the laws of thermodynamics

Most people go wrong by misinterpreting the laws of thermodynamics. The Law of Thermodynamics proves that: To build muscle, you must store energy or increase muscle protein. To lose fat, you must burn energy or reduce stored body fat. The mistake? To assume that: You must be in an energy surplus to build muscle. You must be in an energy deficit to lose fat.

2021 September 6, Tom MacCormick, “The Body Recomposition Handbook”, in T-Nation

These results indicate that people with relatively low and relatively high body-fat can gain fat-free mass to a pretty similar degree in response to resistance training. However, very lean people should accept that fat mass will probably need to increase a little bit to facilitate muscle gain, and they may need to get up to a slightly more comfortable body-fat level before lean mass gains really start to accumulate (for example, every single person under 8% body-fat at baseline had some degree of fat gain, and only one of them gained more than 1kg of fat-free mass). In contrast, people with higher baseline body-fat have a greater capability to “recomp” and are more likely to simultaneously lose a little bit of fat while gaining muscle. It’s important to recognize that these results do not conflict with the laws of thermodynamics or the concept of energy balance dictating weight changes; they simply reflect the fact that the body has both short-term and long-term sensors of energy availability which can influence the regulation of anabolic processes, and that muscle hypertrophy and fat storage are influenced by some distinct regulatory mechanisms.

2022 May 30, Eric Trexler, “Should You Cut Before You Bulk?: How Body-Fat Levels Affect Your P-Ratio”, in Stronger By Science