The calories in the food we eat provide vital fuel energy to maintain our basic metabolic needs, and to power muscles.
To ensure our metabolism has all the fuel it needs, our brain constantly monitors our calorie intake and expenditure, and will - if it thinks there is a shortage of food energy - intervene to conserve energy for survival purposes. So long as we continue eating regularly, there's no problem: we burn calories at our normal speed, and if we create a calorie deficit we will burn stored body fat and thus lose weight. However, if we go too long without eating something, our brain intervenes to reduce our rate of calorie-burning (our metabolic rate). Result? Fat burning slows down or stops.
More calories are needed to maintain muscle tissue than fat tissue. So if our calorie deficit persists and we require more energy, we stop burning fat and start burning muscle tissue instead.
Courtesy : Diet I