When You Lose Weight, Where Does the Fat Go?

Ben Voldman
Ben Voldman / Ben Voldman

Into thin air! While ads for diets, pills, and exercise equipment promise to “burn fat away,” the truth is that our bodies don’t convert fat into energy. That would break the Law of Conservation of Mass. When you lose weight, you’re really just losing atoms—you can’t turn those atoms into energy. Scientists have known this for a long time, but in 2013, Australian physicist Ruben Meerman was waging his own battle of the bulge and wanted to know how much fat we exhale. Meerman did the math and found that for every 10 pounds of fat you lose, 8.4 pounds are exhaled as CO2, while the remaining 1.6 pounds are converted to water, which the body excretes as tears, sweat, urine, or...other stuff.