Background: Alternative, sustainable and adequate sources of protein must be found to meet global demand. Objective: The aim of this study was to assess the effect of a plant-protein blend with a good balance of indispensable amino acids and high contents of leucine, arginine and cysteine on the maintenance of muscle protein mass and function during aging in comparison to milk proteins, and to determine if this effect varied according to the quality of the background diet. Methods: Old male Wistar rats (n = 96, 18 months old) were randomly allocated to four diet groups, differing according to protein source (milk or plant blend) and energy content (standard, with starch, or high, with saturated fat and sucrose), for 4 months. Plasma biochemistry and body composition (echo MRI) were measured every two months. Muscle functionality (Catwalk XT system) was assessed before and after the experimental diet. After 4 months of diet, animals were injected with a flooding dose of L-[1-13C] valine, and hind limb muscles, liver and heart were sampled and weighed. Results: We found that the plant protein blend stimulated muscle protein synthesis, maintained lean body mass, muscle mass, and muscle functionality during aging as well as milk protein. Liver and heart weights were unchanged. The high fat high sucrose diet increased body fat but, assessed by fasting plasma glucose and insulin, had little impact on insulin sensitivity and related metabolisms. Therefore, we could not test the hypothesis that in situations of higher insulin resistance, our plant protein blend rich in leucine, arginine and cysteine may be better than milk protein. Conclusion: Finally, the present findings offer significant proof of concept from the nutritional standpoint that appropriately blended plant proteins can have high nutritional value even in demanding situations such as ageing protein metabolism.