There is an excellent series of Videos online from this years Low Carb Vail conferences – but one of the interesting things about this conference was how significantly experts disagreed about the correct amount of protein people should eat, and in particular how much lean body mass we lose when fasting.
Dr. Jason Fung uses data from Kevin Hall published in “Quantitative Physiology of Human Starvation“. Dr Fung says in Therapeutic Fasting – Solving the two compartment problem –
Are we burning muscle? Hell no! It goes up slightly at the very beginning and then drops. Protein is not a storage form of energy.
Dr. Stephen Phinney in The Case for Nutritional Ketosis – uses data from George Cahills “Starvation in Man” study. Dr. Phinney says
In starvation your muscles are broken down. Even after a month of total starvation – no protein coming in, you are losing 4 oz of lean body mass every day.
What we have here is not just a difference of opinion … but two fundamentally different analyses of human starvation metabolism. Either one or both of them is wrong in general … or they are both right for a specific subset of humans.
So I looked around for any data that might give me a gut feeling for which might be closer to the truth. And found the story of Angus Barbieri (SHOW ME THE SCIENCE) who fasted for 382 days. He started at 456 lbs and ended around 180 lbs, and he could not have lost 4oz of lean body mass every day.
His plasma Urea, which is a marker of his consumption of amino acids (to either turn into glucose, or oxidise for energy) dropped to less than half of the normal rate for a man of his age. This example would appear to support Dr. Fung.
So I decided to do the math … and I apologise in advance for using a bastardization of metric, and imperial units – but nutritionists are most used to dealing with body mass in pounds (lbs), energy in kilocalories (kCal), and food mass in grams (g);
Let’s start with how much energy the Brain uses in a day;
Cahill et al., 1968 showed in “The consumption of fuels during prolonged starvation” that the adult Brain uses between 110g and 140g of glucose per day. Let’s say to be generous: 600 kCal
In the same study George Cahill’s team showed that once adapted to starvation, up to 2/3rd of the brain’s energy can be met from Ketones, leaving only 1/3 to be met from glucose.
So we need 400 kCal of ketones and 200 kCal of glucose to adequately fuel the brain. There are of course other glucose dependant tissue (red blood cells, renal tubules, etc). I don’t have data with which to include those, but when I do I shall.
Converting this to mass, this means we need roughly 50 g of glucose (200 kCal / 4 kCal/g) and 80g of ketones (400 kCal / 5 kCal/g ) – just to keep the brain alive.
I did the math once to work out the ratio of glucose that can be produced through Gluconeogenesis from the glycerol released from a model Triglyceride containing the 3 most common fatty acids in humans (Oleic: mw=282.46, Palmitic: mw=256.43, Linoleic: mw=280.45) and glycerol (mw=92.09) in order to work out the mass ratio of glycerol to that triglyceride … and it turns out that for every 10g of body fat burned there is very close to 1g of glucose that can be produced (10g Triglyceride -> 1.0104g glycerol ->0.988297g glucose).
To produce those 50 g of glucose it turns out that we need to burn 500 g of body fat to produce sufficient glycerol as a substrate for gluconeogenesis, or 1.1 lbs.
So to use ABSOLUTELY no protein to make glucose necessary to fuel his brain Angus Barbieri would have to be burning 1.1 lb/day of body fat. Using the rough figure of 3500 kcal/lb of body fat we can work out that would be 3850 kCal of body fat he would need to be drawing down every day – which is a pretty impressive metabolic rate but that is possible in a 456 lb man who is performing modest exercise.
We know (SHOW ME THE SCIENCE) that the maximum rate we can draw down body fat is around 31.5 kCal/day per lb of body fat. So he would have had to have had at least 122.22 lbs of body fat to be able to draw down 3850 kCal from it – which he probably had.
So I think we can say if he did enough exercise to keep his total daily energy expenditure up above 3850 kCal then, as long as he had more than 122.22 lbs of body fat, he could have got away with consuming NO protein to keep his brain adequately fueled for at least the first part of his fast until his availble body fat dropped below 122.22 lbs.
So a grossly obese subject such as Angus Barbieri would appear to support Dr. Fungs observation.
How about say a 158 lb lean weight lifter, with 10% body fat. He would also need to make 50 g/day of glucose assuming his brain was the same size. The problem for him is that he would only have 15.8 lbs of body fat and therefore he would only be able to generate 497.7 kCal/day from oxidizing body fat (15.8 lbs * 31.5 kCal/lb), which would consume 55.3g of body fat per day. Using the glycerol from fat to make glucose would produce 5.47g of glucose to contribute to his brain’s energy requirements, leaving the shortfall (44.53g) to be made up from other substrates. If that was all protein he would require 89.06g of protein from his body to be converted into glucose every day.
That is a lot closer to Dr. Phinney’s observation, and may be a better reflection of the subjects of George Cahills experiments in the 70s before the low fat revolution.
So yes … Dr. Phinney and Dr. Fung may both be correct, in context.