Taste plays a fundamental role in an animal’s ability to detect nutrients and transmits key dietary information to the brain, which is crucial for its growth and survival. Providing alternative terrestrial ingredients early in feeding influences the growth of rainbow trout (RT, Oncorhynchus mykiss ). Thus, the present study aimed to assess the influence, via long-term feeding (for 8 months from the first feeding), of alternative plant ingredients (V diet for vegetable diet vs. C diet for a control diet) in RT on the mechanism of fat sensing at the gustatory level. After the feeding trial, we studied the pathways of the fat sensing mechanism in tongue tissue and the integrated response in the central nervous system. To this end, we analyzed the expression pattern of free fatty acid receptors ( ffar ) 1 and 2, markers of calcium-signaling pathways (phospholipase C β, Orai, Stim or Serca), the serotonin level (a key neurotransmitter in taste buds), and the expression pattern of appetite-regulating neuropeptides in the hypothalamus (central area of appetite regulation). The results revealed that the V diet modified the expression pattern of ffar1 and paralogs of ffar2 genes in tongue tissue, along with differential regulation of calcium-signaling pathways and a defect in serotonin level and brain turnover, without influencing neuropeptide expression. This study is the first to support that changes in growth performance and food intake of RT fed a V diet could be due to the influence of differences in nutrient sensing and a decrease in hedonic sensation. We revealed that RT have similar fat-detection mechanisms as mammals.