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Body composition and BMR

xxzenabxxxxzenabxx Member Posts: 729 Member Member Posts: 729 Member
Ive always wondered how much does body composition affect BMR and energy expenditure? For example, there are two women aged 27, 5’ 4” and both weigh 150 lbs. Woman A has 35% body fat and doesn’t exercise much whereas Woman B has 25% body fat and does strength training so she’s built much more muscle. How many more calories would Woman B burn more than Woman A at rest (BMR)?

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  • AnnPT77AnnPT77 Member, Premium Posts: 16,800 Member Member, Premium Posts: 16,800 Member
    In my understanding, a pound of body fat burns about two calories per day (it's metabolically active) and a pound of muscle burns about 6 calories per day, theoretically.

    I suspect that, in your example or similar real-world cases, on average the person with less fat and more muscle is going to find daily activities easier, and possibly find exercise activities more achievable/fun; and the person with more fat and less muscle is going to find all types of activity more difficult/challenging, and possibly experience more fatigue from moving that excess fat through the world all day every day. (Just a guess.) To the extent that that speculation is true, I'd expect that to make a more meaninful difference in daily calorie burn than the effect of the purely metabolic differences between fat and muscle tissue.

    If you want a more research-based guess, go to a TDEE calculator that allows you to input a BF% (like Sailrabbit at https://www.sailrabbit.com/bmr/), and see how it estimates it at a practical level.

    For the stats in your example, assuming a sedentary life, Sailrabbit says BMR at high 1400s to low 1600s, and TDEE at mid 1700s to mid 1900s for the 25% body fat, vs. low 1300s to high 1400s BMR and high 1500s to mid 1700s for TDEE.

    But, as I said before, I'm betting BF% influences actitvity level in subtle ways.
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