# When you lose fat, where does it go?

> Burned body fat leaves mostly through the lungs, as carbon dioxide, and the rest as water. Exercise helps, and weight responds slowly to a change in eating.

- Canonical: https://kilocare.in/learn/where-fat-goes
- Publisher: KiloCare (https://kilocare.in)
- Written by: KiloCare Team
- Last updated: 2026-10-29
- Citations: numbers in square brackets refer to Sources at the end; a page or table follows the number where given.

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**In short**

- When the body burns fat, most of its mass leaves through your lungs as carbon dioxide. The rest leaves as water.
- Sweating lowers the scale because you lose water, not fat. Drinking replaces the water, so the scale goes back up.
- In a modelling study, weight responded slowly to a lasting change in eating, over months to years.

## How fat is stored

**Body fat is stored inside fat cells, as a type of fat called triglyceride.**

- Fat cells store extra fat from food as triglyceride. Extra carbohydrate and protein can also be turned into triglyceride and stored there.[1, Weight we want to 'lose']
- To lose body fat, the body has to take that triglyceride out of the fat cells and burn it.[1, Weight we want to 'lose']

## Where it leaves the body

**By the chemistry of burning fat, most of its mass leaves as carbon dioxide that you breathe out, and the rest leaves as water.**

- The authors of a 2014 paper in the BMJ asked doctors, dietitians and personal trainers where fat goes when a person loses weight. Many said it turns into energy or heat. Others said it leaves in the stool, or turns into muscle.[1, Opening paragraphs]
- Burning fat does release energy. The fat's mass cannot vanish, so it has to leave the body as something else.[1, Opening paragraphs]
- Using chemistry, the authors worked out that fat burned with oxygen becomes carbon dioxide and water.[1, Weight we want to 'lose']

**Where burned body fat goes, by mass**

- Fat is stored as triglyceride in fat cells.[1, Weight we want to 'lose']
- The body breaks it down and burns it with oxygen that you breathe in.[1, Weight we want to 'lose']
- About 84% of the fat's mass leaves as carbon dioxide, which you breathe out.[1, Novel calculation]
- About 16% leaves as water, in urine, sweat, breath and other body fluids.[1, Novel calculation]

The paper worked out those shares from the chemistry of fat. It did not measure them in people. The
shares led the authors to call the lungs the main way that fat leaves the body.[1, Conclusion]

When your muscles work harder, as they do when you climb stairs, they burn more fuel, and you breathe
out more carbon, as carbon dioxide.[1, Lifting the veil on weight loss]

## Sweat is water

**Sweating lowers the scale because you lose water, not fat. Drinking replaces the water, so the scale goes back up.**

- Sweat is mostly water and salt, and many other substances in varying amounts.[2, Overview of sweat composition]
- Sports scientists estimate how much a person sweats by weighing them before and after exercise.[3, Abstract] The weight lost is fluid, so sweating more does not mean you burn more fat.
- Some of the water made from burning fat does leave the body in sweat. It also leaves in urine, breath and tears.[1, Novel calculation]

## Exercise, food and the scale

**In adults with overweight or obesity, exercise alone led to small weight losses, and exercise with a change in diet led to more.**

The BMJ paper's sums use an average adult of 70 kg.[1, Lifting the veil on weight loss]

- In its example, jogging for 1 hour in place of rest raises the carbon that adult breathes out in a day by about 20%.[1, Lifting the veil on weight loss]
- For comparison, the paper says a single 100 g muffin gives about 20% of an average person's daily energy need.[1, Lifting the veil on weight loss]
- The BMJ authors still back the familiar advice to eat less and move more.[1, Lifting the veil on weight loss]

In a 2011 modelling study in the Lancet, a modest rise in activity predicted slightly more weight loss than an equal cut in food, calorie for calorie. For larger changes, eating less predicted more weight loss.[4, Modelling of the dynamics of weight change] The Lancet authors add that more activity can lead people to eat more, and that this differs a lot from person to person.[4, Modelling of the dynamics of weight change]

The same study found that weight responds slowly to a lasting change in eating, over months to
years. People with more body fat take longer to reach their new, steady weight.[4, Abstract]

The first weeks can look faster, for a different reason, explained in
[what weight loss looks like over months](https://kilocare.in/learn/what-weight-loss-looks-like).

