This article is for health education purposes only and is intended for informational reference. It does not constitute medical advice. If you have health concerns, please consult a professional doctor.
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You step on the scale after a workout, the number drops by 1 jin, and you think you've lost fat? That's mostly sweat. Sweating a lot doesn't mean you're losing a lot of fat—this is a classic misunderstanding during a fat-loss phase.
Where does fat actually go? A study in the British Medical Journal laid out the math clearly: when you lose 10 kg of fat, 8.4 kg of it becomes carbon dioxide and is exhaled through breathing, while the remaining 1.6 kg becomes water[1]. So fat is mainly "breathed" out, not "sweated" out.
There is almost no fat in sweat. The sweat secreted by sweat glands is mostly water and sodium chloride. Its primary job is to dissipate heat, not to expel oil; its role in eliminating metabolic waste is also far inferior to that of the kidneys and intestines[2]. The weight lost in sweat lodges, sweat suits, or from running wrapped in plastic wrap is all water.
And your body will quickly return that water. According to the consensus of the American College of Sports Medicine: sweating rate varies greatly from person to person and by environment; when water loss exceeds 2% of body weight, it already counts as dehydration and can affect exercise performance, so after exercise you should replenish water and electrolytes[3]. After rehydrating, that 1 or 2 jin naturally returns to the scale—this isn't rebound, it's water.
To truly lose fat, what matters is exercise volume. The same sports medicine consensus gives this amount: 150 to 250 minutes of moderate-intensity exercise per week can prevent weight gain, but the degree of weight loss is limited; only when exceeding 250 minutes per week is it linked to "clinically meaningful weight loss," and it should be combined with moderate dietary control; strength training itself does not reduce weight, but it can help you preserve muscle[4].
Three things you can do today
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Don't use sweat volume as a fat-loss report card. Weigh yourself at a fixed time and look at the weekly trend.
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Do aerobic exercise for more than 250 minutes per week, plus 2 strength training sessions.
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If you sweat a lot, rehydrate promptly. Don't tough out dehydration just to see the scale drop.
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Disclaimer: This article is for health education purposes only and is intended for informational reference. It does not constitute medical advice. If you have health concerns, please consult a professional doctor. Please tailor exercise and hydration plans to your own situation; patients with chronic diseases should consult a doctor first.
Conflict of interest statement: The content of this article is based on publicly published medical literature, and the author has no relevant conflicts of interest.
References
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When somebody loses weight, where does the fat go? BMJ. 2014;349:g7257. PMID:25516540
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Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature (Austin). 2019;6(3):211-259. PMID:31608304
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American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-390. PMID:17277604
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American College of Sports Medicine Position Stand. Appropriate physical activity intervention strategies for weight loss and prevention of weight regain for adults. Med Sci Sports Exerc. 2009;41(2):459-471. PMID:19127177
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