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To compare the effect of 150 min vs. 300 min of weekly moderate intensity exercise training on the activation of the opioid system and apoptosis in the hearts of a diet-induced obesity model. Male Wistar rats were fed with either control (CON) or high fat (HF) diet for 32 weeks. At the 20th week, HF group was subdivided into sedentary, low (LEV, 150 min·week−1) or high (HEV, 300 min·week−1) exercise volume. After 12 weeks of exercise, body mass gain, adiposity index, systolic blood pressure, cardiac morphometry, apoptosis biomarkers and opioid system expression were evaluated. Sedentary animals fed with HF presented pathological cardiac hypertrophy and higher body mass gain, systolic blood pressure and adiposity index than control group. Both exercise volumes induced physiological cardiac hypertrophy, restored systolic blood pressure and improved adiposity index, but only 300 min·week−1 reduced body mass gain. HF group exhibited lower proenkephalin, PI3K, ERK and GSK-3β expression, and greater activated caspase-3 expression than control group. Compared to HF, no changes in the cardiac opioid system were observed in the 150 min·week−1 of exercise training, while 300 min·week−1 showed greater proenkephalin, DOR, KOR, MOR, Akt, ERK and GSK-3β expression, and lower activated caspase-3 expression. 300 min·week−1 of exercise training triggered opioid system activation and provided greater cardioprotection against obesity than 150 min·week−1. Our findings provide translational aspect with clinical relevance about the critical dose of exercise training necessary to reduce cardiovascular risk factors caused by obesity. [ABSTRACT FROM AUTHOR]