Maybe when our bodies get enough of the right fats, we become really efficient because enough of the right fats means that we don't need much else in terms of nutrients. Maybe when someone gains a lot of weight it is because their body is desperate for nutrients so will sacrifice efficiency for extra weight as long as the nutrients are provided. If that makes sense.
I was writing to a woman today who is doing a book on pregnancy nutrition and feeding babies. I told her how crazy it seems to me that we load up these babies with cereal and other carbs. They need....guess what when their brains are growing to 80% of their final size? Fats! Not Cheerios! These friends brought their baby by last night who is 6 months old and all she eats is carbs -- cereal, formula (contains 7% sugars). She was huge! I mean, if an adult had been that fat the floor would have fallen through or something.
It is hard to feed babies fats and have doctors or friends be OK with it. And it's hard to avoid feeding processed grains to your kids.