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Mathematical models are increasingly used to predict the response of an animal to a changing nutrient supply. The objective of this experiment was to provide data that can be used in model development or evaluation and concerns the energetic efficiency with which nutrients are used for protein and lipid deposition. A basal diet (D1), limiting in lysine supply, was fed at 1.7 MJ metabolizable energy (ME)/(kg BW(0.60) x d1) to growing pigs that weighed approximately 60 kg. Four additional diets were formulated: the basal diet and a dietary supplement that consisted of starch (D2), starch and corn gluten meal (D3), starch and casein (D4) or starch and lipid (D5). The latter four diets were fed at 2.55 MJ ME/(kg BW(0.60) x d1) and ensured the same intake of the basal diet across treatments; the difference was supplied by the supplement. Metabolic utilization of the basal diet and supplements was determined using nitrogen and energy balances (indirect calorimetry). The N retention was similar in pigs fed diets D1, D2, D3 and D5 but considerably higher in those fed D4. A data analysis model was developed to account for differences in ME utilization between nutrients. The ME not deposited as protein entered a common pool of energy, which was used for adenosine triphosphate synthesis or lipid deposition. The energetic efficiencies of ME utilization were 0.842, 0.520 and 0.883 for starch, protein and lipid, respectively. Due to the energy cost of protein deposition (or protein turnover), the energetic efficiencies of depositing dietary protein as protein or lipid were similar. [ABSTRACT FROM AUTHOR]

The conventional regression method for partitioning heat production (HP) in growing animals between HP associated with either maintenance or growth assumes maintenance HP to be independent of feeding level (FL). However, there are indications that this assumption is not correct and an alternative method is proposed in this study from a reanalysis of 3 trials. In trial 1, 73-, 152-, and 237-kg calves received one milk replacer at 77, 84, 92, and 100% of their ad libitum metabolizable energy (ME) intake. In trial 2, 70-kg barrows received one diet at 60, 80, and 100% of their ad libitum ME intake {2600 kJ ME/[kg body weight (BW)(0.60) · d]}. In trial 3, 60-kg barrows received a basal diet [1700 kJ ME/(kg BW(0.60) · d)] or 4 diets consisting of the basal diet plus 850 kJ ME/(kg BW(0.60)·d) of starch alone or starch with corn gluten, casein, or vegetable oil. In the 3 trials (n = 48, 18, and 28, respectively), HP and activity-related HP were measured on individuals pigs and calves in respiration chambers for 6 d (fed state) and fasting HP (FHP; at zero activity) was calculated as the asymptotic value of HP kinetics on d 7 (feed-deprived state). The FHP changed by 0.22 kJ in calves and 0.14 kJ in pigs/kJ ME intake change during the previous days. The efficiency of using ME for maintenance and growth [k(mg); 1- (HP - FHP)/ME] was not affected by FL (calves: 84%, pigs in trial 2: 74%). In trial 3, k(mg) varied between diets in connection with variations in efficiencies between nutrients (from 55% for corn gluten to 85% for lipid). This new method of representing partitioning of ME intake considers FHP as variable with FL, does not require estimates of maintenance ME requirements, includes efficiencies that depend on diet characteristics, and is not biased by metabolic adaptations of the animal to FL.

We used the pig as a model to assess the effects of dietary fat content and composition on nutrient oxidation and energy partitioning in positive energy balance. Pigs weighing 25 kg were assigned to either: 1) a low fat-high starch diet, or 2) a high saturated-fat diet, or 3) a high unsaturated-fat diet. In the high-fat treatments, 20% starch was iso-energetically replaced by 10.8% lard or 10.2% soybean oil, respectively. For 7 d, pigs were fed twice daily at a rate of 1200 kJ digestible energy · kg(-0.75) · d(-1). Oral bolus doses of [U-(13)C] glucose, [U-(13)C] α-linoleate, [U-(13)C] stearate, and [U-(13)C] oleate were administered on d 1, 2, 4, and 6, respectively, and (13)CO(2) production was measured. Protein and fat deposition were measured for 7 d. Fractional oxidation of fatty acids from the low-fat diet was lower than from the high-fat diets. Within diets, the saturated [U-(13)C] stearate was oxidized less than the unsaturated [U-(13)C] oleate and [U-(13)C] linoleate. For the high unsaturated-fat diet, oxidation of [U-(13)C] oleate was higher than that of [U-(13)C] linoleate. In general, recovery of (13)CO(2) from labeled fatty acids rose within 2 h after ingestion but peaked around the next meal. This peak was induced by an increased energy expenditure that was likely related to increased eating activity. In conclusion, oxidation of dietary fatty acids in growing pigs depends on the inclusion level and composition of dietary fat. Moreover, our data suggest that the most recently ingested fatty acids are preferred substrates for oxidation when the direct supply of dietary nutrients has decreased and ATP requirements increase.

A framework representing the major biochemical pathways of nutrient metabolism is developed allowing quantification of the energy efficiency of different nutritional scenarios. The model is based on a number of carbon chain pivots (glucose, pyruvate, acetyl coenzyme A, alpha-ketoglutarate, oxaloacetate and serine) and cofactors involved in metabolism. Excess pivots yield acetyl coenzyme A, which may be used for ATP or lipid synthesis. In contrast to previous work of this kind, the framework was constructed so that new insights in nutrient metabolism can be easily incorporated. Traditionally, integral values have been used to quantify mitochondrial ATP synthesis from cofactors (i.e., 3 ATP/NADH and 2 ATP/FADH(2)), but current estimates are approximately 0.20 lower than previously assumed. Based on the latter, the energy expenditure for ATP synthesis from glucose was 91.0 kJ/ATP. For lipid (tripalmitin), 96.3 kJ/ATP was required whereas for amino acids energy expenditures varied between 99.2 (glutamate) and 153.2 kJ/ATP (cysteine). Energy derived from amino acid catabolism is stored and transferred either via carbon chain pivots or cofactors. It is hypothesized that this may affect the ultimate utilization of this energy (e.g., for ATP or lipid synthesis). The energy cost of nitrogen transport appeared relatively modest for most nonessential amino acids. Likewise, the net cost of using dietary glutamate and glutamine for ATP synthesis (e.g., in the viscera) and de novo synthesis of these amino acids in muscle is relatively minor and of similar magnitude as the cost of storing glucose energy as glycogen.