Science Puzzle
Where Does Fat Go When You Lose Weight?
Someone sticks to a diet and exercise plan and loses 10 kilograms of body fat. That fat was real stuff, made of atoms, and atoms can't simply vanish. Most people, including many doctors and personal trainers when they were asked in a survey, say the fat is "burned off" as energy.
Where does the mass of the lost fat actually go?
The Answer
You breathe most of it out. About 84% of the lost fat leaves through your lungs as carbon dioxide; the rest leaves as water.
Fat molecules are made of carbon, hydrogen and oxygen. When your body uses fat for energy, it combines those atoms with oxygen you breathe in, producing carbon dioxide and water. A 2014 study in the BMJ followed the atoms through the sums: to use up 10 kilograms of fat, you breathe in about 29 kilograms of oxygen and produce about 28 kilograms of carbon dioxide and 11 kilograms of water. Of the fat's own mass, about 8.4 kilograms leaves in your breath and about 1.6 kilograms as water, in urine, sweat, breath and tears.
Energy is released, which is the point, but it doesn't carry the mass away. Even the 377 million joules of energy stored in 10 kilograms of fat corresponds, through E = mc², to a mass of a few millionths of a gram. Fat doesn't turn into muscle either; they're different tissues made in different ways.
It's photosynthesis running in reverse. A tree builds its wood mostly out of carbon dioxide taken from the air; your body takes fat apart and hands the carbon back to the air, one breath at a time.
The principle: Conservation of mass in the body. When fat is used for energy, its atoms are rearranged into carbon dioxide and water, and most of its mass leaves the body through the lungs as exhaled carbon dioxide.