I’ve had several conversations in the past few weeks with guys that advocate various forms of fasting. From a sports nutritional standpoint, I simply cannot advocate any forms of fasting. So, I wanted to bring some facts about the dynamics here to the table to allow people to make informed decisions about whether or not fasting is appropriate for them.

During fasting, carbs, fat, and protein are all eventually used for energy – fuel must be delivered to every cell. As the fast begins, glucose from the liver’s stored glycogen and fatty acids from the adipose tissue’s stored fat are both flowing into cells, when breaking down to yield energy thru their respective pathway. Several hours into the fast, however, most of the glucose is used up – liver glycogen is exhausted and blood glucose begins to dwindle and fade. Low blood glucose serves as a signal that promotes further fat breakdown and release of amino acids from muscles.

Glucose needed for the brain:

At this point, most of the cells are depending on fatty acids to continue providing their fuel. But red blood cells and the cells of the nervous system need glucose to perform their respective functions. Glucose is the major energy fuel here, and even when other fuel sources are available, glucose must be present to permit the energy-metabolizing machinery of the nervous system to work. Normally, the brain and nerve cells – which weigh only about 3 lbs – consume about 2/3 of the total glucose used each day (400-600 calories worth). About 1/5 to ¼ of the energy the adult body uses when it is at rest is spent by the brain; for children, it can be up to ½.

Protein meets glucose needs:

The red blood cells’ and the brain’s special requirements for glucose pose a problem for the fasting body. The body can use its stores of fat, which may be quite generous, to furnish most of its cells with energy, but not the red blood cells and the brain & nervous system. Amino acids can yield glucose following deamination thru a process called gluconeogenesis; for the cells to obtain the amino acids necessary for this, body proteins must be broken down. For this reason, body protein tissues such as muscle and liver always break down to some extent during fasting.

The breakdown of body protein is an expensive way to obtain glucose. In the first few days of a fast, body protein provides about 90% of the needed glucose; glycerol about 10%. If body protein losses were to continue at this rate, death would ensue within 3 weeks, regardless of the quantity of fat a person had stored. Fortunately, fat breakdown also increases with fasting – in fact fat breakdown almost doubles, providing energy for other body cells and glycerol for glucose production during the early phases here.

The shift to ketosis:

As the fast continues, the body finds a way to use its fat to fuel the brain. It adapts by combining co-enzyme fragments derived from fatty acids to produce and an alternate energy source called ketone bodies. Normally produced and used only in small quantities, ketones can provide fuel for some brain cells. Ketone production rises until, after about 10 days of fasting, it is meeting much of the nervous system’s energy needs. Still, many areas of the brain rely exclusively on glucose, and to produce it, the body continues to sacrifice proteins – albeit at a slower rate than in the earlier stages of fasting.

When ketone bodies contain an acid group (COOH), they are called keto acids. Small amounts of keto acids are a normal part of blood chemistry, but when their concentration rises, the pH of the blood drops significantly. This is ketosis, a sign that the body’s chemistry is out of whack. Elevated blood ketones are excreted in the urine, so the kidneys are taxed as well. Fluid balances becomes much more essential here.

Suppression of appetite:

Ketosis also induces a loss of appetite. As starvation continues, this loss of appetite becomes an advantage to a person without access to food, because the search for food would be a waste of energy (the survival mechanisms of the human body have always intrigued me). When the person eats again, the body shifts out of ketosis, the hunger center gets the signal that food is available again, and the appetite returns.

Slowing of metabolism:

This aspect, IMO, should be paid attention to most of all… In an effort to conserve body tissues for as long as possible, the hormones of fasting begin to slow metabolism. As the body shifts to the use of ketones, it simultaneously reduces its energy output and conserves both its fat and its lean tissue. Still the lean (protein-containing) organ tissues shrink in mass and perform less metabolic work, reducing energy expenditures. As the muscles waste, they can do less work and so demand less energy, reducing expenditures further. Because of the slowed metabolism, the loss of fat falls to the bare minimum – less in fact than the amount that would be lost on a low-cal diet. Thus, although weight loss during fasting may be quite dramatic, fat loss may be less than when at least some food is eaten.

In summary: When fasting, the body makes a number of adaptations – increasing the breakdown of fat to provide energy for most of the cells, using glycerol and amino acids to make glucose for the red blood cells and central nervous system, producing ketones to fuel the brain, suppressing the appetite, and slowing the metabolism. All of these measures are the body’s ways of conserving energy to minimize losses. In fact, the metabolism slows to such an extent that the loss of fat eventually slows to less than would be achieved with a low-cal diet.

I realize that a fast such as described is highly unlikely for most of, and hopefully all of, us. The points I’m trying to make here is to illustrate the physiological changes that take place, like the deamination of body proteins, which cannot be avoided while on any sort of fast. IMO, this is counter-productive towards most goals in the gym.