Stages of organizing food in the human body:

It is surprising but definitive that the mechanism whereby the human organism reacts to food makes fasting (Siyaam) necessary. Allah, the All-Wise, Most Gracious, has designed the human body to be ready to react to food substances according to a regular and well-organized process that goes in three stages. This is a very important research. 
The first stage: In this stage food is digested in the stomach and intestines,

then food is absorbed, metabolism process takes place, and food is turned into sugar that goes through the blood stream and into other substances used for producing energy and composing tissues. 
The second stage: It is the stage where the surplus energy is stored.

The extra sugar is stored in the liver and muscles as starch, while the extra fat is stored in most parts of the body. 
 So the first stage is that of digestion, absorption and consumption, while the second one is that of storage. 
The third stage: It is the stage where the stores of energy are opened. In this stage the sugar and fat are turned into energy. 

Dear brothers, the special thing about this stage,, is that it does not take place unless a person abstains from eating for a certain period of time. In other words, this important stage of food metabolism does not take place unless one abstains from food!

Human body needs abstaining from food: 

Dear brothers, scientists say that the level of sugar in the blood starts at 80-110 mg/100 cc's, which is the normal level of sugar in the blood. 

After six hours of fasting, this level goes down, which causes a miracle of the human body to take place. The brain sends fast messages to the endocrine glands asking for help and energy supply. 

The supra-renal gland produces a special hormone that stimulates the starch in the muscles and liver to turn into sugar; and the thyroid gland also produces a special hormone that releases the sugar stored in the muscles and liver in order to be consumed. 

This process happens because the level of sugar in the blood goes down after a six-hour fasting. The pancreas also produces a special hormone that releases the stored materials to be consumed.

If man consumes all the sugar stored in his muscles and liver, the fats, stored in the body, are broken down into sugar to be consumed by the body as well in order to give energy. Scientific research confirms that burning fats increases during fasting hours, and so does the consumption of fats accumulated on the different parts of the body. 

Dear brothers, it is amazing that each 1 kilogram of fat tissues needs 3 kilometers of capillaries through which the blood flows. This causes a heavy burden on the heart. If one gains 10 kilograms of weight this means that he has 300 kilometers more of capillaries. 

Scientists confirm that the physiology of the human body requires abstaining from food for certain periods, but do you know why? Because this gives some time for the first and second stages of food digestion to rest and allows the third stage to work well. During fasting the first-stage of food digestion and absorption, and the second-stage of food-storage get some rest, and the third-stage of burning and consumption of the stored energy in the body takes place.

Because fasting is necessary for all people Allah the Almighty has made it obligatory on all nations. Allah the Almighty says:  

(O you who believe! The Fast has been prescribed for you, as it has been prescribed for those before you, so that you may attain self-protection.)

 [Al-Baqarah, 183] 

This scientific research highlights the delicate fact about Siyaam, which is being an act of worship, an act of getting closer to Allah, and keeping more connection with Allah. All deeds that man does are for him except fasting. It is for Allah and Allah rewards him for it. Fasting strengthens man's will

and reminds him of his human weakness. In other words, the outcomes of the mentioned scientific research do not by any means contradict the foremost purpose of fasting, which is being an act of worship, of devotion and of closeness to Allah the Almighty. 
However Allah's Commands have multiple aspects; so it is an act of worship, closeness to Allah, it shows man how weak he is, and how much he is in need to Him.