The process of ketosis is one of the physiological effects of fasting in which the brain (and some other bodily processes) uses ketones produced from fatty tissues as a fuel instead of the usual glucose. This is called "muscle sparing".
When glucose isn't readily available via the diet (in the form of carbohydrates) and the glycogen stores in the liver become depleted, the body could break down muscle to get it. But ketosis is an adaptation that will spare muscle during times of shortage by instead breaking down fat stores and manufacturing ketones for brain fuel. It is said this state is attained at approximately 48 hours of a water fast for women and closer to 72 hours for men.
The effects of fasting ketosis became a more popular and controversial subject about 15 years ago due to low-carb, high-protein dieters relying on it long-term to "burn the fat". Recent iterations of these "ketogenic diets" are fine-tuning fat to protein to carb ratios.
Where ketosis was once considered a "crisis response" of the body and fine only for short durations, there are some diet doctors who now contend ketones are an acceptable alternative fuel, produced and used by the body any time glucose is scarce, which can happen even in non-fasting, non-dieting individuals, such as during intense exercise or during sleep. They are considering it a natural metabolic process where ketone production and use fluctuates constantly in response to the body's needs.
What is so controversial about the dieters' use of ketosis is the long term, artificially produced, use of it. Over long periods of time, their high-protein diet produces excess protein by-products that become a strain on the kidneys to eliminate. Ketosis also creates a mild acidosis of the blood, which, over a long period of time is considered detrimental to our health. One effect being the leaching of minerals from our bones, causing osteoporosis.
Despite this controversy, ketogenic diets continue to grow in popularity. And the adherents would say that the above precautions have been disproved upon further study, or perhaps upon further tweaking of the protocols. Admittedly, new studies are indeed showing promising results in cases of cancer, diabetes, epilepsy, and Alzheimer's disease. No wonder, since it is creating the same bodily environment that the ancient and natural therapy of fasting has always created.
It is beyond the scope of this article to give an opinion on the use of ketogenic diets. A person should study the information available to determine its efficacy in their personal situation.
Unlike long-term dieting, fasting is a short-term endeavor. Even a 21 day fast is short compared to a year-long diet. Excess protein by-products are not being created. And ketosis is being entered by a naturally occurring process.
Also, in fasting, we listen for the body's signal to end the fast, to tell us when its reserves are too low to continue living off them.
Even during complete water fasting, which puts you in the fullest state of ketosis, there is some muscle loss. Dr. Fuhrman, who has fasted thousands, states that it decreases to less than 0.2 kg per day once full ketosis is reached, usually by the third day. There are other bodily/cellular processes that continue to require glucose and some tissue will be broken down to metabolize it.
Fasting methods that include some carbohydrates, like juice or fruit fasting, can produce various degrees of ketosis, and strive to provide enough carbs to prevent any muscle loss. Some juice fasters (as well as low-carbers) adhere to the theory that 400 calories (carbohydrates) will supply enough glucose to prevent muscle loss and aim for that number in their daily diet.
On the other hand, some proponents of water fasting contend the magic number is 1200 calories, and suggest that only complete abstention from food will produce the level of ketosis desired to safeguard muscle. What they don't tell you is that even in water fasting, there is still some muscle wasting.
As unique as each of our bodies and metabolic processes are, it's doubtful any such magic number actually exists.
Because of the inherent tissue loss in water fasting, Fuhrman suggests individuals of normal weight not fast longer than 21 days, or they "can become so thin that they have a long road back regaining their strength." But, we must remember, therapeutic fasting to treat chronic health issues such as Fuhrman was doing, while it might cause some muscle loss, is a trade-off for the improvement in the condition. We make worse trade-offs every day in traditional medicine--just read the warnings and side effects to prescription drugs or the release form you sign before surgery.
Even if we're not fasting for treatment of a serious chronic condition but for simple detox and general improved well-being, we should remember that part of the concept of detox is the breakdown of inferior materials and the rebuilding of new healthier cells. Any muscle lost to a fast isn't lost forever--it can be built back. Respecting the signs of "true hunger", and ending your fast when it appears, no matter whether you've attained the number of days you wanted or not, is your insurance against going too far.
As to the breakdown of tissue, know that any loss isn't arbitrary; the most expendable are used first. To quote Paavo Airola, a proponent of juice fasting:
This makes sense. We see it everywhere about us. Nature doesn't waste, but reuses.
After the initial few days of adaptation, some people experience ketosis as wonderful, with increased energy and loss of appetite. Usually the more overweight an individual, the higher their energy. Thinner people may need more rest than usual. Some fasters show concern that they feel "too good" for anything to be happening.
The loss of appetite serves us well during fasting for obvious reasons, but it also allows for the signal to end the fast to be heard. When hunger suddenly returns after being absent for perhaps days, it's the sign the body now requires additional fuel. It is time to break the fast.
If you are interested in testing yourself for ketones, there are urine testers (sticks) available over-the-counter that will determine if there are ketones present in your urine.
These testers are somewhat unreliable since they only measure excess ketones; if your body is under-producing the necessary amount of ketones, there won't be excess ketones thrown off in the urine, nor will they show in urine once the body has stabilized to the new fasting state and the proper amount of ketones are being produced and used up.
Blood tests are much more accurate since they don't rely on an excess being present, but obviously aren't practical as an at-home test.
Science hasn't yet been able to answer all the questions we have about the effects of fasting and the process of ketosis. But what we do know is that both ketosis and fasting are common and naturally occurring bodily processes. Remember, they have found ketone bodies in the blood of individuals who were under great physical exertion, as well as those who were sleeping.
In generally healthy individuals, the amount of possible muscle loss is considered negligible. But, if you're a 6 foot tall, 95 pound fashion model, this loss may not be so negligible. Nor if you're emaciated by a serious debilitating illness.
Physical bodily reserves are necessary to support the process of ketosis. Most of us have such reserves, but emaciated individuals shouldn't be fasting or, at least, only under the constant care of a physician. (See Who Can Fast.)
If the idea of any muscle loss is highly upsetting to you, if you're an athlete or a body builder, and you're not attempting to treat chronic conditions, you probably shouldn't consider a water fast. You can opt instead for a juice fast, Master Cleanse, or a mono-diet such as a fruit or rice fast. While we don't know the exact number of calories necessary to avoid all tissue loss, the modicum of nutrition in these fasting methods can assist your body in its fuel requirements.
If even these methods concern you, there are always cleansing diets which can provide a full compliment of nutrition, while still allowing the body to catch up on its housecleaning.
The beneficial physiological effects of fasting are well documented. It's been a therapeutic practice for at least 5,000 years that we know of. It doesn't try to manipulate nature, but works with her toward rest and healing. Ketosis is just one of the effects of fasting, all of which come together in leading us toward greater health and vitality.