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Low energy availability (LEA) describes a state where energy intake is insufficient to cover the energy costs of both exercise energy expenditure and basal physiological body functions. LEA has been associated with various physiological consequences, such as reproductive dysfunction. However, the effect of LEA on skeletal muscle protein synthesis in females performing exercise training is still poorly understood. We conducted a randomized controlled trial investigating the impact of LEA on daily integrated myofibrillar and sarcoplasmic muscle protein synthesis in trained females. Thirty eumenorrheic females were matched based on training history and randomized to undergo 10 days of LEA (25 kcal · kg Fat-Free Mass (FFM) -1 · day -1 ) or optimal energy availability (OEA, 50 kcal · kg FFM -1 · day -1 ). Before the intervention, both groups underwent a five-day 'run-in' period with OEA. All foods were provided throughout the experimental period with a protein content of 2.2 g · kg lean mass -1 · day -1 . A standardized, supervised combined resistance and cardiovascular exercise training program was performed over the experimental period. Daily integrated muscle protein synthesis was measured by deuterium oxide (D 2 O) consumption along with changes in body composition, resting metabolic rate, blood biomarkers, and 24h nitrogen balance. We found that LEA reduced daily integrated myofibrillar and sarcoplasmic muscle protein synthesis compared to OEA. Concomitant reductions were observed in lean mass, urinary nitrogen balance, free androgen index, thyroid hormone concentrations, and resting metabolic rate following LEA. These results highlight that LEA may negatively affect skeletal muscle adaptations in females performing exercise training. KEY POINTS: Low energy availability with potential health and performance impairments is widespread among female athletes. We investigated the impact of 10 days of LEA on daily integrated myofibrillar and sarcoplasmic muscle protein synthesis in young, trained females. We show that LEA impairs myofibrillar and sarcoplasmic muscle protein synthesis in trained females performing exercise training. These findings suggest that LEA may have negative consequences for skeletal muscle adaptations and highlight the importance of ensuring adequate energy availability in female athletes. Abstract Figure Legend Ten days of low energy availability in trained females performing supervised exercise training resulted in reduced daily integrated myofibrillar and sarcoplasmic muscle protein synthesis compared to optimal energy availability. Concomitant reductions were observed in lean mass, urinary nitrogen balance, free androgen index, thyroid hormone concentrations, and resting metabolic rate following low energy availability. This article is protected by copyright. All rights reserved.