Here’s What Happens To Fat You “Burn” During Exercise

Photographed by Molly Cranna.
If you managed to snag a treadmill at the gym this week, first of all: congrats! Second, you're probably wondering what all that exercise really amounts to. You know working out boosts your mood and overall health, but what does it really do to you physically? And if you end up losing weight, what happens to the fat you "burn" away? According to a 2014 study in the BMJ's sort-of jokey annual Christmas issue, the vast majority of us get the answer to this question wrong. "Most people believed that fat is converted to energy or heat, which violates the law of conservation of mass," the authors write. "We suspect this misconception is caused by the 'energy in/energy out' mantra and the focus on energy production in university biochemistry courses." So what's really going on, then? Well, we've gotta go through a (very quick, I promise) biochemistry lesson here first. The number of fat cells you have stays pretty much constant through your adult life, which means you're not losing any of them when you work out. Instead, those cells get bigger or smaller depending on a bunch of different factors. And when you're "burning fat," you're really getting rid of triglycerides (chains of fatty acids) that are stored in — and enlarge — your fat cells. Triglycerides are made up of carbon, hydrogen, and oxygen. When you exercise, your body calls upon and breaks down these stores of triglycerides for energy, leaving you with a bunch of CO2 and a smaller amount of H2O as byproducts of that process. That means that, for the most part, you breathe out the remnants of those triglycerides in the form of the carbon dioxide you exhale. The rest of 'em, about 16%, are excreted as water via your sweat, tears, urine, or other bodily fluids. So, yes, you absolutely can "sweat it out." But in this week full of soreness, "crying it out" is probably just as likely.

More from Fitness

R29 Original Series