Today many observant Jews as well as Muslims are hungry. Muslims find themselves just over a week into the month long fast of Ramadan while Jews find themselves observing the 24 hour fast of Tisha B'Av commemorating the destruction of the Temple in 70 CE.
While fasting has always been a component of many faiths the reason behind this practice is not always well understood. At least in Judaism there seem to be two disparate ideas associated with fasting. The first understands fasting as a form of self -mortification, a means of punishing the body so as to affect divine absolution for sins committed. In this school of thought fasting is often joined with other physical privations and mortifications. While still accepted by ascetics this idea lost its hold on the wider population by the start of the modern era. Far more popular today is the mystical notion that fasting is a means towards transcending, if but for a brief time, the material self in the hope of attaining a deeper awareness of the Divine. I'm not sure how many people actually experience the Divine but the notion of at least pursuing it is distinctly more palatable than inflicting self punishment.
Fasting is difficult because you can't ever entirely wean yourself off food. One can, at least in theory, give up smoking cold turkey and never smoke again. The same goes for alcohol, coffee or chocolate. Over time it becomes easier to avoid these substances as the body adapts. The thing with food is you have to keep returning to it and that's what makes it so hard to give it up occasionally. Philosophically speaking fasting and food present a dialectic in which one is forever vacillating between two points. This is particularly true for Judaism and Islam which also prescribe many ritual feasts. And so it's a dialectic of feasting and fasting.
Fasting teaches a person of faith to maintain a creative tension between living in the real material world with all of its imperfections, compromise and disappointments while at the same time trying to transcend it. Neither polarity is desirable or sustainable in the long term but rather they depend on each other for balance. An exclusively materialist reductionist approach to life is devoid of all spiritual meaning while a wholesale rejection of the material world renders faith irrelevant. Yet as with all polarities one can leverage the inherent tension. Practically this means that a person of faith must navigate a dialectic of advance and retreat. Occasionally retreating from, or rising above, the material reality in the hope of experiencing and articulating an ideal vision of reality. Sustained with this vision however one must eventually advance back into the material, imperfect world to contribute to its betterment. Retreat without a corresponding advance can be very seductive for idealistic young believers disillusioned with rampant materialism and so they throw all their weight against one pole upsetting the delicate balance of faith. This is called fundamentalism. By sitting down to a meal this evening at the end of the fast we discredit such skewed and dangerous inclinations. Our retreat over, we commence our advance, reaffirming our commitment to participating fully in this imperfect, troubled world in the inspired hope of making it just a little more habitable each day.