There is no question that insulin causes weight gain. Insulin is a hormone secreted by the pancreas in response to sugar intake usually in the diet. Its role is to drive sugar into the cells of the body where it is used as a source of energy (measured in calories). Insulin therefore pumps calories into cells. If this energy (sugar) is not used by the cells or is more than is needed, it is converted into an energy storage form known as fat. Because of these actions insulin is called an “anabolic” hormone
The word “anabolic” means building up tissue. If a person is using his or her muscles and is physically active, the extra energy is converted into new (larger and/or stronger) muscles rather than fat. In a sense, a person who is sedentary, not using his muscles, getting more calories than he needs and taking insulin is in the midst of a “perfect (metabolic) storm” that will result in weight gain. The issue of insulin causing weight gain has long been a troubling aspect of the treatment for type 2 diabetes. It is not a problem in type 1 diabetes where patients have virtually no circulating insulin and need to receive it from an external source.
But in type 2 diabetes the physiology is quite different. Here the body does make insulin but the tissues are “resistant” to its effects. In fact in the early stages of type 2 diabetes insulin levels can actually be high. This occurs because the tissues are resistant to insulin and higher insulin levels become necessary to drive sugar (glucose) into the cells and thereby drop the sugar level in the blood.The cause of insulin resistance is complex and is still a very active area of research. It appears that a certain type of fat tissue, fat that is contained in the abdomen (also called visceral adipose tissue), produces certain hormones and other substances that together cause insulin resistance. This was a major surprise in medicine when it was discovered only 10 or 15 years ago. Prior to that fat tissue was considered to be “metabolically inert”, which means that it was just a storage tissue and didn’t affect metabolism. This was very far from the truth and visceral fat is now considered to be very active and complex metabolically. It produces a host of hormones (for example leptin, ghrelin and adiponectin) and other factors (cytokines) that have major influences on metabolism.
The discovery that insulin resistance was the central “lesion” in type 2 diabetes led to a whole area of research that resulted in linking type 2 diabetes to high blood pressure, truncal or abdominal obesity, abnormal blood lipids (elevated triglycerides and low HDL cholesterol) and high waist to hip ratio (the “apple” body type). It was originally described by Gerald Reaven, MD a professor of medicine at Stanford University, who named it Syndrome X to underscore how little understood it was and how it was more than just elevated blood sugars that caused disease. He wrote a book for the general public on this called syndrome x, published by Simon and Shuster in 2000. Syndrome X is now also called metabolic syndrome and prediabetes and the majority of people with type 2 diabetes have some or all of its features.
So getting back to the issue of insulin causing weight gain, you can see that using insulin to treat type 2 diabetes is problematic. The person with type 2 diabetes is usually overweight and circulating insulin levels may already be high. Adding additional insulin will certainly cause weight gain and this can actually make the insulin resistance worse. The usual justification is that using insulin will protect the remaining insulin-producing beta cells in the pancreas from having to work overtime. However, only a few months ago this issue was reviewed by one of the leading diabetes authorities in the world, Dr. Ralph DeFronzo. DeFronzo recently gave the prestigious Banting Lecture and it was published in the April 2009 issue of Diabetes. DeFronzo suggests that the American Diabetes Association guidelines for treatment of type 2 diabetes may be misguided and in need of revision.
Regarding insulin-induced weight gain, he notes that when insulin is added to the treatment regimen, “all of these insulin-based add-on studies have been associated with a high incidence of hypoglycemia [low blood sugar] and major weight gain (range 4.2-19.2 lbs, mean 8.5 lbs within 6-12 months or less)….Moreover it is unclear why one would initiate insulin before exenatide [a newer non-insulin drug] since insulin rarely decreases A1C to <7% and is associated with significant weight gain…” (Diabetes, Journal of the American Diabetes Association, April 2009, vol 58(4), page 786)