Lots of people are wondering whatever happened to the insulin index. I wondered too, so I asked Susanna Holt.
Dr. Holt developed the insulin index about a decade ago when she was working on her Ph.D. at Australia's University of Sydney. Her work was exciting but preliminary.
She tested just 38 foods and found that their glycemic index and insulin index values were highly correlated. But there was a big exception.
Their most interesting finding was that "protein-rich foods and bakery products (rich in fat and refined carbohydrate) elicited insulin responses that were disproportionately higher than their glycemic responses."
The American Journal of Clinical Nutrition published her findings in a 1997 article, "An Insulin Index of Foods: The Insulin Demand Generated by 1000-kJ Portions of Common Foods". My 2003 article about the insulin index summarized and popularized that research.
Some people think that the insulin index is even more useful than the glycemic index. It makes sense to compare these two indexes, because Dr. Holt developed the insulin index while working with Dr. Jennie Brand-Miller, who has done the most work on the glycemic index.
We still don't know why there are differences between glycemic and insulin index values and what significance they have. "Some foods (such as meat, fish and eggs) that contain no carbohydrate, just protein and fat (and essentially have a GI value of zero), still stimulate significant rises in blood insulin," Dr. Brand-Miller wrote in her best-selling book, The New Glucose Revolution (New York: Marlowe and Company, 2003, pages 57-58). "We don't know how to interpret this type of response (low glycemia, high insulinemia) for long-term health. It may be a good outcome because the rise in insulin has contributed to the low level of glycemia. On the other hand, it may be not-so-good, because the increased demand for insulin contributes to beta-cell 'exhaustion' and the development of type 2 diabetes."
Because I would love to see work on the insulin index continue, I recently wrote Dr. Holt. She replied that she would have loved to keep working on it, "but the opportunity just isn't there for me."
However, she added an intriguing finding. In addition to milk, she says that chocolate also produces a higher than expected insulin response. This includes plain chocolate, chocolate bars and especially chocolate flavored protein bars and drinks. "I think this is probably due to the combination of sugar and fat (and protein for the protein bars and drinks) and the high palatability of chocolate-flavoured foods," she wrote me.
"One of the last research studies I did was comparing the effects of two different breakfast meals on blood glucose, insulin, and incretin (GLP and GIP) responses," she wrote. "The meal with cow's milk in it produced large GIP and GLP-2 responses, which could be a good thing in people without any insulin resistance, as the GLP-2 is known to be involved in the satiety cascade."
Apparently it is the whey in milk that produces a high insulin response. She directed my attention to an article by Swedish and Danish scientists that The American Journal of Clinical Nutrition recently published, "Glycemia and insulinemia in healthy subjects after lactose-equivalent meals of milk and other food proteins".
So, should we avoid milk and chocolate or eat more of it? I would hardly recommend one or the other until we learn more about the significance of the high insulin responses that they trigger. But it's clear than the glycemic index remains as relevant as ever for determining which carbohydrate foods trigger high blood glucose levels.