>you naturally lose muscle from using your leg muscles less by putting less weight on them, because you weigh less.
That is one cause of muscle loss, but there is a more important factor: when running a calorie deficit, the body removes protein from muscle so that it can turn the protein into glucose for energy. The human body preferentially gets fuel (calories to use to fight infections, digest food, and do the other things needed just to stay alive) from muscle (as opposed to getting it from fat) during times of not-enough-food because it takes more energy (fuel) to keep muscle cells alive than it takes to keep fat cells alive.
We evolved in an environment in which the body had no way to know whether the time of not-enough-food would last so long that it would lead to death. In that scenario, the policy that maximizes the probability of survival is to disassemble most of the muscles first and only then to start to disassemble the fat in the fat cells.
Corollary: most people on weight-loss diets should regularly exercise their muscles especially if they are elderly (because the elderly find it hard or impossible to regain the muscle once it is lost) and should consume a decent amount of protein.
Having a large amount of muscle mass doesn't make you live longer (as far as I know): the case for striving to preserve muscle mass during the aging process is that it gives a higher quality of life.
That is one cause of muscle loss, but there is a more important factor: when running a calorie deficit, the body removes protein from muscle so that it can turn the protein into glucose for energy. The human body preferentially gets fuel (calories to use to fight infections, digest food, and do the other things needed just to stay alive) from muscle (as opposed to getting it from fat) during times of not-enough-food because it takes more energy (fuel) to keep muscle cells alive than it takes to keep fat cells alive.
We evolved in an environment in which the body had no way to know whether the time of not-enough-food would last so long that it would lead to death. In that scenario, the policy that maximizes the probability of survival is to disassemble most of the muscles first and only then to start to disassemble the fat in the fat cells.
Corollary: most people on weight-loss diets should regularly exercise their muscles especially if they are elderly (because the elderly find it hard or impossible to regain the muscle once it is lost) and should consume a decent amount of protein.
Having a large amount of muscle mass doesn't make you live longer (as far as I know): the case for striving to preserve muscle mass during the aging process is that it gives a higher quality of life.