When the Glycemic Index Doesn’t Measure Up

A couple of months ago we wrote about the link - or lack thereof - between dietary Glycemic Index and the risk of developing type 2 diabetes. The Glycemic Index is of interest to those seeking to help prevent or treat diabetes because it measures the effect that a specific food has on a person's blood sugar after the person eats it. Unfortunately, the results of studies assessing the link between GI and diabetes risk have been mixed. As we mentioned in the article, just because the research isn't yet conclusive, that hasn't stopped people from creating commercial diets based on the Glycemic Index.

Today we have another reason for you to treat these diets with some skepticism. An article published in the British Journal of Nutrition describes a small study performed in Finland that essentially compares the theory of the Glycemic Index with reality (2011;106(2):248-253).

The researchers recruited 11 healthy adult men and women of normal weight who also had a normal glucose tolerance based on a standardized test. They were given six different meals that included mashed potatoes in a random order, once a week for six weeks. After they ate the meal, their blood glucose was measured every fifteen minutes for the first hour, then at 90 and 120 minutes after the meal.

The six meals were as follows:

1. Mashed potato with margarine, served with water and ~1.5 ounces cucumber
2. Mashed potato with ~1 ounce rapeseed (canola) oil, served with water and ~1.5 ounces cucumber
3. Mashed potato with ~4 ounces chicken breast, served with water and ~1.5 ounces cucumber
4. Mashed potato with ~4 ounces salad comprised of cucumber, tomato and lettuce
5. Mashed potato with ~1 ounce rapeseed oil, ~4 ounces chicken breast and ~4 ounces salad (as in #4)
6. A slightly smaller portion of mashed potato with ~1 ounce rapeseed oil, ~4 ounces chicken breast, ~4 ounces salad, and a ~4 ounce slice of rye bread

All the meals contained about the same number of carbohydrates (the meals with salad had 4 more carbohydrates than the others). As a reference, each person consumed a standard glucose solution on two occasions during these six weeks so that the researchers could determine the GI for each meal in comparison to the individual's response to the glucose solution.

What they found is rather surprising. Standard reference tables give a range of GI values for mashed potatoes between 71 and 102 (and even up to 106). Having the mashed potato with the rapeseed oil (whose GI is about 0) actually yielded a lower GI value - 37 points less! The meal that most resembled something you might actually eat at a meal, #5, also had a lower GI than the mashed potato alone.

What this means for you

This is an example of something that's useful in the lab, but not necessarily useful in the real world: people eat food, not nutrients. Better to eat great food cooked sensibly than to obsess over every glycemic point.

First posted: June 22, 2011