Can your body just "reduce your basal metabolism"? Where are the energy savings? Is your body temperature lowered? I'm skeptical of the idea that differences in "metabolism" can account for different results arising from the same energy inputs.
Other things that can happen when your metabolism slows down are that you get tired more often and more easily, your thought processes slow down, you sleep more. This happens to everyone as they age, and it can also happen from chronic stress, calorie restriction sufficient to trigger 'starvation mode', and anything else that causes your body to try to conserve energy for survival.