Time-Restricted Ketogenic Diet Combined with Exercise Improves Cardiorespiratory Fitness but Compromises Sleep Quality in Overweight and Obese Young Adults: A Randomized Controlled Trial

BACKGROUND: Obesity and sleep disorders interact bidirectionally, complicating health management. Ketogenic diets (KD) aid weight loss, but their impact on sleep quality, especially when combined with exercise, remains unclear. This randomized controlled trial investigated the effects of short-term non-energy-restricted and time-restricted KD, alone or combined with moderate-intensity continuous training (MICT) or sprint interval training (SIT), on cardiorespiratory fitness (CRF) and sleep quality in overweight/obese young adults. METHODS: 68 participants were randomized into CON, KD-CON, KD-SIT, and KD-MICT groups (n = 17 each), with 59 completing the 4-week intervention. Primary outcomes were peak oxygen uptake (VO₂peak) for CRF and Pittsburgh Sleep Quality Index (PSQI) for sleep quality. RESULTS: All KD groups achieved significant weight loss (5.0-6.0%, d = -2.5 to -1.9). Relative VO₂peak increased in KD-SIT (+13.0%, d = 1.1) and KD-MICT (+7.8%, d = 0.6) groups. However, all KD groups exhibited elevated PSQI scores (12.2-29.8%; d = 0.3-1.0), indicating impaired sleep quality. Daytime dysfunction scores fell in the KD-MICT (-31.3%, d = -0.2) and KD-SIT groups (-20.0%, d < 0.01), but rose in KD-CON (+66.7%). Greater weight loss correlated with larger increases in PSQI (r = -0.5, p < 0.01). Protocol adherence ranged from 82.4% to 94.1%. CONCLUSIONS: Short-term time-restricted KD reduces weight and improves CRF when combined with exercise, especially SIT, but temporarily impairs sleep quality. Exercise partially attenuated daytime dysfunction, suggesting that cardiorespiratory adaptations occur earlier than sleep-related adjustments during early KD intervention.