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Solutions towards decoupling food production from arable land are increasingly promoted to alleviate food system's environmental impacts. It includes solid substrate fermentation (SSF), allowing to nutritionally enhance brous co-products. Yet, it remains unclear whether and to which extent SSF valorization pathways really improve environmental performances of agrifood co-products in comparison to current practices. Using a framework which combines process-based consequential life cycle analysis, uncertainty assessment and biomass resource potential estimation, we quantify and qualify implications of SSF nutritional enhancement through six selected case studies. When used to unlock food markets, SSF yields environmental bene ts for all impact categories. When used to unlock feed markets (low-value streams), SSF pathways are preferable options compared to energy valorization for most impacts except climate change. We show that the environmental impacts of novel food and feed are principally dependent on avoided crops and land use change accounting methods, these requiring further and systematic harmonization. Full TextAchieving generalized and signi cant changes of current food system is essential, not only to meet environmental targets, but also to comply with future food demand 1,2 . Therefore, reducing the dependence on crop importations by developing locally sourced commodities and enhancing the decoupling of food production from land are levers to mitigate food system's environmental impacts 3,4 . Among these solutions, "waste-to-nutrition" pathways, transforming organic wastes, residues and coproducts into edible ingredients, are increasingly promoted 5 . In this study, we assess the conditions under which fungal solid substrate fermentation (SSF) can sustainably be used as an alternative valorization option for agrifood co-products.SSF is based on the colonization of a solid substrate by a biological agent which, in the case of feed applications, allows to enhance or even unlock the nutritional value of brous feedstocks, mainly through lignocellulose degradation, proteins concentration and digestibility enhancement [6][7][8][9] . Applied to