PSVII-1 Milk lactose digestion and a potential role of intestinal carbohydrates in dairy calves.

Abstract The disaccharide lactose is the major milk carbohydrate and is composed of glucose and galactose. It is synthetized by mammary gland lactocytes during lactation. Energy is required for the synthesis of lactose, i.e. for gluconeogenesis, for the synthesis of galactose from glucose, and for the formation of the beta-glycosidic bond of both monosaccharides. After ingestion by the calf, lactose is again cleaved into its monosaccharide components in the small intestine by lactase, and both glucose and galactose are absorbed. Whereas glucose can be immediately used in energy metabolism, the galactose must by re-converted into glucose before used as an energy source. The goal of the present study was to test the hypothesis that lactose, glucose, or galactose are not completely absorbed in the small intestine and may have local tasks in the intestinal tract. If not fully absorbed, we should be able to measure at least portions of the respective carbohydrates in the feces of the calf. Concentrations of lactose, glucose, and galactose in feces, as well as fecal pH and fecal dry matter (DM) were assessed at 4 different age phases (P) from 10 calves each. Feces samples in phase P1 were taken during the first hours of life before any colostrum uptake (meconium), in P2 from calves during week 1-2 of life, in P3 immediately before weaning (on day 96 ± 2 of life, body weight 140 ± 4 kg), and in P4 shortly after weaning from the same calves as in P3. Fecal pH was lowest in P1 (5.4 ± 0.1) and increased gradually via P2 and P3 to P4 (6.4 ± 0.0; P < 0.05). As expected, neither lactose nor glucose could be measured in P1 (meconium). However, galactose concentration reached highest levels in P1 (33.2 ± 3.0 mmol/kg DM) compared the other phases (P < 0.05). Lactose remained below detection limit in P2, P3, and P4, indicating its complete cleavage in the small intestine. Fecal glucose concentration increased from low levels in P2 to highest levels in P4 (1.3 ± 0.3 and 7.9 ± 1.1 mmol/kg DM; P < 0.05) whereas galactose concentration decreased from 23.7 ± 2.3 mmol/kg DM in P2 to lowest levels in P3 and P4 (3.7 ± 0.2 and 4.2 ± 0.7 mmol/kg DM; P < 0.05). Results of the present study indicate that intestinal galactose is of importance in the development of calves. Its role may be pre-biotic which needs further approval. Galactose may be produced locally in the intestine of the calf, considerably in meconium (P1), but to a lower extent also after weaning (P4). The increase of fecal glucose in P3 and P4 is likely caused by intestinal cleavage of polysaccharides and incomplete absorption in the large intestine.