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You have seen many people who are eating too much but not gets fat on the other side there are people if they drink water they will get overweight .If these things ever put you in thought but never get anybody gives you answer ,so you are in right place now.

I am Sachin rana serving in Indian airforce and a certified fitness trainer and a sports nutrionist . In few words I will tell you why these things happens in your body in very simple language.

FIRST WE WILL GO THROUGH THE TERMONOLOGY OF A NORMAL BODY.

In normal body whatever you eat , mainly carbs(glucose) in today scenerio, your body releases a hormone named Insulin from pancreas Beta cells.
Insulin then trap all glucose and carry it to muscle cell of body. On muscle cells there are insulin receptors which trap the glucose and gives entry to glucose in muscle cell and then this glucose coverted and stored as glycogen in muscle cell.
Capacity of muscle cell storage is 200-400 gm.
Now some part of glucose which we ate in form of carbs stored in muscle cell and remaining glucose get stored into liver . On liver there is no insulin receptors so glucose gets direct entry in liver.
Some part of glucose stored as glycogen and some converted into glycerol and free fatty acids also called triglycerides.
Now these tryglycerides moves move towards fat cells by help of lipoprotein also called taxis of triglycerides.
If there is more glucose available these will also get stored in fat cell after liver. fat cells also have insulin receptors for storage of glucose and these glucose is converted into ATP(Adenosine tyi phosphate) and Triglycerides.
So as of now you have seen that glucose is stored in Muscle cell, Liver and Fat cells and Muscle cells and Fat cells have insulin receptors.

NOW LET US TALK ABOUT SECOND CASE WHERE A PERSON GET FAT BY ONLY DRINKING WATER.

There are 2000 to 10000 insulin receptors usually found on muscle cell.
If you have mostly carbs(glucose) in your diet which is mostly in today food. A time comes when your receptors on muscle cell become insulin resistant.
INSULIN RESISTANT means your muscle cell receptors will not work properly and fails to give glucose into your muscle cells.
And if it happens very lesss amount of glycogen stored in your muscle cells that finally means your body have less energy and you will feel lethargic and then you will get hungry in few hours . This is a reason why fatty people eat more in few hours.
But when glucose does not gets entry in muscle cell then they will move toward liver but liver also have a capacity of 80- 120gm to store glycogen after which it will starts making fatty acids .
Then remaining glucose will get stored in fat cells and fat cell size will eventually increase because now mostly glucose try to stores in fat cell.
And if the size of fat ceel increase it will shown up to your body means you will become fat.
Now imagine if liver will also become insulin resistant so very less amount of enery will stored in your body and your condition become pathetic if you will now take help of any sports science nutrionist. you will always feel tired even after eating too much food frequently.
And if liver also become insulin resistant than all glucose will go in fat cell and get stored as triglycerides. then your fat cell size increase and you will become fat.
Now consider one more case if your fat cell also become insulin resistant. Now your body will not able to handle this situation and leads to bacterial infection,retinopathy and neurolathy etc.

NOW LET US IMAGINE CASE IF A PERSON IS IN GOOD DIET(PROTEIN,FAT) OR DOING FASTING.

As of now you know you know carbs spikes your insulin level thats why our body at a period of of time gets insulin resistant and we become in fatty category.
If you are in good nutrition means (protein and fat diet ) or doing intermitent fasting from long time than there will low blood sugar level in your body than glucagon will come into action.
Glucagon is also a hormone secreted by alpha cells of pancreas.
Glucagon have receptors on liver cells.
When there is low sugar level in body glucagon comes and force liver cell to break stored glycogen into glucose and transfer that glucose into bloodstream . than your sugar level become normal.
You notice what happend body is making glucose from it own when you are in good nutrition and intermittent fasting , this process is known as gluconeogenis.
And when glucagon is high in body it will secret 3 enzymes which will break tryglycerides in fat cellinto glycerol and fatty acids.
Then glycerol will move directly into liver without any permission and fatty acids is carrid by L-carnitine to liver.
Now liver will make glucose from glycerol and rest 3 fatty acid is used to make ATP for liver itself and ketones for rest of body.
ketones are like ATP which gives us enery but in large amount.