Seems pretty dumb to talk about them having fast metabolisms and brains all this other forest-for-the-trees. They are going to double their body mass over a year. See how much energy you need if you try that
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.