In fish, the slow-twitch skeletal muscle fibres have an oxidative metabolism, present peripherical location in the red muscle, and are used for sustained movements. They may respond to alterations in food availability differently than the fast-twitch muscle. The aim of this manuscript was to study the slow-twitch muscle fibres of Piaractus Mesopotamicus, a neotropical fish, submitted to 30 days of fasting (D30) followed by one day (D31) or 30 days of refeeding (D60). The treated animals were compared with regularly fed fish. To verify the presence of atrophy and hypertrophy, we performed histological analysis of muscle fibre diameter in D30 and D60, and RT-qPCR gene expression analysis of catabolic (murfa, murfb, mafbx) and anabolic genes (igf-1, mTOR) in D30, D31 and D60. The gene expression of the allergen and Ca2+-carrier parvalbumin (pvalb) was also measured in D30, D31 and D60. The proteome of slow-twitch fibres at D30 and D60 was obtained by shotgun proteomics (digestion of proteins with trypsin followed by LC-MS/MS identification), and the proteins differentially expressed were used to construct protein interaction networks. The histological analysis showed no difference between treated fish in relation to the control. The expression of catabolic and anabolic genes was not changed, except for the negative regulation of igf-1 in D30 and of mtor in D31. The expression of pvalb was not changed in D30 and D60, but it was decreased in D31. The proteomic analysis identified 169 proteins in D30 (24 upregulated and 18 downregulated) and 170 proteins in D60 (17 upregulated and 21 downregulated). Many of them were related to energetic metabolism, including lipid metabolism, as shown by the protein network analysis. These results enlarge our understanding of the acclimation of slow-twitch fibres to changes in nutrient availability and set a path for future research.