Can the benefits of intermittent fasting be achieved without muscle loss? The answer is “yes”, to the best of my knowledge.
Even if you are not interested in bulking up or becoming a bodybuilder, you probably want to keep the muscle tissue you have. As a norm, muscle takes a long time, and effort, to build. It is generally easier to lose muscle than it is to gain it. Fat, on the other hand, can be gained very easily.
Body fat percentage is positively correlated with measures of inflammation markers and the occurrence of various health problems. Since muscle tissue makes up lean body mass, which excludes fat, it is by definition negatively correlated with inflammation markers and health problems.
As muscle mass increases, so does health; as long as the increase in muscle mass is “natural” – i.e., not caused by things like steroids, for instance.
In short-term fasts (e.g., up to 24 h) one can indeed lose some muscle as the body produces glucose using muscle tissue through a process known as gluconeogenesis. In this sense, muscle is the body’s main reserve of glucose. Adipocytes are the body’s main reserves of fat.
Muscle loss is not pronounced in short-term fasts though. It occurs after the body’s glycogen reserves, particularly those in the liver, are significantly depleted. This often happens 8 to 12 hours into the fast, depending on how depleted the glycogen reserves are when one starts fasting.
When the body is running short on glycogen, it becomes increasingly reliant on fat as a source of energy, sparing muscle tissue. That is, it burns fat, often in the form of ketone bodies, which are byproducts of fat metabolism. This state is known as ketosis. There is evidence that ketosis is a more efficient state from a metabolic perspective (Taubes, 2007, provides a good summary), which may be why many people feel an increase in energy when they fast.
The brain also runs on fat (through ketone byproducts) while in ketosis, although it still needs some glucose to function properly. That is primarily where muscle tissue comes into the picture, to provide the glucose that the brain needs to function. While glucose can also be made from fat, more specifically a lipid component called glycerol, this usually happens only during very prolonged fasting and starvation.
You do not have to consume carbohydrates at all to make up for the glycogen depletion, after you break the fast. Dietary protein will do the job, as it is used in gluconeogenesis as well.
Dietary protein also leads to an insulin response, which is comparable to that elicited by glucose. The difference is that protein also leads to other hormonal responses that have a counterbalancing effect to insulin, by allowing for the body's use of fat as a source of energy. Insulin, by itself, promotes fat deposition and prevents fat release at the same time.
When practicing intermittent fasting, one can increase protein synthesis by doing resistance exercise (weight training, HIT), which tips the scale toward muscle growth, and away from muscle catabolism.
This may actually lead to significant muscle gain in the long term. Fasting itself promotes the secretion of hormones (e.g., growth hormone) that have anabolic effects.
The following sites focus on muscle gain through intermittent fasting; the bloggers are living proof that it works.
Muscle catabolism happens all the time, even in the absence of fasting. As with many tissues in the body (e.g., bones), muscle is continuously synthesized and degraded. Muscle tissue grows when that balance is tipped toward synthesis, and is lost otherwise.
Muscle will atrophy (i.e., be degraded) if not used, even if you are not fasting. In fact, you can eat a lot of protein and carbohydrates and still lose muscle. Just note what happens when an arm or a leg is immobilized in a cast for a long period of time.
Short-term fasting is healthy, probably because it happened frequently enough among our hominid ancestors to lead to selective pressures for metabolic and physiological solutions. Consequently, our body is designed to function well while fasting, and triggering those mechanisms correctly may promote overall health.
The relationship between fasting and health likely follows a nonlinear pattern, possibly an inverted U-curve pattern. It brings about benefits up until a point, after which some negative effects ensue.
Long-term fasting may cause severe heart problems, and eventually death, as the heart muscle is used by the body to produce glucose. Here the brain has precedence over the heart, so to speak.
Voluntary, and in some cases forced, short-term fasting was likely very common among our Stone Age ancestors; and consumption of large amounts of high glycemic index carbohydrates very uncommon (Boaz & Almquist, 2001).
Boaz, N.T., & Almquist, A.J. (2001). Biological anthropology: A synthetic approach to human evolution.
Taubes, G. (2007). Good calories, bad calories: Challenging the conventional wisdom on diet, weight control, and disease.