Glycemic index, insulin and weight loss: what are the facts?

The glycemic index (GI) originated as a research tool more than two decades ago. It is a measure of how fast a carbohydrate food is broken down to simple sugars and absorbed into the bloodstream after eating.

GI became popularized through diet books that claim that controlling GI controls insulin and that in turn prevents hunger, fat storage, and weight gain.

The GI-gurus would leave you assuming that only carbohydrate stimulates insulin. However, protein and fat also stimulate insulin. That’s because insulin is required to move protein and fat (as well as carbohydrate) into cells. So while replacing carbohydrates with fat or protein will reduce blood sugar, the effect on blood insulin levels cannot automatically be assumed. For example, a 240-calorie portion of beef, brown rice, fish, multigrain bread, or lentils, have similar affects on blood insulin (while their GI values range from 21 (beef) to 104 (brown rice). [i],[ii]

In fact, none of the claims (preventing hunger, fat storage, and weight gain) stand up to clinical data.[iii],[iv] There are several review articles in the literature looking at the effect of GI on appetite, and they all agree that at this point the majority of studies find no effect. Likewise many studies have looked at insulin resistance and either weight gain or weight loss. These studies show that weight loss occurs at the same rate in insulin resistant persons as it does in persons without insulin resistance.[v] Large long-term studies also find no correlation with insulin resistance and weight gain over time.[vi] So while the overall “glycemic load” of your diet is associated with heart disease and type 2 diabetes, the evidence shows that neither GI nor insulin independently affect subsequent appetite, weight gain, or weight loss.

Question: What has the largest effect on blood on blood sugar?

a)  The glycemic index of the foods
b)  Total carbohydrates (size of  the meal)
c)   Pre-meal blood glucose levels

Answer: The biggest contributors are the (b) size of the meal and your (c) pre-meal blood glucose levels (still elevated from your last meal)—not glycemic index.

The take away message: pay attention to portion sizes (including second servings), and whether you are actually hungry before you start eating.

Once again, the bottom line is that it all comes down to calories and energy balance, not any magic combination of foods, or types of foods.

Best,
-Dorene

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Is there a question about weight loss or dieting that you want answered? Comment here, or email me at dorene@beyonddiets.com.

If this is the kind of information you appreciate, you may also like my book: The NEW Healthy Eating & Weight Management Guide.

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[i] Holt SHA, Brand-Miller J, Petocz P. Am J Clin Nutr 1997;6:1264-76.
[ii] Jennie Brand-Miller PhD, The New Glucose Revolution . New York: Marlowe and Company, 2003, pages 57-58.
[iii] Golay A, et al. Similar weight loss with low- or high-carbohydrate diets. Am J Clin Nutr 1996;63:174-178.
[iv] J Endocrin Metab 1999;84:578-81.
[v] J Clin Invest 1966;45:1648-56.
[vi] J Endocrin Metab 1998;83:3498-3500.

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