system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If our body is trying to conserve energy by losing muscle mass we're not using, why does being sedentary ultimately end up being more harmful?
gpt
Humans evolved in an environment where conserving energy was a definite survival advantage as food availability wasn't guaranteed - so the human body developed a 'use it or lose it' system.

But humans also evolved in an environment where survival involved constant physical activity whether that was hunting, searching, foraging, fleeing, etc.

So the 'use it or lose it' system never needed to develop a safety net, because humans tended to maintain a minimum level of physical activity, especially when food was scarce.

So these days, the 'use it or lose it' still functions, except people don't tend to starve if they're not active. So the musculoskeletal system - bones, tendons, ligaments, joints and muscles - will slowly degrade to the point where your skeleton will struggle to just support your own weight.

The cardiovascular system and your aerobic capacity undergo similar atrophy, and there's also the fact that people who live sedentary lifestyles rarely tend to adjust their calorie intake accordingly which leads to weight gain that exacerbates all the other issues.