The Metabolic Cost of Food Digestion and Its Determinants

An obligatory physiological response following feeding is an increase in metabolic rate stemming from the activities involved in the ingestion, digestion, absorption, and assimilation of the meal.  The accumulated energy expended across these activities is referred to as the cost of digestion, heat increment of feeding, diet-induced thermogenesis, and specific dynamic action.  Across a wide diversity of invertebrate and vertebrate species over a broad range of feeding conditions, this mandatory expenditure is equivalent to 5-30% of the meal’s energy.  Noted contributors to this expenditure include gastric acid production, gastrointestinal motility, substrate catabolism, and biosynthesis. The magnitude of the postfeeding metabolic response and overall expenditure is impacted by characteristics of the meal, environmental conditions, and features of the individual.  Meal size, meal type, and meal composition are well documented determinants of the intensity of the postprandial increase in metabolism.  An increase in meal size is matched closely by corresponding increases in the magnitude and duration of the metabolic response, such that a doubling of meal size result in a doubling in the overall cost of digestion and assimilation.  Meals that are intact or include hard-bodied prey require more time and energy to be broken down within the stomach than meals that are fragmented or soft in structure.  For ectotherms, a change in environmental temperature and hence body temperature will alter metabolic rate and the rate of digestion.  With an increase in body temperature, the postprandial metabolic profile becomes steeper and shorter; however the cost of meal digestion is typically not significantly altered with temperature.   An increase in body size while maintaining relative meal size will generate an increase in the scope of the metabolic response.  Observed for several taxa is that the cost of digestion scales isometrically with body mass.  The cost of meal digestion and assimilation is extremely dynamic and dictated by many different variables that underlie its importance in animal and human energetics.