There are many misconceptions about fasting.  It is useful to review the physiology of what happens to our body when we eat nothing.
Physiology 
Glucose and fat are the body’s main sources of energy. If glucose is not available, then the body will adjust by using fat, without any detrimental health effects. This is simply a natural part of life. Periods of low food availability have always been a part of human history. Mechanisms have evolved
to adapt to this fact of Paleolithic life. The transition from the fed state to the fasted state occurs in several stages.
  1. Feeding – During meals, insulin levels are raised. This allows uptake of glucose into tissues such as the muscle or brain to be used directly for energy. Excess glucose is stored as glycogen in the liver.
  2. The post-absorptive phase – 6-24 hours after beginning fasting.   Insulin levels start to fall. Breakdown of glycogen releases glucose for energy. Glycogen stores last for roughly 24 hours.
  3. Gluconeogenesis – 24 hours to 2 days – The liver manufactures new glucose from amino acids in a process called “gluconeogenesis”. Literally, this is translated as “making new glucose”. In non-diabetic persons, glucose levels fall but stay within the normal range.
  4. Ketosis – 2-3 days after beginning fasting – The low levels of insulin reached during fasting stimulate lipolysis, the breakdown of fat for energy. The storage form of fat, known as triglycerides, is broken into the glycerol backbone and three fatty acid chains. Glycerol is used for gluconeogenesis. Fatty acids may be used for directly for energy by many tissues in the body, but not the brain. Ketone bodies, capable of crossing the blood-brain barrier, are produced from fatty acids for use by the brain. After four days of fasting, approximately 75% of the energy used by the brain is provided by ketones. The two major types of ketones produced are beta hydroxybutyrate and acetoacetate, which can increase over 70 fold during fasting.
  5. Protein conservation phase – >5 days – High levels of growth hormone maintain muscle mass and lean tissues. The energy for maintenance of basal metabolism is almost entirely met by the use of free fatty acids and ketones. Increased norepinephrine (adrenalin) levels prevent the decrease in metabolic rate.
The human body has well developed mechanisms for dealing with periods of low food availability. In essence, what we are describing here is the process of switching from burning glucose (short term) to burning fat (long term). Fat is simply the body’s stored food energy. In times of low food availability, stored food is naturally released to fill the void. So no, the body does not ‘burn muscle’ in an effort to feed itself until all the fat stores are used.
Hormonal Adaptation
Insulin
Fasting is the most efficient and consistent strategy to decrease insulin levels. This was first noted decades ago, and widely accepted as true. It is quite simple and obvious. All foods raise insulin, so the most effective method of reducing insulin is to avoid all foods. Blood glucose levels remain normal, as the body begins to switch over to burning fat for energy. This effect is seen with fasting periods as short as 24-36 hours. Longer duration fasts reduce insulin even more dramatically. More recently, alternate daily fasting has been studied as an acceptable technique of reducing insulin.
Regular fasting, in addition to lowering insulin levels, has also been shown to improve insulin sensitivity significantly. This is the missing link in the weight loss puzzle. Most diets reduce highly insulin-secreting foods, but do not address the insulin resistance issue. Weight is initially lost, but insulin resistance keeps insulin levels and Body Set Weight high. Fasting is an efficient method of reducing insulin resistance.
Lowering insulin rids the body of excess salt and water. Insulin causes salt and water retention in the kidney. Atkins style diets often cause diuresis, the loss of excess water, leading to the contention that much of the initial weight loss is water. While true, diuresis is beneficial in reducing bloating, and feeling ‘lighter’. Some may also note a slightly lower blood pressure. Fasting has also been noted to have an early period of rapid weight loss. For the first five days, weight loss averages 0.9 kg/ day, far exceeding the caloric restriction and likely due to a diuresis of salt and water.
Growth Hormone
 Growth hormone is known to increase the availability and utility of fats for fuel. It also helps to preserve muscle mass and bone density. Secretion is known to be pulsatile, making accurate measurement difficult. Growth hormone secretion decreases steadily with age. One of the most potent stimuli to growth hormone secretion is fasting. Over a five-day fasting period growth hormone secretion more than doubled. The net physiologic effect is to maintain muscle and bone tissue mass over the fasting period.
Electrolytes
Concerns about malnutrition during fasting are misplaced. Insufficient calories are not a major worry, since fat stores are quite ample. The main concern is the development of micronutrient deficiency. However, even prolonged studies of fasting have found no evidence of malnutrition. Potassium levels may decrease slightly, but even two months of continuous fasting does not decrease levels below 3.0 mEq/L, even without the use of supplements. This duration of fasting is far longer than generally recommended. Magnesium, calcium and phosphorus levels during fasting are stable. Presumably, this is due to the large stores of these minerals in the bones. Ninety nine percent of the calcium and phosphorus in the body is stored in the bones. The use of a multi-vitamin supplement will provide the recommended daily allowance of micronutrients. A therapeutic fast of 382 days was maintained with only a multivitamin with no harmful effect on health. Actually, this man maintained that he had felt terrific during this entire period. The only concern may be a slight elevation in uric acid that has been described in fasting .
Adrenalin
Adrenalin levels are increased so that we have plenty of energy to go get more food. For example, 48 hours of fasting produces a 3.6% increase in metabolic rate, not the dreaded metabolic ‘shut-down’. In response to a 4 day fast, resting energy expenditure increased up to 14%.   Rather than slowing the metabolism, instead the body revvs it up. Presumably, this is done so that we have energy to go out and find more food.
This is really quite interesting.  Fasting, but not low calorie diets results in numerous hormonal adaptations that all appear to be highly beneficial on many levels.  In essence, fasting transitions the body from burning sugar to burning fat.  Resting metabolism is NOT decreased but instead increased.  We are, effectively, feeding our bodies through our own fat.  We are ‘eating’ our own fat.  This makes total sense.  Fat, in essence is stored food.  In fact, studies show that the epinephrine (adrenalin) induced fat burning does not depend upon lowering blood sugar.
Recall our previous discussion of How Insulin Works.  Fat is food stored away in the long term, like money in the bank.  Short term food is stored as glycogen, like money in the wallet.  The problem we have, is how to access the money in the bank.  As our wallet depletes, we become nervous and go out to fill it again.  This prevents us from getting access to money in the bank.
Fat is stored away in the ‘bank’.  As our glycogen ‘wallet’ depletes, we get hungry and want to eat.  That makes us hungry, despite the fact that there is more than enough ‘food’ stored as fat in the ‘bank’.  How do we get to that fat to burn it?  Fasting provides an easy way in.
Start with Fasting Part I
Continue with Fasting part 3
Start with Calories Part I
Watch the lecture “The Fast Solution” – The Aetiology of Obesity 4/6