Where does fat go when you lose weight?

Someone standing on a scale with hand weights nearby
Where does fat go when you lose weight? Does it turn into muscle? We bust some common myths about where fat goes when it leaves your body (Image credit: Westend61)

Where does fat go when you lose weight? When you’re putting a lot of miles in your new road running shoes or hiking boots, does it transform into heat energy the way your high school biology showed you using a bunsen burner and a potato chip? When you’re lifting weights, does it turn into muscle? We’ve written about both running and hiking for weight loss and explained how both activities might be helpful for those goals, but until now we’ve never actually answered the question – where does fat go when it goes away? 

Despite lots of years working in the fitness industry, this question didn’t come up for me until a few years back when I was debating the potential merits of hot yoga with a fellow yoga teacher. A fan of the 98 degree room, she proposed that it was great for both “sweating out toxins” and “burning fat.” I was studying for a master’s degree in Integrative Medicine at the time and reasonably certain that the only thing you sweat out doing downward dog in Death Valley-like conditions is water and salt, but I realized I had no idea where fat goes when it goes away. Can you sweat it out? Being curious (and wanting to win an argument) I did a little research, and the answer amazed me. 

Woman wiping sweat from forehead

Can you release metabolized fat through sweat? (Image credit: Erik Isakson)

Can you turn fat into muscle?

Before we get into the journey that a fat cell takes when it leaves your body, let’s start with a commonly held myth: when you’re working out, you turn fat into muscle. It’s something you often hear when you get on the scale after working out like crazy for three months only to discover you’ve gained three pounds. 

“Oh, that’s just because you’ve turned all that fat into muscle,” remarks your personal trainer, helpfully.

It sounds good, but you cannot turn fat into muscle, according to Live Science. They are totally different tissues; muscle is made from blocks of proteins called myofibrils, and fat is composed of carbon, hydrogen and oxygen. One can never become the other. 

You can, however, burn fat and build muscle at the same time, which might, somewhat annoyingly, leave you weighing more than when you started. However, with a healthy amount of muscle tissue you’ll also be better protected against injuries, improve your balance, posture and strengthen your bones, all of which will become more important as you age. All good reasons to put the scale away and try some resistance band workouts.

Should runners lift weights

You cannot turn fat into muscle, or muscle into fat (Image credit: Getty)

Where does fat go when you lose weight?

Back to the question at hand – fortunately for me, a couple of years before my friendly debate on this topic, in 2014, a couple of researchers had published a paper in the British Medical Journal explaining where fat goes when we lose it. In it, they reveal the results of a survey that demonstrates why we’re all so confused about this – nearly 70% of doctors and dieticians and half of all personal trainers believe that fat is transformed into energy that we then utilize to get down the trail – and they’re all wrong.

The key to understanding it actually lies in the components of fat: carbon, hydrogen and oxygen. When it’s broken down into these components, it can get released by the body by processes that expel those components, and so it turns out my hot yoga-loving friend was a little bit right. Some metabolized fat (triglycerides) does get released from your body via fluids such as urine, feces, sweat, breath, tears, or other bodily fluids, but only to the tune of about 16%. The remaining 84% is released from your body through your breath as carbon dioxide, making your lungs the primary organ involved in weight loss. 

This helps us to understand why the lower heart rate zones (zones two and three) we achieve in lower intensity activities like jogging and hiking are most associated with weight loss, where you’re still able to breathe and get oxygen into your body to oxidize fat. Once you pick up the pace in interval training or other high intensity workouts, your body bypasses the fat storage and switches to carbohydrates for energy instead. 

A female jogging for exercise

84% of metabolized fat is released from your body through your breath as carbon dioxide (Image credit: Jordan Siemens)

So can you just breathe your way thin? Well the authors of the paper are quick to point out that while aerobic training is associated with weight loss, your goals can be easily foiled by relatively small quantities of excess food, so any weight loss plan based around exercise always needs to be undertaken with attention to diet. But with that in mind, picking up the pace and breathing just a little bit heavier is a good starting point.

And there you have it. Most of the fat you burn is released by your exhales. If you want to breathe heavily in hot yoga to get there, you can, but I’m going to stick to room temperature yoga, trail running and hiking trails.

Julia Clarke

Julia Clarke is a staff writer for Advnture.com and the author of the book Restorative Yoga for Beginners. She loves to explore mountains on foot, bike, skis and belay and then recover on the the yoga mat. Julia graduated with a degree in journalism in 2004 and spent eight years working as a radio presenter in Kansas City, Vermont, Boston and New York City before discovering the joys of the Rocky Mountains. She then detoured west to Colorado and enjoyed 11 years teaching yoga in Vail before returning to her hometown of Glasgow, Scotland in 2020 to focus on family and writing.