Right. Belly fat is stored there because a central location is best for the body (although it was sufficiently rare in our evolutionary ascent to afford 40 pounds of belly fat that our bodies didn't adapt well to it, which is why it's so bad for us). In terms of structural imposition on the body, the backpack is worse.

Incidentally, anyone who's hiked will tell you that the best position for heavy weight is at the very top of the pack. The reason is that this puts most of the weight on your hips rather than your back.

Women seem to prefer weight lower in the pack than men, even if equal height, because of a generally lower center of gravity. I nearly broke my nose carrying a male friend's pack briefly while hiking -- tripped on a tree root and was so unused to the high center of gravity that I smashed my face into the ground. My own pack was heavier (yay food!) but sized and packed very differently.

Shouldn't it be at the bottom of the pack then ?

No, the bottom of the pack is completely outside (behind) the hips. If you put it at the top of the pack, you'll naturally bend forward slightly to an equilibrium between your torso and your backpack, the top of the pack will be roughly above the hips.

Exactly why you see the head basket carrying in many countries that have to do long haul water carrying.

What I meant in my other comment is: can you help me figure out how to map structural imposition to lifespan probability in a meaningful way?

How about wearing a diver's weight belt? The weight is all at your hips and is more or less equally distributed between front and back.

Completely different physics, because buoyancy cancels out gravity. The weight belt is distributed around your center of mass because it has different buoyancy (higher density) than you, but subject to the same gravity, and you don't want it affecting your orientation underwater.