It works best when your heart rate reaches the fat-burning zone.
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燃 (rán, 'burn') + 脂 (zhī, 'fat') — direct calque of 'burn fat'. Often appears as compounds: 燃脂区间 (fat-burning zone), 燃脂运动 (fat-burning exercise), 燃脂训练 (fat-burning training). Heavily used in fitness apps like Keep.
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Marketing/fitness term — you'd see it on workout videos, supplement labels, and gym promo. Conversational equivalent is 减脂 (jiǎnzhī, 'reduce fat'), which is the actual outcome (losing fat) vs 燃脂 (the metabolic process).
