If diabetes is caused by a lack of insulin, and patients receive injections of insulin, then why do they get these complications? This is an important question. At least three possible hypotheses exist to explain this observation. First, perhaps the complications were caused by the use of beef and pork insulin, which are slightly different than human insulin. Second, the beta cells in the pancreas, the cells destroyed in diabetes, may secretes other important factors besides insulin (for instance the c chain). Third, the complications might be cause by differences in delivery. Therapeutic insulin was delivered via a bolus several times per day under the skin, rather than continuously released into the blood stream, as needed by the pancreas.
Each of these hypotheses have been tested.
The first hypothesis was tested when recombinant human insulin was developed.
The second was tested several times. Most recently by Cebix. http://asweetlife.org/feature/is-insulin-enough-developing-c-peptide-therapy/