The Quran has termed "siyaam" as compulsory (2:183). It means to abstain from morning till night from eating, drinking or having sex 92:187). These are the days of fasting during the month of Ramadan in which the Quran started being revealed. A man who is a resident (not traveling), and healthy (not sick) and his physique is such that fasting does not make him suffer, then "siyaam" is a must for him (2:184), but he who suffers during fasting should instead feed a needy (2:184).
Fasting is actually a sort of military training for making the momineen capable of bearing the difficulties of jihad, which is the struggle for establishing Allah's way of life.
The Quran has itself said:
2:183 - So that you become capable of looking after the laws of Allah
2:185 - so that you can establish the laws of Allah as given in the Quran and replace the system based on man made laws
2:185 - so that your efforts can produce full results
"saa'imun": one who abstains from the wrong path, or one who fasts (33:35), or one who controls himself, and one who stays within the limits defined by the Quran. The Quran explains the purpose of its injunctions and in the case of fasting it declares that the purpose is to become righteous as defined in the Quran e.g. (2:183, 2:177, 23:1-9, 17:31-39). At an individual level one should asses the level of self-development on the first of Ramadan and then asses that after completion of the training through fasting what improvements have been made in the personality. The Quran recognizes the difficulty of keeping fast and emphasis's its importance through the effects it will have on the character development. Therefore it is important to understand the reason behind this value to get maximum benefit and not to follow it as a mere ritual as the alter will not contribute to any improvement despite fasting and remaining hungry/thirsty will result in no self-development.