Results from various studies demonstrate that, compared with enzymatically digestible starch, inclusion of NSP in pig diets causes changes in the GI tract with regard to retention time, rate and type of fermentation and physical conditions. This can alter microbial activity and this has consequences for pH and number of pathogens in the gut. In sows and in growing pigs, some (e.g. SBP and resistant starch), but not all NSP sources reduce energy expenditure on physical activity. The effect could not be demonstrated by an increase in gut fill achieved by adding wheat straw to the diet. The reduction in physical activity was observed for both individually and group-housed pigs. A behavioural study (open-field) revealed that pigs fed SBP showed reducedlocomotor activity, but not at the expense of explorative or escape behaviour. Further research, which should include (behavioural) observations on group-housed pigs, is necessary to elucidate how and why a NSP diet affects behavioural responses and, possibly, the welfare of pigs. Additionally, it remains to be investigated if the results found in literature are related to fermentable NSP in general, or are specific for NSP from SBP. The observed effects of fermentable NSP on physical activity have an impact on its feeding value. Accounting for the effect of NSP from SBP on eg, physical activity in a net energy system would increase its NE value to a value close to that of starch. Dietary effects on physical activity are, however, difficult to accountfor in a NE system, because they are generally considered to be a part of maintenance energy expenditure. It is, however, important to account for this effect in a feed evaluation system, because it represents an economic value of a feedstuff, and several options are discussed. For future feed evaluation, an increase in knowledge on the biological mechanisms behind the effects of DF on physical activity are important. Quite likely, these effects depend on the chemical and physical properties of the NSPfraction.
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