Where energy is the first limiting resource in a food it is proposed that an animal will try to eat the food at the rate that is needed to just meet its requirement for energy. An immature animal in a thermally neutral environment is assumed to need energy only for maintenance, including some level of physical activity, protein retention and lipid retention. The energy needed for units of each of maintenance and protein and lipid retentions is assumed to be constant i.e. the partial energetic efficiencies are all constant. The rates of maintenance, protein retention and lipid retention in non-limiting conditions can be expressed as functions of the current protein weight of the animal and a small set of parameters which are assumed to be sufficient descriptions of the genotype of the animal. These are its mature size, expressed as protein, Pm , its rate of maturing assuming a Gompertz growth function, B per day, and its lipid:protein ratio at maturity, Q. On these assumptions the rate of intake of a food with a known energy content can be predicted for a known animal as a function of time only. Between animals B varies directly with Pm raised to the -0.27 power so that the scaled rate parameter B* = B.P 0,27 m is not correlated with Pm . The pattern of food intake, appropriately scaled to mature size, with time is shown to depend only on the values of B* and Q. Initial estimates of the parameters are presented.