16 And when you fast, do not look gloomy like the hypocrites, for they disfigure their faces that their fasting may be seen by others. Truly, I say to you, they have received their reward. 17 But when you fast, anoint your head and wash your face, 18 that your fasting may not be seen by others but by your Father who is in secret. And your Father who sees in secret will reward you.
Fasting is an often neglected discipline by today's believers, in spite of the fact that Jesus assumed his hearers would fast ("when you fast"). I've personally found great personal reward through fasting and try to make it at least an annual discipline in my life. The start of a new calendar year is an ideal time to begin a fast. There are several books written about fasting and it benefits; I simply want to give some personal experience insight.
Why I fast:
I don't fast because I am a highly spiritual person. In fact, it is just the opposite. I find that I need fasting in order to get my life back on the right track. Fasting is a cleansing and purifying process that can bring clarity to my spiritual condition. It brings to the forefront the issues in my life I need to deal with. I replace my temporary dependence on food and depend on God instead and the strength He supplies. I began my fast this year on January 4 (for practical reasons, I did not begin on January 1); in addition to my daily readings on YouVersion.com, I'm re-reading John Piper's Why I Don't Desire God: How to Fight for Joy. Fasting, for me, is like hitting the Reset Button in my life. It is a fresh start.
Because I know some might think it, especially having read the Scripture above, but "doesn't going public about your fasting negate the spiritual benefits ('they have received their reward.')?" I don't feel this is true, and it has not been my experience. The Pharisees were fasting intentionally "to be seen by others." For me, fasting is inevitably going to be seen by others; so instead of disguise my fasting with deceit (making other excuses for why I'm not eating), I've found it valuable to be honest. In today's culture, fasting is less likely to make people think you are more spiritual, and more likely to make people think you are weird. I pray that my motives are sincere in that I fast for the reward of the Father.
To take it further, "That's what you may say, but isn't it going a little far to advertise your fasting in a blog post and on Facebook and Twitter?" A good point that I've wrestled with. However, if you knew a secret that could make other people's lives better (e.g. the Gospel), do you keep it to yourself? In this case, my "going public" is with the motive of discipleship and instruction. I'd love to start some conversations that would see people's spiritual lives re-tuned, revamped, and revived as a result of fasting. I would love to encourage others in their personal spiritual journey.
Fasting also has many mental benefits as well. In the beginning, this is not the case. Initially, your body is having to wrestle with the release of toxins back into the blood stream which can bring fatigue (physical and mental) and a "cloudiness" to the mind. Focus isn't always easy. There can be an increased irritability with friends and family, which relates to the physical processes going on as well as the spiritual revelations. However, after the first couple of days, a significant clarity is experienced, the "fog" will be lifted, and hopefully an improved mood.
There are obvious physical "side-effects" to fasting. It is not wrong to fast solely for physical benefits. The most obvious benefit is weight-loss. It is not unusual to experience a couple of pounds lost per day in the first couple of days. Overall, I find my weight-loss to be around an average of a pound a day. I also find that my net loss after recovering from a fast tends to be half of what I initially lost (i.e. 1/2 lb per day).
In the first few days, there will be a significant decrease in energy. I try to get increased rest during this time. Your digestive system will shut down, this will enable rest and healing to begin to occur throughout your body. There will be a detoxification taking place that will ultimately result in higher energy in the later days of the fast.
There is no biblical prescription or rules around fasting. There are several examples of biblical fasting and variations. I believe we have liberty in this area to fast in a way that would benefit us. I very my "rules" at different times. Typically, I like to start out with water only. If I find my energy dropping too low, I may resort to juices. It is important (for me) to stay away from proteins or anything that might really engage my digestive system. I want to give my body as much of a break and limiting non-water intake is important. If the point of your fast is mostly for physical benefit, then the "rules" become more central. However, if you are seeking spiritual growth, the concessions are not as important, as long as you keep the right perspective and focus.
How long can you fast?
This is, of course, a better question for a licensed physician. However, it is much longer than people think. Consider that at the beginning of a fast, you have a day or two's worth of energy stored through food consumed in the previous days. Once your body exhausts that supply, it will begin to break down fats. Body fat for a "normal" man is between 15-18%. Honestly, when I started, my body fat was in the 20+% range. This is a combination of subcutaneous fat, fat that is found on the outside of the body just under the outer skin layer, and visceral fat, the more unhealthy/dangerous fat that accumulates around the body's organs. My starting weight was 203. Considering that I had 20% body fat (although it was certainly higher), I have 40lbs of fat on my frame. A pound of fat renders approximately 3500 calories. Assuming that with the slow-down of activity and metabolism that is experienced through my fasting, I expend a liberal 2500 calories a day; I could go up to 56 days before my fat supply would be exhausted. I should note that it is unhealthy to have 0% body fat. The body is smart to use stores of fat before it begins breaking down other (e.g. muscle) tissues.
My longest personal fast has been 14 days. I did not stop because I was hungry; I stopped because I knew it was time. From my research, once your hunger subsides, it will not return until your body reaches the point of starvation. It is at that point that the fast should be broken.
A new perspective on food:
A several points in time throughout the first few days, I realize that I could easily be satisfied by much less food. Cravings are decreased so that I could be satisfied by much more ordinary food. I don't need the Chinese buffet, I'd be happy with just a bowl of rice. I don't need a candy bar, carrots would satisfy. If I do well, this perspective continues when I break the fast. Having killed many cravings, I can be much more sensible in how I fuel my body.
I'd love to start a conversation with anyone about this topic, their experiences, or field any questions. I'd also encourage anyone interested to do more research. There is a lot of information on the web (mostly secular), but I'd recommend Piper's book, A Hunger for God.