Why do we lose lean muscle when on a deficit?

Lost about 20 pounds and not one of it muscle.
It's all in how you do it.

My trick is high protein, and high volume weightlifting.

To boil it down...

Muscle and fat both get burned. If you go too fast (read reduce too many calories) then more muscle gets burned up. If you do not lift muscle gets "chewed" up and your metabolism suffers as your body thinks it is starving. Thousands of years of evolution made us this way for obvious feast or famish style diet humans have had.

That's the basics in laymen's terms.

All metabolic processes are constantly running, but the equilibrium between the forward and reverse reactions shifts. For example, when you haven't eaten for several hours, the tendency is to break down fatty acids to get energy. However, because of the chemistry and the way we evolved, there is still a tiny bit of fatty acid synthesis going on, though the net result is fatty acid breakdown. The reactions for breaking down and building up muscle are also running at the same time, but eating in a deficit favors the breakdown reactions. The larger the deficit, the more you favor breakdown. You preferentially break down fatty acids, but because our bodies can't (reasonably) stop the muscle breakdown reactions (it would be very bad if we did anyway), there's still a little bit that has to happen when eating in a deficit. (Source: I just took a class on the biochemistry of metabolism)

RunRutheeRun wrote: »

Its virtually impossible not to lose muscle along with the fat, too agressive a deficit will really see the muscle being lost as well, so slower loss will mean less muscle loss. Protein does help as does resistance training with keeping muscle loss to a minimum.

It is not impossible to mitigate muscle loss depending on the deficit size, amount of available body-fat, and training experience of the individual.