] 0.01) with casein infusion compared to control or glutamic acid infusion. Large intestinal starch digestion was similar (P = 0.28) among the treatments. Nitrogen retention was greater (P [less than] 0.01) in response to casein compared to control and glutamic acid. With increase in N retention and similar C retention, retained C:N ratio was least (P [less than] 0.01) among calves provided casein, compared to glutamic acid or control. Gross energy intake was similar (P = 0.45) among the treatments, while digestible energy was increased (P = 0.03) with casein infusion compared to control and was intermediate to similar with glutamic acid infusion. Similarly, metabolizable energy was increased (P = 0.01) with casein infusion compared to control and glutamic acid infusions. Retained energy tended to increase (P = 0.07) with casein infusion compared to other treatments. The increase in retained energy was 34 percent more than that of control. Tissue energy was greatest (P = 0.03) among calves provided with casein compared to glutamic acid and control. The majority of increase in tissue energy occurred in fat tissue, where calves provided with casein had greatest (P = 0.02) energy from fat tissues compared to control or glutamic acid. The increase in tissue energy in protein tissue was greatest (P [less than] 0.01) for casein compared to glutamic acid and control. Glucose turnover rate, mmol/min was similar (P = 0.17) among treatments, while glucose turnover rate as a function of body weight (mmol/min/MBW) tended (P = 0.15) to be 12 percent greater among calves provided glutamic acid, compared to casein or control. Consequently, glucose rate of appearance was greatest (P [less than] 0.01) among calves provided casein, compared to glutamic acid or control. Palmitate fractional synthesis rate (FSR) from omental fat (P = 0.88) and subcutaneous fat (P = 0.89) were similar among treatments. Palmitate FSR from intramuscular fat tended (P = 0.11) to be greater among calves provided casein, compared to control or glutamic acid infusion.