Recent years have seen a marked increase in our awareness of the need to incorporate greater physiological realism into studies of parasitoid behaviour and population dynamics. Quantification of the number of eggs produced as a result of a host–feeding event, the host–feeding gain, is essential for predicting when a parasitoid should bypass an opportunity for current reproduction (i.e. laying eggs) in order to feed from the host and, thus, increase its chances for future reproduction (i.e. producing further eggs). Using radioactively labelled amino acids, one of the main constituents of insect haemolymph, we followed the incorporation of a known quantity of nutrients into each individual egg laid over a long period of time relative to the average life span of the parasitoid. Although the maximum incorporation of nutrients obtained by the female from a discrete feeding event occurs within a short period of time, a large proportion of nutrients are stored and used gradually for egg production throughout the life of the parasitoid. We therefore provide novel experimental evidence showing that feeding gain is not a discrete event in time occurring shortly after feeding, as has so far been assumed, but is instead spread throughout the parasitoid's lifetime. This has important consequences for calculating the increase in lifetime fitness as a result of a feeding event, a common currency of models that aim to predict feeding and oviposition behaviour in parasitoids.