Fat tissue produces many hormonal substances called adipokines. Most of these substances are known to have harmful effects on health. However, fatty tissue can also provide benefits, mainly when practicing high-intensity physical exercises such as weight training. In this article, we will see the role that fatty tissue plays in the development of muscular strength.
Fat Promotes Muscle-Building
Fatty tissue emits various messengers, which are grouped under the generic term adipokines. One of the main adipokines is leptin, mainly known for its appetite-regulating effects, thanks to the feeling of satiety it confers. It also participates in the metabolism of glucose and lipids and is involved in immunity, reproduction, or the control of blood pressure.
It acts via receptors located on the surface of cells. Leptin also plays a crucial role in muscles, as evidenced by the abundant presence of its receptors there. Studies in animals allow us to understand better the function it fulfills there. There is a strain of mice in which the leptin-producing gene has been knocked out.
In these animals, deficiency of this compound is associated with lower muscle mass than usual. As part of a study carried out in Spain, researchers observed the effects of administering leptin to these animals, focusing on their muscles. They found that the intervention allowed the mice to gain muscle mass. They have uncovered some underlying mechanisms.
Anti-Catabolic Effect
Leptin blocked the action of a compound, FoxO3a. This is a transcription factor, a combination capable of governing the activity of specific genes. It activates two of them, in particular, MAFbx and MuRF1. These genes are responsible for the production of enzymes and ubiquitin ligases, which destroy muscle fibers. The administration of leptin thus made it possible to delay this phenomenon. Furthermore, the production of myostatin, a protein that opposes muscle growth, was decreased, while that of PGC-1α, which exerts beneficial effects on muscle, increased.
Anabolic Effect
Another team confirmed the impact of leptin on muscle mass and strength. Although she did not observe a protective effect of leptin against muscle catabolism as in the previous study, she highlighted its ability to activate muscle protein synthesis, which is commonly called “anabolism.”. These data echo previous work carried out on muscle stem cell cultures. They revealed the synergy between leptin and leucine, an amino acid with anabolic power, to regulate protein metabolism within muscles.
Control Of Muscle Contraction Force
Leptin is decisive not only for muscle mass but also for their functioning. Abnormalities in calcium flows at the level of muscle fibers were detected in mice deprived of the action of leptin, which caused premature fatigability. The work carried out by the American team revealed this type of anomaly in mice devoid of fatty tissue. In these animals, a drop in maximum contractile tension of approximately 20% was recorded in muscles containing a significant proportion of fast-twitch muscle fibers. These type II fibers are crucial in bodybuilding because they enable explosive and powerful movements.
When the researchers reintroduced fat to a portion of these animals, to the tune of approximately 10% of their mass, contractile abilities were thoroughly restored. They were able to prove that leptin secreted by adipocytes was responsible because this experiment carried out in animals that did not react to it proved unsuccessful. This work is entirely in line with observations that have always been made in the sporting field: when you lose a lot of fat (for example, in the context of cutting ), strength drops sharply, even without a change in lean muscle mass.
A Balance To Find
If the presence of fatty tissue appears crucial for muscle development and strength, its excess is, on the contrary, harmful. Being overweight leads to increased production of leptin. Analyzes have shown that its average blood concentration is just over 30 ng/ml in obese people, compared to 7.5 ng/ml in thin people. This influx of leptin causes the body to become desensitized to its action. Its receptors are produced in lower quantities, both in the brain and muscles. In these circumstances, it can no longer exert its muscular benefits optimally.
Furthermore, the increase in the volume of fats in the body promotes insulin resistance. This hormone, produced by the pancreas to regulate blood sugar, has favorable effects on muscle development. It stimulates protein synthesis and fights against their degradation. However, many weightlifters or “strong men” are overweight or even obese. This paradox could be explained by the fact that practicing intense physical activity protects against insulin resistance and leptin resistance. These athletes thus manage to obtain the benefits of high leptin levels without having all the disadvantages.
Fat Assists In Muscle Regeneration
Muscles have great regenerative capacities, which make it possible to repair damage caused by sports practice or an injury. They are based on the existence of stem cells, satellite cells located on the surface of muscle fibers. They can multiply and differentiate to replace damaged tissue. However, they do not work alone; they are notably assisted by another type of cells, fibro-adipogenic progenitors (FAP), also residents of the muscle. When intact, they are in a dormant state. In the event of muscle damage, they begin to increase and stimulate the multiplication of stem cells, which will rebuild the muscle.
Fat Is A Reserve Of Stem Cells
In this process, fatty tissue plays a significant role that has remained unknown for a long time. Work recently published in the journal Nature describes it. The researchers who conducted them started from a singular observation. In the event of an injury, the proliferation of FAP present within the muscle tissue only occurs three days after its occurrence.
However, cells of this type that are already activated are visible much earlier, within a few hours. So, where do they come from? Noting that fatty tissue located under the skin contains cells strongly resembling FAPs – a type of stem cell called adipose stromal cells or ASCs – they hypothesized. The subcutaneous fatty tissue would act as a reservoir of these cells with healing effects.
Migration Of Fatty Tissue Toward Muscle
Several experiments were conducted to test this hypothesis. One of them showed that when a muscle undergoes injury, the increase in the amount of FAP in the power goes hand in hand with the decrease in the amount of ASC in the subcutaneous fat. This first analysis argues in favor of a transfer of cells from one tissue to another. This migration was then confirmed using fluorescence labeling of the ASCs, which made it possible to visualize their infiltration within the injured muscle. It is, therefore, the fat that provides the power to the cells, which will facilitate the repair of damage in the early stages following the injury.
Protection Against Injuries
In this same study, researchers also showed that blood taken from athletes after training has a robust and attractive effect on these ASCs. It could be mediated by a messenger, GDF-15, which reflects the level of muscle fatigue. The ASCs of the fatty tissue would thus be mobilized as soon as the muscle begins to emit distress signals, with the aim of preserving its integrity and avoiding damage.
A Makeover For Aged Muscles
Fat tissue stem cells have a strong potential in terms of regeneration thanks to another mechanism. Like many cell types, they emit extracellular vesicles. These particles ensure the transfer of signal molecules (proteins, small RNA, and DNA) between cells. They arouse particular enthusiasm for regenerative medicine.
One study examined the effects of administering extracellular vesicles from adipose tissue stem cells taken from young mice to aged mice. Animals thus treated showed fewer signs of fragility than their peers of the same age. They performed better on tests assessing their physical abilities. The quality of their muscles was improved, with more numerous and thicker fibers and fewer fatty deposits. Levels of oxidative stress and inflammation were also reduced.