SECOR, S.M.: The source and sense of the metabolic response to feeding

Specific dynamic action (SDA) is the collective energy expended on the ingestion, digestion, absorption, and assimilation of a meal. Known is that SDA is initially fuel by endogenous energy stores, that SDA varies with meals size and composition, and that the magnitude of SDA is apparently greater following the up-regulation of a previously quiescent gut. Unknown is why SDA differs among meals and varies with the magnitude of gut up-regulation. For example, why does an increase in meal size result in a disproportional greater SDA response? A 4-fold increase in meal size produces a 5- and 7-fold increase in SDA, respectively, for pythons and toads. Also, why (independent of meal size) does SDA vary with meal composition? Toad SDA almost doubles from an earthworm to a cricket meal and for pythons increases by 25% from a steak to a rat meal. And how much does the up-regulation of the gut contribute to the overall SDA response? The answers to these questions undoubtedly lies in the assumed differences among meals in the energy output of each organ and tissue during digestion and assimilation. An increase in the demand on a gut tissue can be met with an increase in tissue performance, thus elevating the tissue’s metabolism. For example, in response to meals, 5%, 25%, and 65% of body size, pythons elevate intestinal active glucose uptake capacity by 12-, 27-, and 60-fold, respectively, which would contribute to the respective increases in SDA. While a down-regulated gut during fasting may significantly lower basal metabolism, the cost of its up-regulation with feeding may be an insignificant component of the total SDA response. Overshadowing the cost of up-regulation is the expenditure on gastric function (acid and enzyme production) and body growth (protein synthesis), combined with the added cost of intestinal, pancreatic, liver, and renal function.