Type of milk substitute influences growth of the gastrointestinal tract in artificially reared rat pups.

Growth of the gastrointestinal (GI) tract was studied in rat pups that were reared normally (mother-reared, MR) or were artificially reared (AR) by intragastric infusion of milk substitutes from postnatal day 5. Two milk substitutes were used: one high in carbohydrate and low in protein compared with rat's milk (Messer) and the other close in composition to rat's milk (Auestad). Pups reared on these formulae are termed ARM and ARA, respectively. Pups were killed at 7, 12, and 20 days for the determination of the wet and dry weights of stomach and caecum and the lengths of the small and large intestines (SI and LI). At 7 days AR pups showed deficits in several GI measures compared with MR rats. However, by 20 days they showed enhanced growth of various parts of the GI tract. Stomach weight was greatest in ARA rats; caecum weight was greater in AR than in MR rats; SI was longer in AR than MR rats, with the effect more pronounced in ARM rats. These effects were already evident, or becoming so, by 12 days. In an additional experiment, rats were AR on rat's milk from 5 to 12 days. Their GI tracts did not differ from MR, suggesting that the AR procedure per se was not responsible for the above GI growth effects.