Is it true that muscle turns into fat

"Quit sport and your muscle will turn to fat" is one of the most enduring myths in fitness. Many people have seen former athletes who gained weight after ending their careers and conclude there was a "transformation." The editorial team explains why this is physiologically impossible, what actually happens to the body after training stops, and how to avoid it.

Why muscle cannot become fat

Muscle and fat are different types of tissue, with different cells, origins, and functions. Skeletal muscle consists of multinucleated muscle fibers filled with the contractile proteins actin and myosin. Fat tissue consists of adipocytes — cells that store energy in the form of triglycerides.

One mature cell cannot turn into another mature cell of a different tissue, just as bone does not become skin. A muscle fiber can decrease in volume (atrophy) or increase (hypertrophy), but it cannot be "reworked" into a fat cell.

Chemically, muscle is mostly water and protein, while fat is triglycerides. Proteins can break down into amino acids, some of which are used to obtain energy or glucose, but the conversion of protein into fat in the human body is an extremely inefficient pathway and has no practical significance with ordinary eating.

So the phrase "muscle turned into fat" describes not a process but the visible result of two independent changes: a decrease in muscle mass and a simultaneous increase in fat. Together they give the impression of a "transformation," especially if body weight has barely changed.

What happens after training stops

After you stop training, the body gradually reduces the "expenditure" on what is no longer used. Endurance declines first: VO2max and the efficiency of aerobic metabolism begin to fall as early as 2–4 weeks. The reviews by Mujika and Padilla describe these changes as detraining — a partial or complete loss of training adaptations.

Strength and muscle mass are preserved longer. According to studies, in trained people strength barely changes during the first few weeks of a break, and noticeable atrophy develops gradually. Part of the initial "deflation" of the muscles in the first days is related to a smaller store of glycogen and water, not to a loss of protein.

The rate of loss depends on age, training level, nutrition, and whether the person remains generally active. In older people atrophy proceeds faster, and complete immobilization (a cast, bed rest) sharply accelerates muscle loss — even in the young, a noticeable portion of the leg's muscle mass is lost in one to two weeks.

Important: even after a long break, the level of muscle mass usually remains higher than in a person who has never trained. Structural changes, in particular an increased number of nuclei in the fibers, can persist for a long time.

Where "getting flabby with fat" comes from

The main cause of fat gain after training stops is energy balance. A person is used to eating a lot to support training and muscle growth, and after quitting sport eating habits often stay the same. Energy expenditure falls, intake does not, and the excess is deposited as fat tissue.

Expenditure decreases for several reasons: there is no energy spent on the workouts themselves, spontaneous physical activity declines, and the loss of some muscle mass slightly reduces resting expenditure. The contribution of muscle to basal metabolism is often overestimated, but combined with the absence of training the difference becomes noticeable.

In professional athletes the situation is complicated by a change of routine, stress after ending a career, and sometimes injuries that limit movement. This is exactly why former athletes are the best-known "proof" of the myth, although physiologically they demonstrate an ordinary calorie surplus against a background of reduced activity.

A separate case is people who built mass using anabolic steroids. After stopping the drugs and against a background of suppressed natural testosterone, the loss of muscle and gain of fat can be faster. But here too the fat comes from an energy surplus, not from "reworked" muscle.

Muscle memory: how quickly does fitness return

The good news: restoring lost fitness is much easier than gaining it for the first time. The phenomenon of "muscle memory" has several explanations. The first is neuromuscular: the skill of performing exercises and the coordination are retained for a long time, so strength returns quickly.

The second is cellular. In animal experiments (Bruusgaard et al., 2010) it was shown that the nuclei muscle fibers receive from satellite cells during hypertrophy are retained even after atrophy. These "extra" nuclei can speed up repeated growth. In humans the data are less clear-cut, but the concept is being actively studied.

The third is epigenetic. A study by Seaborne and colleagues (2018) in humans showed that after a period of strength training, a break, and repeated training, the muscles responded more strongly to the second cycle, and certain methylation changes associated with the earlier training were retained in the DNA.

In practice this means: even after several months of a break, you should not assume that all your efforts are lost. A return to training with a gradual increase in load usually quickly restores a significant part of your previous fitness.

How to reduce losses during a break

If the break is forced or planned, a few simple steps will help preserve the result. A minimal workout is better than none: studies show that maintaining strength and mass requires a much smaller volume than developing them — sometimes one or two short workouts a week at sufficient intensity.

• adjust calorie intake to match the new activity level to avoid a surplus;
• keep protein intake high — roughly 1.2–1.6 g/kg per day to preserve muscle;
• keep up daily movement — walking, everyday activity, strolls;
• in case of injury, train the uninjured parts of the body if your doctor does not object;
• plan your return with a gradual increase in volume.

It is useful to track not only weight but also waist circumference and photos: this makes it easier to notice a trend toward fat gain in time, before the changes become significant.

For older people, maintaining activity is especially important, because sarcopenia — the age-related loss of muscle mass — accelerates with inactivity. Even a short break due to illness can have noticeable consequences, so activity should be resumed as early as possible with a doctor's permission.

Editorial conclusions

Muscle cannot turn into fat: they are different tissues. After training stops, two separate processes occur — muscle atrophy and, with a calorie surplus, fat accumulation.

The main culprit of "getting flabby" is eating that was not adjusted to the new activity level. Controlling calorie intake, protein, and a minimum of physical activity can significantly soften the consequences of a break.

Thanks to muscle memory, the return of fitness usually happens faster than gaining it the first time.

We also recommend: "Is it harmful to train every day," "Should you drink protein without training," and "Obesity and estrogen in men."

References

1. Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part I: short term insufficient training stimulus. Sports Med. 2000;30(2):79–87.
2. Mujika I, Padilla S. Detraining: loss of training-induced physiological and performance adaptations. Part II: long term insufficient training stimulus. Sports Med. 2000;30(3):145–154.
3. Bruusgaard JC, Johansen IB, Egner IM, et al. Myonuclei acquired by overload exercise precede hypertrophy and are not lost on detraining. Proc Natl Acad Sci USA. 2010;107(34):15111–15116.
4. Seaborne RA, Strauss J, Cocks M, et al. Human skeletal muscle possesses an epigenetic memory of hypertrophy. Sci Rep. 2018;8(1):1898.
5. Bickel CS, Cross JM, Bamman MM. Exercise dosing to retain resistance training adaptations in young and older adults. Med Sci Sports Exerc. 2011;43(7):1177–1187.
6. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition position stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20.