I don't think you're understanding this. As long as person A has more bread than they're going to eat, and person B still wants more bread, it is still optimal to trade. A well fed individual can do more labor than a minimally fed person who can in turn do more labor than a dead person. As person B's quality of life improves, they are in an ever better position to trade. The bread is worthless to person A, it doesn't matter how little person B gives for it, it is still optimal.

It does not become Pareto efficient until person A is worse off by trading bread, by which point they no longer have enough to completely satisfy their needs.