[q-rews]

> your body still used up 1800 calories

If you don't eat enough, you'll feel lethargic and unable to perform normally, so I doubt it will still use 1800 calories. According to the article, if you eat 800 calories and your body stores them all, you'll feel as hungry as if you never ate at all.

Of course this likely isn't so drastic ("50%") but it would explain why many people keep gaining weight over their lifetime (or until the body reaches a balance).