Time to ditch the Glycemic Index?


GI is a selling tool for many fad diets

The glycemic index was originally conceived 30+ years ago as a simplistic way to rate foods according to their effect on blood glucose.  The thinking went this way: if a food raises blood glucose too much, it must be bad.  Why?  Because, as the thinking went, blood glucose should not go up too much.  GI became a popular tool for planning diabetic diets, because diabetics want to keep blood glucose under control.  It’s also used to promote fad diets and processed diet foods.

Here’s how a food’s glycemic index is determined:

  1. a portion of a food is measured out to contain 50 grams of carbohydrate
  2. a research subject eats that amount of the food — and nothing but that food –, on an empty stomach
  3. the person sits around for 2 hours, while blood glucose is measured
  4. the GI for that food is calculated based on how much the person’s glucose went up, compared to how much it would rise after eating an equivalent amount of pure glucose.
  5. the higher the GI, the worse the food.

Never mind that:

  • Frequently people do not eat a portion of a food with 50 grams of carbohydrates.  For some foods, it’s a huge amount; for others it’s small.
  • People don’t eat in laboratory settings, sitting around for 2 hours getting blood draws.
  • People eat mixed meals, not single foods.

I’ve never been a fan of the GI for those reasons.  The glycemic index of a single food just has nothing to do with how people actually eat.  But even I was surprised by the results of a new, very well-designed diet study that looked at the effect of glycemic index on several disease risk factors.  The researchers designed 4 different diets:

  1. low carb/low GI
  2. low carb/high GI
  3. high carb/low GI
  4. high carb/high GI

The assumption was that the low GI diets would be beneficial: increasing insulin sensitivity and lowering blood lipids and blood pressure.  In fact, that’s not at all what happened.  For example, the low GI/high carb diet actually decreased insulin sensitivity compared to the high GI/high carb diet.  For the two low carb diets, changing the glycemic index didn’t have much impact of disease risk factors.

Dr. Robert Eckel MD, of the University of Colorado, who wrote an accompanying editorial, also expressed some surprise at the results.  He believes the results “suggest that the concept of glycemic index is less important than previously thought.”  Dr. Eckel’s advice: when it comes to heart disease risk, the whole diet is important and a Mediterranean-style diet is the best choice.

If you’re curious, you can find long lists of foods’ GI scores here.  This shorter list quickly reveals some of the nonsense behind these scores: potato chips are “healthier” than plain potatoes?  High fat premium ice cream is “healthier” than rice cakes?   A Snickers bar is “healthier” than fresh watermelon?  I’ll stick with a Mediterranean diet.

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