Re: Water Fasting & Exercise: which does the body burn first, muscle or protein?

Hello Chris,
the body will always burn fat before muscle and or protein while water-fasting.
This occurs in the process of ketosis where the body makes the transition from burning glucose as its main energy source (in the absence of the intake of carbs & other external nutrients) to that of using fat as its main energy source.

However, the body will still need tiny amounts of glucose for some functions, so a very small amount of protein, 18-24 grams, is still catabolized to supply it - from 1/2 to 1 ounce of muscle tissue per day.
Over a 30 day water-fast a person generally loses a maximum of 1-2 pounds of muscle mass. This conservation of the body's protein is an evolutionary development that exists to protect muscle tissue and vital organs from damage during periods of insufficient food availability.
From the third day onward the rate of the breakdown of fatty acids from adipose or fat tissue continues to increase, hitting its peak round about the tenth day. This seven day period, after the body has shifted completely over to ketosis, is where the maximum breakdown of fat tissue occurs.
As part of protein conservation, the body also begins seeking out all non-body-protein sources of fuel: nonessential cellular masses such as fibroid tumors and degenerative tissues, bacteria, viruses, or any other compounds in the body that can be used for fuel. This is part of the reason that fasting produces the kind of health effects it does. Also, during this period of heightened ketosis the body is in a similar state as the one that occurs during sleep - a rest and detoxification cycle. It begins to focus on the removal of toxins from the body and the healing and regeneration of damaged tissues and organs.
This is the case whether you are resting or exercising, but the body will soon tell you whether or not it is capable of exercising and even force you to rest, just as the body will force you to sleep under conditions of sleep deprivation.

Regards

Chrisb1.