When you lose weight, where does it go?

“The pounds you shed are essentially the byproducts of that process. They are excreted in the form of water—when you sweat and pee—and carbon dioxide, when you exhale. In fact, the lungs are the primary excretory organ for fat.”

The linked-to article (https://edition.cnn.com/2018/03/26/health/lose-weight-where-...) is way clearer:

“The correct answer is that fat is converted to carbon dioxide and water. You exhale the carbon dioxide and the water mixes into your circulation until it's lost as urine or sweat.

If you lose 10 pounds of fat, precisely 8.4 pounds comes out through your lungs and the remaining 1.6 pounds turns into water. In other words, nearly all the weight we lose is exhaled.”