Effects of replacing lactose from milk replacer by glucose, fructose, or glycerol on energy partitioning in veal calves

Calf milk replacers contain 40 to 50% lactose. Fluctuating dairy prices are a major economic incentive to replace lactose from milk replacers by alternative energy sources. Our objective was, therefore, to determine the effects of replacement of lactose with glucose, fructose, or glycerol on energy and protein metabolism in veal calves. Forty male Holstein-Friesian calves (114 ± 2.4 kg) were fed milk replacer containing 46% lactose (CON) or 31% lactose and 15% of glucose (GLUC), fructose (FRUC), or glycerol (GLYC). Solid feed was provided at 10 g of dry matter (DM)/kg of metabolic body weight (BW0.75) per day. After an adaptation of 48 d, individual calves were harnessed, placed in metabolic cages, and housed in pairs in respiration chambers. Apparent total-tract disappearance of DM, energy, and N and complete energy and N balances were measured. The GLUC, FRUC, and GLYC calves received a single dose of 1.5 g of [U-13C]glucose, [U-13C]fructose, or [U-13C]glycerol, respectively, with their milk replacer at 0630 h and exhaled 13CO2 and 13C excretion with feces was measured. Apparent total-tract disappearance was decreased by 2.2% for DM, 3.2% for energy, and 4.2% for N in FRUC compared with CON calves. Energy and N retention did not differ between treatments, and averaged 299 ± 16 kJ/kg of BW0.75 per day and 0.79 ± 0.04 g/kg of BW0.75 per day, respectively, although FRUC calves retained numerically less N (13%) than other calves. Recovery of 13C isotopes as 13CO2 did not differ between treatments and averaged 72 ± 1.6%. The time at which the maximum rate of 13CO2 production was reached was more than 3 h delayed for FRUC calves, which may be explained by a conversion of fructose into other substrates before being oxidized. Recovery of 13C in feces was greater for FRUC calves (7.7 ± 0.59%) than for GLUC (1.0 ± 0.27%) and GLYC calves (0.5 ± 0.04%), indicating incomplete absorption of fructose from the small intestine resulting in fructose excretion or fermentation. In conclusion, energy and N retention was not affected when replacing >30% of the lactose with glucose, fructose, or glycerol. Increased fecal losses of DM, energy, and N were found in FRUC calves compared with CON, GLUC, and GLYC calves. Postabsorptive losses occurred with the urine for glucose and glycerol, which caused a lower respiratory quotient for GLUC calves during the night. Fructose was oxidized more slowly than glucose and glycerol, probably as a result of conversion into other substrates before oxidation.