[gaius]

Not true because a) within reason, using any of the body's systems makes them stronger. That's the point of exercising. And b) effective exercise isn't about burning calories directly, it's about raising your resting metabolic rate.

[Alex3917]

I've heard it suggested though that the extra calories cause your cells to divide more frequently, which means you run out of telomeres faster. Not sure if that's really true or not though.

[stcredzero]

"using any of the body's systems makes them stronger"

Only partly true. This is true in the short term for younger people. It's not true in the long term for older people. Your metabolism results in different kinds of intracellular junk and intercellular junk accumulating. After a certain point, it reaches the level of pathology. But before that happens, using those metabolic pathways does tend to make them stronger.

[gizmo]

Not eating: bad - organs deteriorate. Overeating: bad - organs die too quickly.

Your liver grows stronger when you drink lots of alcohol, but that doesn't mean you're not killing it in the long term.

Same for muscles and joints. Consider somebody who works at a farm his entire life. Healthy when young. All joints break down at 45 because of the hard labor.