I've seen lots of older brains in the MRI, and it is not pretty. A lot of atrophy is probably due to cardiovascular-related disease and degeneration. Seeing enough of those brains finally got me off my chair and jogging. I feel great because of it too.
“Our data showed that, rather than actually increasing the size of the hippocampus per se, the main ‘brain benefits’ are due to aerobic exercise slowing down the deterioration in brain size. In other words, exercise can be seen as a maintenance program for the brain"
Though, the article also says:
"Overall, the results – published in the journal NeuroImage– showed that, while exercise had no effect on total hippocampal volume, it did significantly increase the size of the left region of the hippocampus in humans."
It's a meta review of other articles, with all the caveats such meta reviews come with.
"[T]his meta-analysis found no effects of exercise on total hippocampal volume, but did find that exercise interventions retained left hippocampal volume significantly more than control conditions."
Everyone is free to define normal for him or herself, but I'd normalize activity and say lack of exercise atrophies the brain.
Logically the same, but motivates me differently.
Granted, the article gives you more than a bumper sticker.
Work out more = eat more.
Overtime your metabolism goes up. Then you eat even more. Since energy can't be created or destroyed, you're technically just letting it pass through you –in and out. Since over time, the amount of energy going through your body increases, I'd argue that you gain more energy, both in and out through exercise.
I was taking poetic license with the notion that the equation for mass is almost entirely dependent on motion.  That's why I listed the equation first. Ah, well.