You might have seen the graphs on treadmills claiming you'll increase fat oxidation you keep your pulse relatively low when doing cardio. While there is evidence that the “fat burning zone” exists where you increase your fat oxidation (oxidation is the scientific term for “burning”), a higher fat oxidation doesn’t necessarily mean that you will end up with less fat on your body. Your body will balance this by burning more carbs and less fat throughout the day after your workout! In the end, fat stores can only really be reduced long term by being in a caloric deficit!
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One must remember that the mass of fat on your body is reduced by fat oxidation but also increased by fat storage. Even if you maximize your fat oxidation, having a higher fat storage will still leave you gaining fat. This happens in a caloric surplus.

The basic principle is that the body will “burn” more of the type of energy mostly available; fat or carbs. The takeaway is that your body is smarter than you think and that it doesn’t really matter if you eat more carbs or fats the long term balance of your body’s fat and glycogen stores will balance out!
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THE EVIDENCE:: Melanson EL. When energy balance is maintained, exercise does not induce negative fat balance in lean sedentary, obese sedentary, or lean endurance-trained individuals. J Appl Physiol (1985). 2009 Dec;107(6):1847-5