A 2006 Cochrane review looked at 43 trials in adults with overweight or obesity. Exercise on its own led to small weight losses. Exercise with a change in diet led to more weight loss than diet alone.[5, Abstract, main results] The same review found that exercise went with better blood pressure, blood fats and blood sugar, even when no weight was lost.[5, Abstract, main results and authors' conclusions]

A year-long US trial, published in 2012, studied 439 inactive women past menopause with overweight or obesity, average age 58. The groups that ate less lost more weight than the group that only exercised. The group that did both lost the most.[6, Abstract]

Exercising one part of the body did not take fat off that part in 2 small trials, both in young
adults.

- In a 2011 US trial, 24 inactive adults aged 18 to 40 were split into 2 groups. One group did stomach exercises 5 days a week for 6 weeks. The fat under the skin of the belly did not go down, though the stomach muscles gained endurance.[7, Abstract]
- In a 2013 trial in Chile, 11 young adults, average age 23, trained 1 leg for 12 weeks. Fat mass fell in the arms and trunk, and did not change in the trained leg.[8, Abstract]

For a rough guide to the calories your body needs to keep its weight, try the
[daily calorie and macro calculator](https://kilocare.in/learn/daily-calorie-and-macro-calculator).
Then look at any change in your weight over months, with your doctor.

When to check with a doctor:

- Anyone who takes medicine for diabetes should check with their doctor before changing how much they eat. The same applies to blood pressure medicine.
- If you have been inactive, or have a heart, joint or back problem, see a doctor before you start strength exercise.
- If you get chest pain, or breathlessness that is new or worse than usual, stop and see a doctor.

> **Medical disclaimer:** This article is general information. It does not replace advice from your doctor about your own health. Prescription required. Consult your doctor. Results may vary.

## Frequently asked questions

**Where does fat go when you lose weight?**

The body burns stored fat with oxygen. Most of the fat's mass leaves as carbon dioxide, which you breathe out. The rest leaves as water, in urine, sweat, breath and other body fluids.

**Does sweating burn fat?**

Sweat is mostly water and salt. Sweating lowers the scale because you lose water, not fat. Drinking replaces the water, so the scale goes back up. Sweating more does not mean you burn more fat.

**Will stomach exercises take fat off my belly?**

In a small US trial in adults aged 18 to 40, 6 weeks of stomach exercises did not reduce the fat under the skin of the belly. The exercises did improve the endurance of the stomach muscles.

## Sources

1. Meerman R, Brown AJ. When somebody loses weight, where does the fat go?. BMJ. 2014;349:g7257. doi:10.1136/bmj.g7257. PMID: 25516540. Available from: https://www.bmj.com/content/349/bmj.g7257
2. Baker LB. Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature (Austin). 2019;6(3):211-259. doi:10.1080/23328940.2019.1632145. PMID: 31608304. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC6773238/
3. American College of Sports Medicine, Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-90. doi:10.1249/mss.0b013e31802ca597. PMID: 17277604. Available from: https://pubmed.ncbi.nlm.nih.gov/17277604/
4. Hall KD, Sacks G, Chandramohan D, Chow CC, Wang YC, Gortmaker SL, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-37. doi:10.1016/S0140-6736(11)60812-X. PMID: 21872751. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC3880593/
5. Shaw K, Gennat H, O'Rourke P, Del Mar C. Exercise for overweight or obesity. Cochrane Database Syst Rev. 2006;2006(4):CD003817. doi:10.1002/14651858.CD003817.pub3. PMID: 17054187. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC9017288/
6. Foster-Schubert KE, Alfano CM, Duggan CR, Xiao L, Campbell KL, Kong A, et al. Effect of diet and exercise, alone or combined, on weight and body composition in overweight-to-obese postmenopausal women. Obesity (Silver Spring). 2012;20(8):1628-38. doi:10.1038/oby.2011.76. PMID: 21494229. Available from: https://pubmed.ncbi.nlm.nih.gov/21494229/
7. Vispute SS, Smith JD, LeCheminant JD, Hurley KS. The effect of abdominal exercise on abdominal fat. J Strength Cond Res. 2011;25(9):2559-64. doi:10.1519/JSC.0b013e3181fb4a46. PMID: 21804427. Available from: https://pubmed.ncbi.nlm.nih.gov/21804427/
8. Ramírez-Campillo R, Andrade DC, Campos-Jara C, Henríquez-Olguín C, Alvarez-Lepín C, Izquierdo M. Regional fat changes induced by localized muscle endurance resistance training. J Strength Cond Res. 2013;27(8):2219-24. doi:10.1519/JSC.0b013e31827e8681. PMID: 23222084. Available from: https://pubmed.ncbi.nlm.nih.gov/23222084/

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