Many of our predictions about fish growth, size and performance rely on an understanding of digestive physiology. Studying digestive physiology in fish, however, often requires researchers to bypass feeding behaviour altogether by delivering meals directly to the stomach through a method known as intragastric gavage, or force feeding. This approach offers exceptional control over what, when and how much a fish eats, but raises a fundamental question. Does force-feeding trigger the same digestive response as letting a fish eat on its own? We tested this in sand flathead (Platycephalus bassensis), comparing fish that consumed intact pellets voluntarily with fish given an equivalent meal as a slurry via gavage, alongside fasted and sham-handled controls. We quantified the postprandial rise in metabolic rate associated with digestion and assimilation, known as specific dynamic action (SDA), and assessed the changes in blood acid-base status that accompany gastric acid secretion, commonly termed the alkaline tide. Voluntarily fed fish showed a substantially larger digestive response, with SDA more than three times greater and lasting roughly 37 hours longer than in gavaged fish. Much of this difference was attributable to the handling and anaesthesia required for gavage, which altered postprandial physiology independently of digestion itself. Voluntarily fed fish also displayed a pronounced alkaline tide, the first evidence of this response in the species, but gavage fed fish did not. Together, these findings show that feeding method can profoundly shape the physiological responses used to quantify digestion in fish, with important implications for the design and interpretation of digestion studies and for how confidently experimental results can be applied to aquaculture, fisheries, and conservation.