Milk and yogurt constitute a major source of dietary protein. The nutritive value of dietary proteins is linked to subsequent postprandial amino acid availability in the portal blood (Rérat, 1988). Portal absorption of nutrients cannot be studied in humans, but pigs provide a valid model for studying protein digestion in humans (Rowan et al. 1994).

Since stable isotopes are suitable to distinguish the exogenous from endogenous protein fraction in the intestinal lumen, intrinsic isotopic labelling of milk proteins has been considered a useful technique for nutritional investigations (Gaudichon et al. 1995; Gaudichon et al. 1999; Mahé et al. 1994). Recently, the use of 15N-labelled milk proteins were used to distinguish exogenous from endogenous N fractions in the human intestine after ingestion of 15N-milk or 15N-yogurt (Gaudichon et al. 1995). These authors pointed out that the jejunal flux of 15N was different for milk and yogurt. It is known that milk proteins and lactose undergo preliminary hydrolysis during lactic fermentation (Tamine & Deeth, 1980). It is also suggested that lactic fermentation enhances the nutritional value of milk proteins (Vass et al. 1984).