In the context of the input sentence, determine the relationship between 'beta cell' and 'insulin'.
Options: operator, owner of, product/material produced, headquarters location

In contrast, patients receiving a single course of teplizumab demonstrated a relative increase in C-peptide levels, suggesting not only a delay in beta cell destruction, but also the recovery of sick or dying beta cells and the restoration of their functionality and insulin secretion.
product/material produced