Traditional feeding systems calculate energy transactions from fasting heat production and energy density of the feed. Body composition is calculated from fixed relationships between fat and protein content of weight and weight gain (Freer et al., 2007). However, this empirical approach fails to adequately capture the relationship between energy density of feed and prior growth pattern on the gain of protein and fat.
We have devised a simple dynamic model that mechanistically represents protein and fat in the body to estimate heat production and composition of gain. It uses the observation that heat production is proportional to protein content and gain of the body, and that the protein in visceral organs accounts for at least 50% of energy expended in the body (Oddy et al., 2019). Energy not deposited as protein or lost as heat is deposited in fat.