Heat Increment: A Static or Dynamic Dimension in Bioenergetic Models?

Abstract

Bioenergetic models of growth endeavor to integrate food-processing transactions with energy expenditures imposed by abiotic and biotic factors. Some components of these models have lagged behind others in the synthesis of new information. A case in point is heat increment, a measure of metabolic work primarily for the postabsorptive processes that follow the ingestion of food. Importantly, the energy requirements for grasping, chewing, and swallowing food are technically distinct from those for heat increment but are experimentally difficult to separate from them. In order to take special account of these mechanical aspects, we suggest modifying the term “heat increment” (also known as specific dynamic action) to the less-precise “apparent heat increment.” Bioenergetic models invariably incorporate the assumption that apparent heat increment, relative to food intake, is independent of other variables. Laboratory studies have revealed that apparent heat increment is not always a fixed proportion of gross energy intake. Values reported range from 3 to 41% for fish fed natural diets and from 11 to 29% for those given formulated diets. Generally, apparent heat increment increases with meal size and body weight, but declines with body weight when food intake is fixed. Apparent heat increment increases with temperature, It is related to the proportion of dietary protein either directly or in an asymptotic manner. In some cases, increases in dietary lipid reduce apparent heat increment by reducing protein catabolism. In most modeling situations, consequently, the practice of adopting a fixed value for apparent heat increment relative to ingested or digestible energy leads to spurious outputs. We recommend that creators of future bioenergetic models consider the findings reviewed in this article.