Well its not for everyone especially those people with a higher basal metabolism who are waking up in the morning already hungry and with low blood sugar. If blood sugar is already low in the morning (e.g from low carb diets or low total intake the day before or excessive exercise), then because the body is always trying to get to its normal homeostasis this low blood sugar may drive Glucagon and leptin which then drive the individuals genetic hunger response (called their set point). This may make them crave anything which will help them to get the blood sugar up, e.g. sugar.
When the body is in a fasting state, it has been well researched that your fasting enzymes (lip-protein lisapse) kick in. They are your survival (famine) enzymes that function to assist tryglycerides to be stored in adipose tissue. This explains part of the reason why many anorexics have very little muscle mass, but if you do skin-folds they tend to have increased fat mass – why? Because their body is in starvation mode and therefore their survival genes kick in to take whatever food they do have and put it into storage for another famine. Again this is driven by hormones.
If people are trying to increase muscle mass or they are an athlete then everything about Recovery strategies is about trying to maintain muscle levels of glycogen. This is called Glycogen – sparing. Therefore if an athlete is in a fasting state, then they will go into their training or competition and more rapidly use up their stored glycogen. When this runs out they will then use stored fat and then protein to assist liver turnover of glycogen to get to the working muscles – gluconeogenesis. This makes it harder for Recovery as now the body has to play ‘catch- up’ during the day for restoring muscle glycogen. If enough carbs and protein and Omega 3 fats aren’t eaten then the athlete will enter the next training session already in a depleted state (just ask all those mothers who have to manager swimmers competing in a 5 day competition for Nationals and how their kids accumulate fatigue, because they haven’t got their ratio of recover carbs and protein correct from day to day.
Fasting has been used for centuries as the many say as a form of religious celebration. Generally then, people are quite sedentary. As well after the period of fasting it is well known that there is a Feast. Say No More.
So while i dont disagree with the concept of Fasting as a way to manage total energy intake over a day. therefore it may make sense for some people in the population what I do disagree with is the opinion that everyone should do it.