The internet is full of weight loss methods ― from eating cinnamon to fibermaxxing. And the rise in GLP-1s like Ozempic has helped fuel a weight loss craze, with approximately 1 in 8 American adults currently taking a GLP-1 medication.
Weight is not always a direct indicator of health, despite what our ever-present diet culture would lead you to believe. That’s why it’s vital to talk to your doctor if you’re going on any type of weight loss journey ― and it could also be useful to find out the science behind what’s happening in your body when you’re on one.
No matter how you lose weight, it’s usually not clear where weight goes once it’s shed. There’s some misinformation about what happens, including claims that fat turns into muscle, leaves our bodies as urine, or that fat cells disappear as the number on the scale decreases.
“Many myths surround where fat goes after we lose weight. But it’s a way simpler answer than people think,” said Dr. Katie Buckner, a family medicine and obesity medicine physician with Kaiser Permanente Southern California.
HuffPost talked to doctors to help explain where weight actually goes once it’s no longer a part of our bodies.
Your body eliminates weight through bodily functions like breathing and, yes, sometimes peeing.
Most of us know that weight loss occurs when our bodies use more calories through everyday movement and exercise vs what we consume through food and beverages.
To make up the difference between the calories we consume and our energy output, our bodies tap into stored energy reserves, explained Dr. Adrian Dobrowolsky, director of the Bariatric Weight Loss Program at Hoag, a healthcare system in Southern California. These energy reserves are primarily glycogen, stored carbohydrates and body fat, he said.
However, “over time, fat becomes the primary fuel source,” Dobrowolsky explained. “As fat is broken down, it undergoes a series of chemical reactions.”
The byproducts of that conversion are carbon dioxide and water, which are then eliminated from the body through normal bodily functions.
Where weight goes when we lose it is the same, no matter how we create a calorie deficit. For example, “many people assume that exercise directly ‘melts’ fat,” Dobrowolsky said. “In reality, exercise increases the body’s energy demands, which can help create the calorie deficit needed for fat loss. The actual process of eliminating fat still occurs through metabolism, resulting in the subsequent release of carbon dioxide and water.”
Even doctors didn’t fully understand this process until about a decade ago. It wasn’t until Australian researchers published a study about weight loss in 2014 that we had a definitive answer to this question. Before this study, most doctors thought that fat left the body by being converted into energy or heat, turned into feces or changed into muscle ― none of which is correct.
What actually happens is that approximately 84% of fat lost is converted into carbon dioxide, which is exhaled through the lungs.
“Many people are surprised to learn that the lungs are the body’s primary organ for fat loss” and that we lose most fat by breathing it out, Dobrowolsky said.
Although most fat is lost through respiration, about 16% is turned into water. As we lose weight, we primarily excrete this water through sweat and urine.
“Sweat contributes to the elimination of some of the water produced during fat metabolism,” Dobrowolsky said.
He noted, however, that losing weight by turning fat into a small amount of sweat is different from losing weight by purposefully inducing sweat.
“A person can lose several pounds through sweating during a workout or sauna session, but most of that weight is water and returns when fluids are replaced,” which is a completely different process from fat loss.
Additionally, some of the water our bodies produce during weight loss is excreted in urine. Dobrowolsky clarified that “the fat itself is not converted directly into urine. Urination is simply one route by which the body removes water generated during weight loss.”
There’s a big reason why understanding this process matters.
Understanding the science behind weight loss can help people focus on what actually works and avoid gimmicks if they need to lose weight for health reasons.
Dobrowolsky hopes that by gaining a deeper understanding of where weight goes when we lose it, his patients will focus on building habits that promote sustainable weight loss and healthier behaviors. This mindset “shifts the focus away from detoxes, cleanses and other quick fixes toward sustainable habits such as healthy eating, physical activity, adequate sleep and stress management,” he said.