Changes in Liver and Gastrointestinal Tract Energy Demands in Response to Physiological Workload in Ruminants

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Abstract
Liver and gastrointestinal tract weights (ingesta- and adipose-free) appear to increase or decrease in direct proportion to dietary intake within and across physiological stages of maintenance, growth, fattening or lactation. Liver and gut mass increase ∼15 and 30 g per unit of liveweight raised to the 0.75 power (Wt0.75) for each multiple of 500 kJ/Wt0.75 [∼1 × maintenance (M)] increase in metabolizable energy (ME) intake, with linearity indicated up to the highest recorded level (4.5 × M). Extrapolation from in vivo arteriovenous O2 measurements across splanchnic tissues and from the previously cited weight information indicates that liver and gut tissue oxidize ∼3.5 and 1.0 kJ of ME/g of fresh tissue daily, in contrast to whole-animal rates of 0.1 kJ/g. Thus, energy use by the relatively small amount of liver and gut accounts for 45 to 50% of whole-animal heat energy. On a differential basis, increases in energy use by these tissues appear to account for up to 70% of the heat increment of ME use above maintenance.