Let's answer your question from another point of view
Let's assume that we have changed the human body such that at that first moment it was capable of being entirely self sustaining. What are a few of the problems?
Weight gain/loss Which happens for more reasons than eating/not-eating too many carbs. As you exercise, fatty tissue is lost and muscle mass increases. Your self-sustaining body could, perhaps, do this perfectly — right up until it needs to add more muscle mass than it has fatty tissue to dissolve/consume. The change in mass from that first moment must come from, or go to, somewhere. The loss is relatively easy to explain, it's the gain that's the problem.
Skin cells Some of the dust in your house comes from you! Dead skin cells that flake off. There's also dead hair, clipped finger/toe nails, lost eye lashes, sweat... the problem here is that they're lost to the self-sustaining system, which means the mass to recreate them is coming only from the internals of the body. It would be a great way to stay thin, but you'd die early due to lack of mass to continue replacing skin cells.
Heat loss Your body is a furnace. Simplistically, it creates heat to stay warm, to move, to breathe, to do pretty much everything. The unfortunate problem with heat (which others have discussed), is that it requires some form of combustion (chemical reaction) that's destructive. You can't even come up with a "perfectly recycled" heat system because the amount of heat generated must change depending on what the body is doing.
Aging We start young and small, add mass as we grow to maturity, then it's a bit of a roller coaster until the end. During that time you work through two sets of teeth, become injured and slowly heal, the length of your bones changes...
And we're going to stop there, because that one word is the fundamental problem: change. As the immortal (and self-sustaining) Data once said:
My legs are exactly eighty-seven point two centimetres in length. They were eighty-seven point two centimetres the day I was created. They will be eighty-seven point two centimetres the day I go off line. My operation depends on specifications that do not change. (Source)
The only way to be 100% self-sustaining is for nothing, absolutely nothing, to change. If your perfect robot climbs a mountain, the energy to do that must come from somewhere. And something was destroyed to provide it.