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If I work out and do treadmill to become fitter with the goal of losing weight, how do I know if I’m losing muscle mass or fat if I’m losing weight? Thanks.

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  • If you lose the fat slowly whilst continuing to do weight training with a high protein diet, most of what you lose should be fat.
    – Tom
    Commented Feb 25, 2023 at 14:31

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You're losing both, the only question is in what proportion. You could measure this using DEXA, but it really isn't practical to do so. If you're already moderately lean then you may also be able to get a vague estimate of where lost weight is coming from by using body circumference measurements (e.g. wanting to see waist circumference reductions but not arm or thigh circumference reductions).

Realistically though, the best approach is to maximise your chances of preserving muscle while losing weight by engaging in resistance training multiple times per week and ensuring your diet is sufficiently high in protein. You shouldn't need to know what proportion of weight you're losing is coming from muscle, because what could you do with that information anyway? If you were already doing everything you could to preserve muscle and discovered that you were still losing substantial amounts of muscle mass, then the only remaining means of stopping that would be to stop losing weight.

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    He's losing muscle mass by working out and running on the treadmil?
    – fectin
    Commented Feb 24, 2023 at 13:21
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    @fectin I have some experience of dropping from 110kg to 85kg and grow back to 110kg later. I'd say yes, when you lose a lot of weight, you lose both. Reverse is also true, they both grow back even if you don't exercise much. It's quite visible. At 110kg my belly is significantly bigger than the 85kg me but the muscle in my arms is also much thicker, even though I have never purposefully exercised my arms.
    – user3528438
    Commented Feb 24, 2023 at 15:56
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    According to my experience, I think that if I don't exercise much, my belly can become significantly bigger if I increase my weight yet the muscle in my arms can stay the same thickness.
    – HelloDarkWorld
    Commented Feb 24, 2023 at 17:41
  • Just curious: There are smart scales that claim to be able to give you a rough estimate of the proportion of fat, water, etc in your body (presumably by measuring electrical resistance or the properties of pressure wave propagation through the body). Are they useful in the context of this question?
    – Andreas Rejbrand
    Commented Feb 25, 2023 at 12:32
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    @AndreasRejbrand No, BIA (Body Impedance Analysis) scales are notoriously inaccurate, and produce hugely different readings for the same person based on things other than fat levels, such as hydration status.
    – David Scarlett
    Commented Feb 26, 2023 at 10:29

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