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CHEAT: Aerobic exercise needs to performed for 30 minutes plus.

DEETS: Fat is like the money in a high-interest linked savings account – you can't just withdraw it. First you need to use up what's in your direct transaction account (preferred macromolecules) – starting with creatine phosphate and glycogen.These provide quick energy for short-term, high-intensity exercise, or fuel for the early stages of moderate-intensity workouts.

The type and duration of exercise determines which macromolecule is metabolised first.

Why is half-an-hour the magic number?

When you keep busting a sweat past the halfa mark, the body switches from primarily metabolising glucose to favouring fatty acids. In plain speak, that means it can efficiently tap into free fatty acids (FFAs) derived from lipids in body fat. Tip: the brain can't metabolise fatty acids during the first 30 minutes of exercise.

There's also an order of events that can't be overwritten, no matter how clever you think you are. Fat and amino acids derived from muscle protein are the last fuel in the chain after stored glycogen and residual glucose in blood.

The takeaway is that sub-30 minute aerobic exercise can prevent new fat stores but not erode the old ones (and possibly decrease muscle mass – bad). To really reap the perks of a fat-burning workout, 40 minutes is the holy grail. Then the body feasts primarily on fatty acids for fuel, reducing body fat. Bonus: since most fuel is now FFAs, muscle protein is largely spared.