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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.

KiloCare Team · · 5 min read · How we write and review

A man in a navy kurta stands at the top of a terrace stair in golden light, one hand on the rail, looking out over the rooftops.

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]
  • To lose body fat, the body has to take that triglyceride out of the fat cells and burn it.[1]

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]
  • Burning fat does release energy. The fat's mass cannot vanish, so it has to leave the body as something else.[1]
  • Using chemistry, the authors worked out that fat burned with oxygen becomes carbon dioxide and water.[1]
Where burned body fat goes, by mass
  1. Fat is stored as triglyceride in fat cells.[1]
  2. The body breaks it down and burns it with oxygen that you breathe in.[1]
  3. About 84% of the fat's mass leaves as carbon dioxide, which you breathe out.[1]
  4. About 16% leaves as water, in urine, sweat, breath and other body fluids.[1]

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]

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]

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]
  • Sports scientists estimate how much a person sweats by weighing them before and after exercise.[3] 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]

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]

  • 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]
  • For comparison, the paper says a single 100 g muffin gives about 20% of an average person's daily energy need.[1]
  • The BMJ authors still back the familiar advice to eat less and move more.[1]

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] The Lancet authors add that more activity can lead people to eat more, and that this differs a lot from person to person.[4]

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]

The first weeks can look faster, for a different reason, explained in what weight loss looks like over months.

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] The same review found that exercise went with better blood pressure, blood fats and blood sugar, even when no weight was lost.[5]

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]

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]
  • 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]

For a rough guide to the calories your body needs to keep its weight, try the 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.

Common questions

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.

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.

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: bmj.comCited atWeight we want to 'lose'Weight we want to 'lose'Opening paragraphsOpening paragraphsWeight we want to 'lose'Weight we want to 'lose'Weight we want to 'lose'Novel calculationNovel calculationConclusionLifting the veil on weight lossNovel calculationLifting the veil on weight lossLifting the veil on weight lossLifting the veil on weight lossLifting the veil on weight loss
  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: pmc.ncbi.nlm.nih.govCited atOverview of sweat composition
  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: pubmed.ncbi.nlm.nih.govCited atAbstract
  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: pmc.ncbi.nlm.nih.govCited atModelling of the dynamics of weight changeModelling of the dynamics of weight changeAbstract
  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: pmc.ncbi.nlm.nih.govCited atAbstract, main resultsAbstract, main results and authors' conclusions
  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: pubmed.ncbi.nlm.nih.govCited atAbstract
  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: pubmed.ncbi.nlm.nih.govCited atAbstract
  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: pubmed.ncbi.nlm.nih.govCited atAbstract