Summary The intricate architecture of plant metabolic networks and the dynamic fluxes of elements through these networks are fundamental determinants of how carbon (C) is partitioned among growth, reproduction, storage, respiration and the synthesis of secondary metabolites. While these C fluxes are critical to cellular function and plant life, their routine measurement remains a significant challenge. This review aimed to highlight the substantial potential of hydrogen (H) isotopes of plant carbohydrates to bridge this methodological gap by serving as a flux‐based proxy for primary C metabolism. This potential is demonstrated from both a theoretical perspective and by summarising available evidence at the whole‐molecule and position‐specific levels. The utility of this proxy is significant for understanding species' metabolic plasticity, assessing plant responses to environmental change and selecting superior metabolic phenotypes in agriculture and forestry. However, for this proxy to be fully realised, several fundamental questions remain. This includes the identification of specific metabolic reactions associated with isotopic variation and their relationship to plant performance. We outline several approaches to advance the development of an H‐isotope based plant metabolic proxy for plant performance.