The 'first 20 minutes of cardio don't count' myth

The idea that your body flips a switch to fat-burning at minute 20 isn't quite right

The popular claim: your body supposedly burns only glycogen for the first 20 minutes of cardio, only switching to fat afterward — so short workouts are supposedly pointless for fat loss. There's a real physiological fact underneath this myth: the ratio of fat to carbs used for fuel really does shift gradually with duration and intensity. But the switch happens smoothly and continuously, not on a clock at some specific minute — fat gets used as fuel from the very first minutes of any aerobic activity, just in a smaller proportion early on.

More importantly, your total weekly fat loss depends far more on total calories burned than on the exact moment-to-moment ratio of fat vs. carbs being used — the body ultimately replenishes spent glycogen from its overall energy balance, including fat breakdown at other times of day.

Practical takeaway: a 15–20 minute workout isn't 'a pointless warm-up before the real fat-burning starts' — it's a genuine contribution to your total weekly calorie burn, especially if you're doing several of them a week.