Skeletal muscles have a remarkable plasticity to readily expand their size in response to increased activity and mechanical loading. The adaptive process of muscle hypertrophy is energetically expensive but is retained even in conditions of low dietary intake. Our preliminary data now show a surprising new phenomenon, i.e. that this muscle hypertrophy in trained muscles is accompanied with simultaneous muscle atrophy in some untrained muscles. This could hint towards a muscle mass reallocation process, which seems to occur mainly when energy and/or protein availability is limited. The aim of this project is to investigate muscle mass reallocation in different modes of nutritional deficit: energy or protein. This new type of untargeted research is enabled using novel artificial intelligence powered magnetic resonance imaging to individually segment 150 human muscle volumes. We will perform two 10-week resistance training studies with isolated exercises and strictly controlled diets in young and elderly people while examining metabolic pathways driving these muscle changes. It is hypothesized that some specific muscles will experience atrophy to ‘feed’ the hypertrophy of recruited muscles during inadequate energy and/or protein consumption. This will initiate a new era for our scientific understanding of muscle mass regulation and dietary recommendations, which is also highly relevant for nutrition companies, the elite sports sector, and (para)medical practitioners.