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Transition is defined as the period from 3 weeks before parturition until 3 weeks postpartum and is characterized by changes in endocrine status and by major alterations in the requirement for nutrients, vitamins and minerals. At parturition, the dry period diet is replaced by an ad-libitum-offered lactation diet, which causes changes of rumen fermentation because of its higher energy density and the overall increase of dry matter intake. Hence, ruminal fermentation parameters such as pH, short chain fatty acid (SCFA) concentrations and osmotic pressure are changed. This causes alterations in the rumen epithelium generally termed adaptation: the modulation of enzyme activities, an increase in the number and size of papillae and enhanced transport rates of ions. These effects are well-documented when sheep or cows are shifted from a low-energy (roughage) to a concentrate diet. Only a few studies have been conducted in cows during transition and, thus, data on diet-dependent effects on postpartum rumen acidosis are absent and information regarding number and size of papillae is inconsistent. Furthermore, investigations of gene expression related to SCFA absorption or metabolism have not shown diet-dependent alterations. However, evidence exists that the stimulation of fermentation before parturition enhances the transport rates of Na, Cl and SCFA across bovine rumen epithelium. The underlying mechanisms for epithelial adaptation to dietary alterations are poorly understood at present. Luminal factors such as increased SCFA concentrations (particularly butyrate) and the hormonal effects of insulin and of insulin-like growth factor-1 have been considered and partly confirmed by corresponding experiments. More research is needed in order to determine the factors causally inducing adaptational responses and the reason that adaptational responses apparently level off at a certain intake of energy. Despite these uncertainties, an increase of energy and protein intake undoubtedly improves the barrier and transport functions of the epithelium. Consequently, at least for two weeks before parturition, a diet should be offered that is easier and more rapidly fermented than the diet in the early dry period. Furthermore, efforts should be directed towards an improved voluntary food intake before and after parturition, with special attention being paid to the mechanisms enabling the epithelium to cope with regular diurnal variations in pH, SCFA and osmotic pressure, mechanisms that seem to fail in the pathophysiology of subacute rumen acidosis.