In pigs, starch is digested into glucose in the small intestine. Undigested starch, i.e. resistant starch (RS), serves as substrate for microbial fermentation. The aim of this thesis was to improve understanding of underlying processes affecting the utilization of digested and fermented starch to ultimately predict the effect of starch digestion kinetics on pig performance more accurately.
Despite the assumption that fermented starch yields less energy than digested starch, growth rates of pigs fed low RS and high RS diets are often the same when feed is available ad libitum. In Chapter 2, we hypothesized that RS affects nutrient digestion and digesta passage rate, and consequently feeding behavior, which ultimately affects growth performance of pigs under ad libitum conditions. In this study, pigs were fed either a high RS (HRS) or low RS (LRS) diet. Despite a substantial reduction in enzymatic digestible starch (LRS: 98% vs. HRS: 74%), carcass gain, slaughter quality parameters, and feed efficiency used for carcass gain were not affected by diet. Because dietary RS concentration did not affect digesta passage rate nor feeding behavior, we suggested that the difference in energy intake between fermentable and digestible starch was compensated for post-absorptively.
The aim of Chapter 3 was to investigate maintenance energy requirements and the efficiency in which energy is used for growth (incremental energy efficiency) in slow- (SG) and fast growing (FG) pigs, which were fed either a slowly (SDS) or a rapidly digestible starch (RDS) diet. A slower rate of starch digestion was hypothesized to reduce fat depositing in pigs and be beneficial for slow growing pigs that are associated with a reduced insulin sensitivity. Gross energy intake was 6% greater in FG-pigs than in SG-pigs, whereas incremental energy efficiencies and fasting heat production were unaffected. We concluded that a lower energy intake rather than greater maintenance requirements or a lower energy efficiency explains slow growth of SG-pigs. No beneficial effects of SDS on energy utilization of SG-pigs were observed. RDS increased incremental use of energy for fat retention (2 %-units), which was most likely explained by greater levels of postprandial fat deposition.
Microbial activity in the small intestine has shown to exist, which may result in an overestimation of starch digestion when not accounted for. In Chapter 4, we aimed to quantify both ileal and total tract starch fermentation and investigated the effect of RS on bacterial biomass formation and microbiota composition. A method based on the contrast in natural 13C-enrichment between starch and non-starch dietary components was used to quantify total tract fermentation of starch. Pigs were fed either a high RS (HRS) or low RS (LRS) diet. Microbiota composition in rectal digesta, but not in ileal digesta, slightly differed between diets. Total tract starch fermentation was 18 %-units greater in HRS-fed pigs than LRS-fed pigs (P<0.001). Large intestinal starch disappearance was 24 %-units greater in LRS-fed than HRS-fed pigs (P<0.001), implying that ileal starch fermentation was 6 %-units greater in HRS-fed pigs than LRS-fed pigs (P=0.046). The 13C-method used to estimate starch fermentation was reasonably close to colonic starch disappearance, but largely overestimated ileal starch fermentation. Our results suggested that ileal starch digestion in pigs can be overestimated with 1-7% when based on ileal starch disappearance.
In Chapter 5, we hypothesized that alterations in feeding behavior to changes in RS intake may be dynamic, depending on the adaptation of processes involved when shifting from starch digestion to fermentation or vice versa. To test this hypothesis, we interchanged HRS and LRS in 5 steps, either in upwards (low to high; LH) or downwards (high to low; HL) direction. Complete substitution of LRS with HRS increased the proportion of starch fermented, which was greater in LH pigs (17.6%) than HL pigs (8.18%) and decreased feed intake (106 g/d) and meal size (12.6 g) of LH pigs, but not of HL pigs. We concluded that pigs adapt more slowly to increasing dietary supply of digestible starch than to RS, which resulted in fermentation of potentially enzymatically digestible starch. Furthermore, feed intake decreased only in pigs poorly adapting to RS; hence, the adequacy of adaptation, rather than RS itself reduced feed intake of pigs.
Misalignment of the day/night rhythm with circadian feeding rhythms (circadian misalignment) has been shown to increase fat deposition and the risk for metabolic disorders in humans and rodents. In Chapter 6, we investigated the effects of circadian misalignment on energy expenditure in pigs. Pigs were fed either during the day (10.00h - 18.00h; diurnal feeding: DF) or night (22.00h - 06.00h; nocturnal feeding: NF), bihourly the same sequential meals, representing 15, 10, 25, 30 and 20 % of a similar daily allowance. Heat production was 3% lower for NF-pigs than DF-pigs increasing fat retention by 7% in NF-pigs. Methane production was 30% greater in NF-pigs than in DF-pig. We concluded that circadian misalignment has little effect on nutrient digestion, but alters nutrient partitioning, ultimately increasing fat deposition. The causality of the association between circadian misalignment and methane production rates remained to be investigated.
In Chapter 7, I discussed the impact of starch digestion kinetics on growth performance, energy utilization, and meal patterns of pigs, by combining the results described in this thesis with existing literature. Rapidly digestible starch favors postprandial lipogenesis ultimately leading to an increase in fat deposition in pigs, whereas the extent of starch digestion, hence RS, did not affect growth performance of pigs. Consequently, net energy values of RS and digestible starch seem to be similar, particularly under ad libitum conditions. Finally, meal patterns may be determined by both dietary and animal-intrinsic effects. Substantial changes in RS intake (30% RS), however, did not affect meal patters of growing pigs.