Human body is a machine, something like a car whose engine cannot be switched off. The heart will beat, the lungs will expand and recoil the blood will continuously flow, the intestines will keep on moving the food forward-digesting the food, the kidneys will keep on filtering continuously. If you want to move your hands and legs- where most of the muscles are situated many more muscle cells would start working. All these activities need energy- like our car needs (in the form of petrol). This energy comes from the food that we eat.
Now, how much energy does our body need per day? The unit of this energy is calorie (more scientifically we call it kilocalorie). If we do not use our body muscles to a great degree (which means we only sit and sleep) our body needs about 1000 to 1200 calories in 24 hours.
A little activity (equal to a sedentary person) will need about 1600 calories. A manual labourer- who has to keep on moving most of his hand and leg muscles-need about 3000 calories and a hard-working Eskimo-staying in Greenland may require about 5000-7000 calories per day.
For this energy we need food. Our stomach and intestine will digest this food – the broken particles will go to the blood after absorption, every cell of our body will get them from the blood and produce energy in their powerhouses called mitochondria. Billions of our cells can be fed with this 1600 odd calorie food for the whole day. If we consume foods worth more calories- the extra calories will be converted into fat and stored in our cells especially your fat cells, which now increasing in number and size. On the other hand, if we do not eat anything for the whole day, these stored fats will be utilized to mobilize these 1600 calories, which our body needs throughout the day.
This is the basic and simple concept of weight gain and loss. It is something like putting petrol in our car. If our car consumes about 10 litres of petrol and we put 10 litres in the car daily the storage tank will remain as it is. But if we keep on putting 15 litres per day, the fuel tank will first fill up and after a few days it will start overflowing. Same concept applies with our body – if we need 1600 calories and eat food worth 1600 calories the bodyweight will remain the same. But if we put 2000 calories worth food in our body, our body will have to store these extra four hundred calories in our body, our body will have to store these extra four hundred calories in our fat cells. It is something like our body’s fuel tank is stretchable- it will not waste the calories but will expand the storage area or capacity all over the body. The fat cells will increase in size and number to accommodate these extra calories. If intake of this extra calories goes on- soon there is a substantial storage of fats and the result is obesity.