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
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).