You crash and (no longer) bum...

Ever wondered why crash diets don’t work? Part of the problem is that they’re tough to maintain; severe deprivation tends to lead to bingeing. But also, switching to an ultralow-calorie diet actually slows down your metabolism by up to 50 per cent, laying the groundwork for weight gain later on, says Larson. Starved of the calories it needs, the body shifts into survival mode. The result? You’ll be depriving yourself of calories but still not seeing the scales budge, and you’ll feel depleted even before you run, unable to get the calorie-burning and muscle-building benefits. Even after you return to normal eating habits, your body will still be in survival mode, clinging to as many calories as it can. And because you’ve probably lost muscle, you’ve further hampered your fat-burning capacity.