Dr Joseph R Kraft was one of the pioneers who turned this belief upside down. As early as 1975 stated that diabetes is a disease of excess insulin.
He showed that patients could test for normal blood sugars, but have elevated insulin levels. Fasting blood glucose tests did not reveal the true picture. His insulin assay demonstrated the real situation. 2 hours after glucose intake, if the insulin levels are at a high plateau (and continue), the person is at risk of arteriosclerosis.
Why does this matter?
Dr Kraft also demonstrated a direct link between diabetes and heart disease. When diabetes remains undiagnosed, heart disease may develop causing sudden death from a heart attack.
Why didn't his approach receive wider acceptance?
Dr Kraft found that insurance companies were unwilling to change their approach to diabetes diagnosis and treatment. Colleagues told him that members of the public were not willing to receive a diabetes diagnosis, even though they could treat it with a change in diet and lifestyle. Dr Kraft found that a reduced intake of carbohydrates helped reduce diabetes and heart disease.
He has published a new book on the subject, which is well worth reading:
He is interviewed here (with subtitles for clarity):
Youtube: KRAFT - Father of the Insulin Assay
Other Youtube videos present some of the material from the book: