Thanks for all the info guys.
I also found another interesting thing:
Differences in fat, carbohydrate, and protein metabolism between lean and obese subjects undergoing total starvation
M Elia, RJ Stubbs and CJ Henry
Addenbrooke’s Hospital, Cambridge.
Despite extensive experimental studies on total starvation, many of the findings relating to protein, fat (plus ketone body), and carbohydrate metabolism remain confusing, although they become more consistent when considered in relation to the degree of initial obesity. During prolonged starvation, protein loss and percent energy derived from protein oxidation are 2- to 3-fold less in the obese than in the lean; percent urine N excreted as urea is 2-fold less in the obese; and the contribution of protein to net glucose production is only about half in the obese compared to lean subjects.
During short-term starvation (first few days) the following differences are reported: hyperketonaemia is typically 2-fold greater in lean subjects, but associated with a 2-fold lower uptake of ketone bodies by forearm muscle; glucose tolerance becomes impaired more in lean subjects; and both protein turnover and leucine oxidation increase in the lean, but may show no significant change in the obese. It is no longer acceptable to describe the metabolic response to starvation as a single typical response. The differences between lean and obese subjects have important physiological implications, some of which are of obvious relevance to survival.
From: http://www.obesityresearch.org/cgi/content/abstract/7/6/597
Ha! Someone else (back in 1999) asked a similar question:
There are three stages in the starvation process with the first taking place in a very short time - even just between meals for us. Glucose is the fuel that the body uses if available, with over half of the daily glucose requirements going to the brain. The rest of the glucose fuels e.g. the function/production of muscle tissue and red blood cells. The body can obtain glucose from the breakdown of glycogen (first choice with small quantities being stored in the liver) and fat. After the glycogen reserve is used up - still in this short term situation - the body begins to draw on fat to maintain blood glucose levels. Of the fatty acids and glycerol derived from the fat breakdown, the glycerol is transformed into glucose that can go to the brain (fatty acids can not enter the brain.) The fatty acids can be used directly by all other body cells. The use of fatty acids in the cells swithches off the use of glucose in the same cell - with the result that parctically all the glucose available is used in the brain.
So during early starvation (say, 2-3 days after eating the last meal), we make extensive use of fatty acids as a fuel. Glucose is conserved and recycled and is reserved for almost exclusive use by the brain. However, with the brain using 120 g glucose per day it is not going to take long for our carbohydrate stores to run out - what a pity we can’t get the brain to make use of our vast stores of fatty acids!
Although the brain cannot use fatty acids, it can use short derivatives of fatty acids called KETONE BODIES. These are small, 4-carbon “mini-fatty acids” which CAN get into brain cells and can be split to give 2 molecules of acetyl CoA. Ketone bodies are produced from fatty acids in the liver after about 3 days of starvation. Essentially they are produced as acetyl CoA production reaches a maximum in this tissue. Their oxidation by the brain, cuts the brain’s requirement for glucose from 120 g per day to about 30 g per day. Although the production and utilisation of ketone bodies is therefore a great help in relieving the pressure on our carbohydrate stores, it is still not really good enough - we still need to make that 30 g of glucose everyday from something! True we get 20 g of new glucose from glycerol everyday, but it still leaves a shortfall of 10 g - the difference must come from protein
After several days of exposure to low blood insulin concentrations, ALL our cells start to breakdown protein. This releases amino acids into the bloodstream which, as with glycerol and lactate, can be converted into glucose in the liver. Although this gives us all the glucose that we need for our brain, it should be realised that most cellular protein is ESSENTIAL to the survival of that cell. A cell cannot tolerate the loss of very much protein. Furthermore, the breakdown of proteins in cells is not very discriminatory. Prolonged low insulin levels are just as likely to breakdown lung cells as muscle cells. In addition, about 2-3 g of protein has to be broken down to synthesise 1 g of glucose - so over 30 g of pure protein is broken down each day just to keep our brain alive.
Eventually, after about 40 to 50 days of starvation, the loss of body protein reaches a stage at which important organs are irrevocably affected, and death results, regardless how many fat reserves are still present.
If the fat store has been depleted earlier, the body enters the final stage three in the starvation process earlier As the main source of energy switches to protein muscles deteriorate and cell function is impaired. The brain once again adapts to utilize some amino acids.
The ultimate cause of death would probably be due to arrhythmia or cardiac arrest brought on by tissue degradation and electrolyte imbalances, if an infection didn’t attack first.
So the bodies of both the fat person and the lean person would respond the same to starvation. First the short term glucose/glycogen supply from the liver would be used with some energy coming from proteins and fats. Then fat tissue becomes the primary supplier. The fat person would hold at this stage two longer than the lean person could. At stage three, both would finish up their protein reserve.
Who is going to live the longest? - As soon as one has fat reserves for more than 2 months (and aery single normal healthy american has) there is no difference between the lean an fat persons. Of couse, if one has only few fat reserves (mostly people in 3rd world countries) then a few gram of fat can prolong your survival time during starvation considerably.
From: Re: Can a 'fat' person survive longer without eating than a skinny person?