Lean means lack of excess flesh. Clinically thin means lack of flesh due to deficiency.
Most pediatricians are not taught this difference. Forget about parents, they are brainwashed by media. For parents, fat or excess flesh is sign of health.
As per my observations, lean but active, with no visible sign of any type of deficiency are children with high emotional and intellectual capacity.
When parents of such kids approach me, I suggest them no food advices but prescribe to spend more time with Mother Nature so that kids Sattva is nurtured. Naturally lean spend more energy at brain then fat conversion.
But not all kids are fortunate. They are prescribed feeding formulas. Extra digestion burden. Some of them lose nature’s gift. Like Hanuman’s amnesia, they too forget their real potential due to parental and doctor’s ignorance.