Why does the human body create solid waste?

Answer 1

An active person will excrete as waste the excess of food that they eat.

As backpacker, who has been on trips of several weeks carrying all supplies on the back, I assure you that you excrete little.

This is partly because the food is probably low-residue and carefully chosen, but if you're exercising a lot (as a backpacker does), then your excess is minimized.

On a 21-day trip, our party of 6 went OK (including 7 days snow-caving) on a diet of 24oz per day each. A little less than 700g/day.

[One slightly overweight member, who lost his tobacco during the trip was certain that he'd lost weight, (for his belt was more slack) so on reaching civilization again, his first move was to weigh himself. And was disappointed to find he'd gained weight!. In a 21-day period you can lose fat and put on muscle, which is more dense than fat.]