This is approximately correct and should not be downvoted because it was written by an asshole‡.

The disproportionate energy expenditure in children is almost certainly explained by cell division and growth.

They are literally fashioning body mass from consumed food.

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‡ We would do well not to ignore assholes if they are stating something of consequence.

But newborns grow faster than infants and the increased energy expenditure was not seen in them.

Might have something to do with the fact that newborns barely move. Lots of kids will get increasingly chunky right up until the moment they start crawling/walking, when they start slimming down. I can’t imagine there’d be increased energy expenditure when you literally can’t roll yourself over, let alone move across a room.

We should also not call someone that wrote a message in a certain way an asshole. You very likely don’t know that person and whether they’re an asshole.

15 months old infants do not double their body mass in a year. That takes about 4-5 years.