I have thought that it might be easier, to eat a prehistoric diet. Your tribe just killed a mammoth and are eating as much of the meat as possible, before it gets to be spoiled.
After 5 days, you might only drink liquids. You might not eat for 24 hrs and your body would think it is starving, so it would cause you to eat more, to have reserves, when you didn't kill another animal.
Another variation on this would be to exercise as you ate and don't on the 6 th and 7th days,when your body would be expecting to need to replace the calories you would normally need for the exercise.

You have to watch for bakery goods like pies in the store, that are 5 days old, as I didn't notice and was sick.
The problem with dumpster diving, is what has been thrown on top of the food and can it be cleaned off.
I was reading about stores donating expiring food to food banks, which was still edible. It might be better to see if you can make a deal with a small store, buy their food and have a weight gain orgy.