Consistent with this idea neuraminidase activity in rats and mice is highest during the suckling period and in particular in the middle and distal thirds of small intestine. Dietary Sia is able to get into brain and other tissues however the mechanism involved and whether Sia is taken up as such or whether catabolic products reach the brain are largely unexplored. The small intestine is thought to be a major site of Sia uptake and as previously mentioned Sia has been proposed to be catabolised to ManNAc and pyruvate with Hederagenin-3-O-alpha-L-rhamnopyranosyl putative subsequent resynthesis of Sia. Not all milk sialyloligosaccharides however, or indeed other milk oligosaccharides, are Orcinol-glucosid digested in the small intestine. The majority of human milk oligosaccharides seem to reach the colon where they are available for catabolism by both the microflora and host, or pass unaltered into the feces. Yet their precise roles and the amount of fermented or colonic digested oligosaccharide structures are largely unknown. Given the importance of the suckling period in the development of the newborn we sought to gain further insight into the metabolism of Sia through gene expression profiling. Our results support the view that the high concentration of Sia found in milk early during lactation favours its catabolism in the colon of the suckling pup whereas the low concentration seen at weaning stimulates the expression of genes for its synthesis and subsequent use. We collected non-stimulated milk and analysed its content in lactose, sialyllactose and total Sia. Lactose in milk increased throughout the first week of lactation, dropped slightly at 10 days postpartum and then increased further towards weaning. Besides the 39- and 69sialyllactoses no other oligosaccharides were detected. 39Sialyllactose was the major oligosaccharide detected and was found at an ever increasing level during the first week of lactation but thereafter dropped steadily. 69Sialyllactose was found at about ten times lower levels and also showed an increase over the first week of lactation. It then remained at a constant although low level until weaning. The total amount of Sia in milk showed a similar profile throughout lactation to that of 39sialyllactose and was consistent with previously published results.