Usually if person take more calories with food than he spends, he is gaining weight.

But if he has fast metabolism, his weight stays the same.

Where these calories go? Maybe food is not completely digesting?

PS. How it related to Basal metabolic rate?

  • $\begingroup$ "fast metabolism" usually refers to the daily energy expenditure, also known as the metabolic rate (en.wikipedia.org/wiki/Basal_metabolic_rate) Incomplete uptake of nutrients ("not completely digesting") is distinct from metabolic rate. So it sound like your question is whether metabolic rate or nutrient uptake is the key factor? $\endgroup$ – Roland Mar 4 '17 at 10:37
  • $\begingroup$ @Roland, I just want to understand how this metabolism works, why people can stay slim. So you saying that they waste more energy, so they have low energy conversion efficiency? $\endgroup$ – Alexan Mar 4 '17 at 16:17
  • $\begingroup$ No, I'm not saying that (and I don't know what you mean by "conversion efficiency"). I'm trying to figure out more precisely what you're asking. If a person has a high caloric intake and does not gain weight, then obviously there is either high energy expenditure ("fast metabolism"), or there is incomplete uptake of nutrients, or both. This varies between individuals. Are you asking for population data / studies on metabolic rate, or on digestion, or ... ? $\endgroup$ – Roland Mar 4 '17 at 16:42
  • $\begingroup$ @Roland, I'm interested in fast metabolism mechanics, how it's actually works in simple terms. About Energy conversion efficiency: en.wikipedia.org/wiki/Energy_conversion_efficiency $\endgroup$ – Alexan Mar 4 '17 at 17:08
  • $\begingroup$ probably relation with Basal metabolic rate $\endgroup$ – Alexan Mar 4 '17 at 17:49

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