system
Given the following post, respond as if you're commenting on Reddit:
human
How does weight loss actually work?

Specifically, the idea of "if calories in > calories out, weight gained. If calories in < calories out, weight lost." Is this to say that if I ate something, say a Greek yogurt that was 340 calories, would I need to run 2 miles (assuming 1 mile=170 calories lost) just to maintain my weight? Why is it that doctors suggest that somebody who lives an inactive lifestyle still consumes ~1500 calories per day if calories in then obviously is not less than or equal to calories out?
gpt
Not taking into account losses due to incomplete digestion and all other funky biological stuff, those 1500 calories per day are used up running your body. Moving muscles in the heart and lungs requires energy, keeping your eyes open, raising your temperature when you are cold, all these things don't just happen, they need energy input. In fact, the brain takes a very large amount of energy to keep running, controlling your body, analyzing sensory information, thinking about how many calories you ate today; all that work uses up a lot of energy. This is where the majority of your food intake energy goes. You can even notice how little energy is used up by mechanical processes like running and lifting weights compared to your basal metabolic rate (energy use when not doing physical activities).