Nothing that does not exist in edible form in nature can be counted as a necessary part of the human diet. Bread is a human-invented product. We lived just fine for centuries, millennia, even millions of years (however long you think we've been here) without it.
I know she's your wife, but don't let her off easy. Tell her to explain to YOU what is in bread that we can only get from bread. She won't be able to.