Fasting is a safe and effective tool for health promotion and fasting for up to three days is something anyone can do on their own. However, fasting longer than three days should not be undertaken without the supervision of a physician. It is always a good idea to consult your doctor before undertaking any fasting program to make sure there are no underlying issues.
The focus of this article is on a fast for 24 to 72 hours or 1 to 3 days as the benefits of a fast do not start until at least 12 hours and only increase during the first 24 to up to 72 hours. Fasting is not starvation but rather a controlled abstention from food for a short period of time. The most effective approach to fasting is to only have water during the fasting period. That is not to say that programs such as the 5:2 fast diet or one off one on approach are not beneficial, it is just to say that the more restrictive the caloric intake the quicker the body is brought to a state where beneficial processes are engaged.
When abstaining from foods and liquids other than water, the body begins to seek its needed glucose from alternate sources through the conversion of fats, nonessential tissues, foreign substances and organisms, and digestive enzymes. Muscle is also used to a degree as muscle tissue is itself a reservoir for readily available fuel. Only after a prolonged period after these nonessential sources are depleted are the metabolic needs fulfilled from more essential tissues such as muscle and vital organs. It is at this point that starvation is occurring. However, even a thin person has sufficient reserves to fast for up to 40 days and not experience symptoms of starvation.
As can be expected, abstaining from caloric intake results in rapid weigh loss. The first couple of days of a fast can result in upwards of 2 to 4 lbs of weigh loss depending on ones initial condition. Much of this is water as the body begins to release salts. When fasting is initiated, the body goes through metabolic changes that are focused on conserving body mass while drawing its energy needs from its least essential sources. The two primary sources are fats and muscle and the ratio of consumption various based on initial fat stores, metabolic conditions and availability of other sources.
The most important source of energy for the body is glucose, which is the primary fuel for the brain as well as being needed for many other body systems. Once food intake is suspended, the first and most readily available source of additional glucose is from glycogen stores found in the liver. The liver however only stores from 100 to 125 grams of glucose in the form of glycogen and this is rapidly depleted usually within the first day of the fast. The brain typically consumes up to 80 percent of the body's resting energy requirements which can represent up to 180 grams of glucose per day. So based on this paramount need for glucose, one can see that the liver acts as a buffer or reservoir for glucose supply. As we eat, glucose is provided directly to the bloodstream as well as converted by the liver to glycogen and stored as a reserve. As glucose levels drop in between meals, the glycogen is converted back to glucose as needed. This is why, in my opinion, a fast does not really start to engage until at least 12 hours without any caloric intake as it is at least this much time that is needed to deplete the liver stores of glycogen.
Once we are beyond the 12 hour point of a fast the body starts to convert fat stores into fatty acids to be used as a fuel source for muscle tissue and organs. However, glucose is still needed for the brain and red blood cells to function properly which cannot use fatty acids as a fuel. The body can manufacture glucose through two metabolic pathways.
- Creation of glycerol from fat tissue
- synthesis of amino acids in muscle tissue to create glucose
The conversion of fat occurs through the following transformation, fat to triglyceride to glycerol, to glucose. However, this process cannot produce sufficient glucose once the liver stores of glycogen are depleted.
This is where the catabolism of muscle tissue steps in as an interim supply through the conversion of amino acid stores in the tissue into glucose. Similar to the liver, muscle tissue can also considered an organ due to its metabolic contributions and provides buffering capacity for balancing glucose needs. This is one of the main reasons body builders and fitness enthusiasts believe that there needs to be a near constant intake of food to keep up with glucose demands and to avoid any depletion of hard earned muscle tissue. Energy needs derived from muscle would require over a pound of muscle tissue per day. Amino acid conversion from muscle also creates acidosis in the system.
Muscle depletion is only temporary as by the third day of the fast the liver begins to generate ketones through fat conversion. As these ketones enter the bloodstream, they provide an alternative to glucose thereby reducing the demand for glucose and reducing the demand for conversion of amino acids in the muscle tissue. The brain, muscle and other organs begin to use ketones instead of glucose as a fuel resulting in a reduction of muscle wasting to less than half a pound per day. At this stage of the fast, muscle mass is conserved and the maximum breakdown of fatty tissue and removal of non-essential and waste substances starts to accelerate. Ketosis will start to occur within 48 to 72 hours of a fast with it being quicker for women compared to men. This is a safe process for normal individuals compared to diabetics where ketosis can reach dangerously high levels. For people with normal function the body will control the level to ketones being produced for fuel.
The above is a summary of the metabolic processes that occur during a short 2 to 3 day fast. It is important to also consider how you prepare in advance of a fast and how to break a fast, and that will be presented in a future article.
