The digestive process requires that you always have 20 pounds of shit inside you.
When you die, the floodagtes open.
Fat is the last thing to worry about.
You lose twenty pounds upon death, so matter what you've done to preserve yourself your "firmness" you will still look like a partially deflated balloon from the indiscreet and immediate absence of 20 pounds post mortem.
It's more important to make sure you have a pact with everyone you know to make sure that you're rolled onto your front so that there's no undue spillage once you become an open vessel of poop.
Or better yet plan your death and die on the toilet like Elvis.
They should sell "I'm going to die soon" super diapers to the elderly who need it.