> [When you loose weight], your body looks for ways to conserve energy. Your muscles work more efficiently, for example, Ard says, so walking that normally burned 100 calories might now burn only 90. By making you want to eat more and burning fewer calories, your body is eventually able to slow weight loss down to zero.
How exactly is your body able to accomplish the same task with less energy? What are you giving up? Do you become tired more easily, or wear out your muscles more quickly? Or does loosing weight just turn you into a more efficient super-human (albeit perhaps not in a way you would like)?
Would it be desirable for, say, an ultra-marathon runner (who I assume needs to consume as little energy as possible) to spend a year gaining and then loosing a bunch of weight, to make their body more efficient?