Lack of food does have an effect in the beginning of an extended fast in terms of the hunger signals the body sends. The body really does not like to consume itself and complains loudly in the beginning. Most people experience the beginnings of this on a regular basis. The myth is that those signals continue at that same frequency for the duration, they do not.
I have had two extended stays in the wilderness with little or no food, both very physical. The first was an eight days "off the land" survival exercise in Pennsylvania, the second was a ten day canoe trip in Canada done on extremely small rations. I have also done countless four day trips under survival conditions on a ration of about 300 calories per day.
After the initial intense hunger pain subsides you pass into a phase in which the hunger message is always present but the siren is turned off. The lack of energy seems to come and go, you do want to rest but that rest really does pay off and you feel better shortly. Unless you have some underlying medical condition extended fasting is not a serious threat.
One thing I find that helps a great deal with the energy level is to eat strategically. If you have some food resource then discipline yourself to not eat it until after the hunger pains have gone away. In that economy a little bit of food goes a long way if it is eaten before you have to work hard. Forget about "mealtimes" and only eat a portion of the food you have or find to fuel up for strenuous exercise. It is much easier to sleep on an empty stomach than it is to hike several miles. Mac
