BTW, there are a pile of reasons beyond stress for intestinal issues in
young kits, from infection exposure on, including that there are aspects
of intestinal development which are not mature enough for the rigors of
full weaning and kibble only diet.  (Immaturity issues are also true for
some tracheal secretion changes, and some other things; for example early
weaning causes skull morphology differences -- with the ones due to soft
diet different from the ones due to hard diet.  Yes, you can find these
studies in Pubmed.) For an intestinal example, see:
 
> Developmental changes in the ferret ileum during the postnatal period.
>
> Asari M, Sasaki K, Igarashi S, Amasaki T, Moriya H, Amasaki H.
>
> Department of Anatomy, Azabu University School of Veterinary
> Medicine, Kanagawa, Japan.
>
>The development of the ferret ileum was studied in order to determine
>the pattern of morphological maturation during the postnatal period.  In
>the newborn, intestinal villi were short, and lined by simple columnar
>enterocytes with striated borders and small vacuoles.  At one week old,
>the equivalent cell had large vacuoles that were shown on TEM to be
>related to a tubulovesicular system in the apical region of the
>cytoplasm.  At two and three weeks old, the epithelial cells at the
>same site showed a more extensive tubulovesicular system, and vacuoles
>occupied major areas of both the supra- and subnuclear cytoplasm.  At
>six weeks old (weaning), although the enterocytes contained a few small
>intracellular vesicles, they appeared histologically more mature than
>in previous stages.  At nine weeks and in the adult, the villi were
>longer and the enterocytes no longer contained vacuoles.  In TEM, the
>epithelial cells had long microvilli, and the apical tubulovesicular
>system and vacuoles had completely disappeared from the cytoplasm.
[Posted in FML issue 5022]