I thought the same thing you did, until your question prompted me to do some research!
Apparently, it's the other way around - your body will start by eating from it's fat and only move onto muscle after periods of prolonged starvation. As for a calorie number, I found information (below) that seems to say we can avoid "starvation mode" - the point where we need to turn our own body into fuel - by consuming enough glucose to feed our brain's daily 120g need. Below that amount, we pull from our glycogen reserves in the liver and muscles, and after that, our body eats our fat, and our brain eats our fat too, but only after being processed by ketone bodies produced by our liver. It isn't until day four without any food that muscles proteins will be used. I assume if some food is eaten throughout the four days - though not enough to meet the 120g need, it will take longer still before our muscles are used for nutrients.
The bit about metabolism... I'm not sure on this. Lots of conflicting information out there. A lot of legitimate looking sources say that's a myth and nothing can change your basal metabolism. Others say you should never dip below 1000 calories if you want to keep your metabolism steady - but then sticking to at least 1000 calories a day seems like good advice even without considering metabolism.
GREAT QUESTION! I learned a few new things today!
Interesting tid-bits below:
"Starvation mode is a state in which the body is responding to prolonged periods of low energy intake levels. During short periods of energy abstinence, the human body will burn primarily free fatty acids from body fat stores. After prolonged periods of starvation the body has depleted its body fat and begins to burn lean tissue and muscle as a fuel source."
"Ordinarily, the body responds to reduced energy intake by burning fat reserves first, and only consumes muscle and other tissues when those reserves are exhausted. Specifically, the body burns fat after first exhausting the contents of the digestive tract along with glycogen reserves stored in muscle and liver cells."
"After prolonged periods of starvation, the body will utilize the proteins within muscle tissue as a fuel source. People who practice fasting on a regular basis, such as those adhering to energy restricted diets, can prime their bodies to abstain from food without burning lean tissue."
"The body uses glucose as its main metabolic fuel if it is available. About 25% of the total body glucose consumption occurs in the brain, more than any other organ. The rest of the glucose consumption fuels muscle tissue and red blood cells...Glucose can be obtained directly from dietary sugars and carbohydrates. In the absence of dietary sugars and carbohydrates, it is obtained from the breakdown of glycogen. Glycogen is a readily-accessible storage form of glucose, stored in small quantities in the liver and muscles. The body's glycogen reserve can provide glucose for about 6 hours.After the glycogen reserve is used up, glucose can be obtained from the breakdown of fats. Fats from adipose tissue are broken down into glycerol and free fatty acids. Glycerol can then be used by the liver as a substrate for gluconeogenesis, to produce glucose.
Fatty acids can be used directly as an energy source by most tissues in the body, except the brain, since fatty acids are unable to cross the blood-brain barrier. After the exhaustion of the glycogen reserve, and for the next 2–3 days, fatty acids are the principal metabolic fuel. At first, the brain continues to use glucose, because, if a non-brain tissue is using fatty acids as its metabolic fuel, the use of glucose in the same tissue is switched off. Thus, when fatty acids are being broken down for energy, all of the remaining glucose is made available for use by the brain.
However, the brain requires about 120 g of glucose per day (equivalent to the sugar in 3 cans of soda), and at this rate the brain will quickly use up the body's remaining carbohydrate stores. However, the body has a "backup plan," which involves molecules known as ketone bodies. Ketone bodies are short-chain derivatives of fatty acids. These shorter molecules can cross the blood-brain barrier and can be used by the brain as an alternative metabolic fuel.
After 2 or 3 days of fasting, the liver begins to synthesize ketone bodies from precursors obtained from fatty acid breakdown. The brain uses these ketone bodies as fuel, thus cutting its requirement for glucose. After fasting for 3 days, the brain gets 30% of its energy from ketone bodies. After 4 days, this goes up to 70%.
Thus, the production of ketone bodies cuts the brain's glucose requirement from 120 g per day to about 30 g per day. Of the remaining 30 g requirement, 20 g per day can be produced by the liver from glycerol (itself a product of fat breakdown). But this still leaves a deficit of about 10 g of glucose per day that must be supplied from some other source. This other source will be the body's own proteins.
After several days of fasting, all cells in the body begin to break down protein. This releases amino acids into the bloodstream, which can be converted into glucose by the liver. Since much of our muscle mass is protein, this phenomenon is responsible for the wasting away of muscle mass seen in starvation.
However, the body is able to selectively decide which cells will break down protein and which will not. About 2–3 g of protein has to be broken down to synthesise 1 g of glucose; about 20–30 g of protein is broken down each day to make 10 g of glucose to keep the brain alive. However, this number may decrease the longer the fasting period is continued in order to conserve protein.
Starvation ensues when the fat reserves are completely exhausted and protein is the only fuel source available to the body. Thus, after periods of starvation, the loss of body protein affects the function of important organs, and death results, even if there are still fat reserves left unused. (In a leaner person, the fat reserves are depleted earlier, the protein depletion occurs sooner, and therefore death occurs sooner.)"
The ultimate cause of death is, in general, cardiac arrhythmia or cardiac arrest brought on by tissue degradation and electrolyte imbalances."