The vast majority of doctors, dietitians, and personal trainers believe that when you burn fat during exercise, that fat is being used up as fuel for energy or heat. Some believe it’s excreted through urine or feces, while others think the fat is turned into muscle.
All of these ideas are to some degree incorrect, according to Ruben Meerman, a physicist, and Andrew Brown, a biochemist specializing in lipids, who say there’s "surprising ignorance and confusion about the metabolic process of weight loss."
When You Lose Weight, Where Does the Fat Go?
“Excess dietary carbohydrates and protein are converted to a type of fat called triglyceride. When people attempt to lose weight, they are attempting to metabolize these triglycerides while keeping their fat-free mass intact…
Triglycerides are comprised of three types of atoms: carbon, hydrogen and oxygen. Triglyceride molecules can be broken down only by unlocking these atoms, through a process known as oxidation.
The researchers chose to follow the path of these atoms when leaving the body. They found that when 10 kg of fat were oxidized, 8.4 kg were converted and excreted as carbon dioxide (CO2) via the lungs, and 1.6 kg became water (H20).
In order for 10 kg of human fat to be oxidized, the researchers calculated that 29 kg of oxygen must be inhaled. Oxidation then produces a total of 28 kg of CO2 and 11 kg of H20.”
The researchers note that this is not new to science—rather the process has simply been misunderstood. The equation does involve release of energy; it’s just that the process isn’t as direct as one might think. According to the law of conservation of mass, it’s actually quite difficult to convert matter into energy.
As noted by The Atlantic:4 “If you were able to convert your fat stores [directly] into energy, you would explode in a glorious, catastrophic spectacle…” According to their calculations, you basically exhale 84 percent of your lost fat. The remaining 16 percent is metabolized into water, which is excreted through sweat and urine.
The authors estimate that by substituting one hour of sedentary lounging with one hour of moderate exercise—to increase your respiratory rate—your metabolic rate is increased sevenfold. However, they note that you can easily hamper any potential weight loss by eating too much food—and I would stress, by eating the wrong kinds of foods.