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What Happens to the Fat When I Lose Weight?
By: Scottsdale Weight Loss Center
Published: September 17, 2025
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A common question I hear in clinic is: “When I lose weight, where does the fat actually go?”
Some common misperceptions include:
- Fat turns into For the chemistry enthusiasts out there, this would violate the law of conservation of mass.
- Fat turns into muscle. This would be ideal, but unfortunately it’s
- Fat leaves the body in bowel movements. Not true
Don’t worry, many health professionals are surprised by the real answer as well.
So where does the fat go?
The truth is that fat is converted to carbon dioxide and water. You exhale the carbon dioxide through your lungs, while the water enters your bloodstream and is eliminated as urine or sweat.
Here’s a surprising fact: if you lose 20 pounds of fat, 16.8 pounds leaves your body as carbon dioxide and 3.2 pounds leaves as water. This means nearly all of the weight you lose is literally breathed out. Amazing!
This process isn’t unique to fat. Nearly all carbohydrates, alcohol, and dietary fat you consume are ultimately broken down the same way, into carbon dioxide and water. Protein is similar, with the exception of a small amount of nitrogen containing components that are excreted in urine. The only real exception is fiber, which passes through undigested and leaves the body as stool.
One of my patients once asked me: “If fat turns into carbon dioxide, does that mean losing weight contributes to global warming?”
No, it does not.
Be well.
Nick
Nick's Story
"I wish I would have come here first. We spend all our time reacting to the next great weight loss tip or program. It might work for a while, but does not last and we end up yo-yoing from one fad to the next – as we gain more weight." Read More
Gina
Lost 44.5 lbs
"I had no trouble adhering to the meals and they really helped catapult my progress! The classes were so beneficial to me because they helped change my thinking about how I viewed health, nutrition, and fitness.
I will always have my relationship with Scottsdale Weight Loss Center. They truly helped me change my life forever." Read More
Lauren
Lost 60 lbs
"Dr. Ziltzer and everyone who I have come in contact with at the Scottsdale Weight Loss office have been so encouraging and I look forward to seeing them each week. It is now a year and 2 months later since I started. I am down 60 lbs., which is the average amount a 9-year-old weighs, it is crazy to think I lost a whole person! I feel amazing, I hit my goal weight in November 2021 just before Thanksgiving. Nothing on my body hurts anymore..." Read More
Chuck
Lost 66 lbs
"The best thing I did was call the Scottsdale Weight Loss Center. I have a lot more energy, I'm not hungry, and it feels great.
You can take it from me that this is a program that produces results. The staff is always positive, supportive, and never judgmental.
Your visits always result in positive reinforcement, no matter if you had a good week or a bad week." Read More
Emily
Lost 101 lbs
"Here I am at the lowest I have been since high school weighing in at 155lbs. I have lost 101 lbs, BMI of 39 when I started, to 21.6, and of course at body fat % of 47.1 to 26.5%. I told myself by my 30th birthday I wanted to be confident in the way I look and I owe it all to SWLC. The entire staff is so welcoming and they are rooting for you just as much as you are rooting for yourself...." Read More
Reviews
Meet Our Team
Robert Ziltzer, MD, FACP, FAAP, FOMA
Obesity Medicine Certified Physician
Diana Bone, FNP-C
Specializing in Obesity Medicine
Craig Primack, MD, FACP, FAAP, MFOMA
Obesity Medicine Certified Physician
Kristen Ash, FNP-C, FNP-BC
Nurse Practitioner
Alicia Earley, FNP-C
Family Nurse Practitioner
