The regulation of feed intake in fish is dependent upon different neuroendocrine and metabolic mechanisms including amino acid sensing in the gastrointestinal tract (GIT). However, there is little information regarding the impact of diets on such mechanisms. Therefore, in this study, we fed rainbow trout (Oncorhynchus mykiss) with 3 diets: a diet with high content of fishmeal and low content of soybean protein concentrate (SPC) (HF), a diet with a reduced content of fishmeal and high content of SPC (LF), and the LF diet supplemented with an umami tastestimulating additive (LFU). Fish were fed ad libitum once a day for 4 weeks, with no significant differences being registered in feed intake among groups. At the end of the feeding trial, we collected samples of different areas of the GIT (stomach, proximal and distal intestine) and hypothalamus at different times: after 48 h of fasting (time 0), and 1 h, 4 h, and 24 h after feeding. We evaluated the activity of pepsin in the stomach and trypsin and chymotrypsin in the proximal intestine, as well as mRNA abundance of transcripts encoding amino acid transporters and taste receptors, intracellular signalling molecules, and hormones. Moreover, we assessed the hypothalamic mRNA abundance of neuropeptides involved in feed intake regulation. Feeding rainbow trout with LF did not result in marked alterations in parameters related to digestive function and amino acid sensing in the rainbow trout GIT, nor in the expression of gastrointestinal hormones (except cck) and hypothalamic neuropeptides. In contrast, supplementation of the LF diet with an umami taste-stimulating additive resulted in a general improvement of digestive or absorptive function (increased protein, dry matter and energy digestibility, and earlier peak in plasma amino acid levels) and activation of gut-brain axis mechanisms involved in feed intake regulation through the transcriptional activation of amino acid transporters, taste receptors, signalling molecules, and hormones. These results demonstrate that the dietary inclusion of umami receptor stimulants has the potential to improve fish physiological responses to the rise in levels of vegetable protein in the diet.