Where does the weight *go*?

why does the gas gauge in your car go down?

It got burned for energy.

When you eat a calorie deficit your body starts taking the extra weight (fat and sometimes muscle) and turning it into energy.

Interesting fact: Calories are determined by how much they can heat water when they are lit on fire (there's that burned thing again). http://www.straightdope.com/columns/read/680/how-is-the-calorie-content-of-foods-determined

It's all one big energy balance. Scientific types call it thermodynamics.

When you're in a calorie deficit, your body has to go to its reserve (fat and to some extent, lean tissue) to get the ATP it needs for your cells to perform their vital functions (metabolism, respiratory, brain function, breathing, digestion, heart, cellular repair etc). The tissue is broken down into ATP which cells use for energy. Since mass can't be created or destroyed, just changed form, the extra energy is given off in the form of body heat and escapes into the atmosphere. (Someone feel free to fill in the gaps or correct me - I'm trying to remember biochemistry from 2 years ago.)

Thanks for replying. It makes sense now. I majored in physics so I understand the concept of matter being converted to energy, just didn't think to apply that thinking to weight loss. Though just saying "Adenosine triphosphate" is not that helpful, as I have no idea what that is.

That's ATP - what cells use for energy.