OK having busted a few common myths, where do we being to think constructively about fasting? It’s surprising how much there is about food in the Bible once you start thinking about it. Quite a few of Jesus’ parables feature banquets; miracles are based around food (e.g. feeding of 5,000). In fact Jesus Himself came ‘… eating and drinking...’ (Lk.7:34); both in the Garden of Eden and in the New Creation, our relationship to food is highlighted. Food features in the rhythm of our worship as we share communion. Feasting is an intrinsic part of worship throughout the Old Testament, as the Church focussed on the coming of Christ (Lev.23); and when people offered sacrifice they were to eat part of it themselves in the presence of the Lord. The Law was given in the context of an incredible feast (Ex.24:9-11), and commanded Israel to praise the LORD when they had eaten and were satisfied (Dt.8:10). More sobering is that our relationship to food is radically altered after the fall, and is an aspect of the curse (Gen.3:17-19). It was a principle part of the temptation to return to Egypt during the Exodus (Num.11:15), and of Jesus’ experience of temptation in the wilderness (Matt.4:3-4). You can pretty much chart out the whole story of creation with food and what is eaten by whom.
Given the deep connections – positive and negative – between our relationship with food, and our relationship with God, it is unsurprising that fasting (& feasting) is so prevalent. Many fasts in the Bible are for a day (e.g. Jdgs 20:26; I Sam.7:6; Jer.36:6), although examples of longer fasts include 3 days (Esther 4:16), 7 days (I Chron.10:12), or fasts of undetermined length (Jonah 3:5; Neh.1:4; Lk.2:37). I suppose the most famous fasts are 40 days (by Moses in Dt.9:9 & 10:10; Elijah in I Kings 19:7-8; and of course Jesus Himself in e.g. Matt.4:2). Jesus expected the Church to fast, though not till He after He had ascended (Lk.5:35; Mt.6:16-18).
Even from this handful of references we can see that the practise of fasting is deeply established in the life of the Church, but massively varied: in terms of the length of time people fasted, the purpose for which they fasted, and whether it is an individual or corporate discipline; regular and planned (as in Zech.8:19), or spontaneously reacting to situations that develop. Over the years I’ve read a number of attempts to crystallise the meaning and effect of fasting. Often people try to reduce fasting to a single idea; but to my mind, it echoes in a number of themes, for example: the love of the Lord is better than life, and spiritual nourishment is more important than physical (Ps.63:3-5); we do not live by bread alone, but by every Word that comes from the mouth of God (Matt.4:4); hunger for God and His righteousness (Matt.5:6); humbling ourselves before the Lord (Ps.35:13); the means through which the Lord exposes what lies in our hearts, and through which He can discipline us so that we may learn to trust His providence (Deut.8:1-5); practised with intercession / supplication (Ezra 8:21); averting disaster (II Chron.20:3); response to suffering brought about by the Lord in discipline or times of spiritual crisis (Joel 2; Acts 9:9; II Cor. 11:27); seeking wisdom and discernment from the Lord (Jdgs 20:26); spiritual power and effectiveness in ministry (Mk.9:29 see NIV footnote); it underpins times of worship and is the context in which the Church receives guidance for ministry (Acts 13:1-3; 14:23); mourning (Ps.42:3) and repentance (I Sam.7:5-6; Jonah 3:5); anticipation of the coming age (Lk.5:33-39). It seems to intensify the effectiveness of prayer and response to God, if done with integrity and in the context of wider life of righteousness (Is.58; Zech. 7:3-14 – note that when the prophets attack fasting, it is not the practise itself, but the hypocrisy they harangue; see also Matt.6:16f; Col.2:16-23).
But why does it have this effect of intensifying the dynamics of our relating to God? I’ve never read a theology that fully explains this. My best guess is that it confronts us with the reality of our own mortality, and of our immediate dependence on God for life and the provision of what we need to continue in life. The point of fasting might be that it brings front and centre our own intrinsic created-ness and therefore our dependent-ness. Thus humbled, and acutely aware of our reliance on the Lord for life itself, we are well-placed to approach the throne of grace in an appropriate frame of mind, heart and soul. Within hours of beginning a fast we can begin to feel cold, hungry, weak, tired, and irritable. It is tempting to try to turn it into show of strength, and to speak of it as if it demonstrated titanic spiritual heroism. But it is a grotesque parody of fasting that leads to self-righteous pride (Lk.18:9-14). To fall foul of this would be to catastrophically miss the point and to critically undermine its purpose (Matt.6:16-18). It is meant precisely to expose our weakness, vulnerability and impotence before our God, and so our reliance on Him (Ps.109:24). It may expose more than this, but it should not expose less.
 Something similar might lie behind the sacrifice of sleep for the purposes of Bible study & prayer (e.g. Ps.63:6; 119:148 etc). It is – similar to fasting – the sacrifice of something critical, and confronts us forcibly with the fragility of our createdness. It is this purpose in fasting that I think means the vocabulary of fasting is restricted to those things on which we depend for life. Giving up TV, or meat, or chocolate might be a good thing to do, but it isn’t fasting. However important we might feel these things are, it isn’t life-threatening to go without them.