Interesting info about Glycemic Index


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Four Years of Frigidity
Notes from the 2003 Ontario Exercise Physiology Conference
by John M Berardi

Glycemic Index of Different Breakfast Cereals is Not Due to Glucose Entry into Blood But by Glucose Removal by Tissue (Schenk et al.)

As many of you know, the glycemic index is calculated by the total amount of glucose that appears in the blood after a test meal divided by the amount of glucose that appears in the blood after a reference meal (white bread or glucose). However, the factors contributing to the blood glucose rise and fall often aren't discussed, presenting an incomplete picture of the glycemic index.

After eating a meal, blood glucose will rise in proportion to the rate of appearance (Ra) of glucose in the blood (how fast the meal was digested and absorbed). However, the rate of appearance isn't the only determinant of glycemia. During the time that glucose is appearing, the amount of glucose we can measure in the blood is also determined by the amount of glucose that's concomitantly taken up by the tissues, or the rate of disappearance (Rd). Therefore, the glycemic index/blood glucose = Ra - Rd.

Usually, when a food is high GI, we assume that it's rapidly digested and absorbed, leading to a high glucose response. In addition, we assume that insulin will rise in proportion to this measured glucose rise, leading to antilipolytic effects and lipogenesis. Likewise, when a food is low GI, we assume that it is slowly digested and absorbed, leading to a low glucose response. Again, leading to a small insulin response and minimal impact on fat storage or fat loss.

As you can see, the assumption is that Ra, or the rate of entry of the carb into the blood, determines GI. In other words, a high GI meal has a high Ra while a low GI meal has a low Ra. This study points out that the assumption commonly made is far too simplistic.

In this investigation, two different breakfast cereals were studied. The first cereal was a high-GI cereal (corn flakes; GI = 151) while the second cereal was a low-GI cereal (all bran; GI = 54.5). After consuming 50g of each available carbohydrate, blood glucose and insulin concentrations were measured for two hours.

Of course, the total glucose measured in the blood reflected the large difference in GI. The corn flakes promoted a much larger blood glucose rise than the all bran.

Interestingly, though, when labeled glucose was used to trace the glucose Ra and Rd, the glucose Ra into the plasma wasn't different between cereals, meaning that both cereals were digested and absorbed at the same rate.

How, then, might the glycemic index have been different? Well, the Rd from the blood was much faster in the all bran group, meaning that the reason that the glucose concentrations in the blood were lower with all bran was not that it was slower to digest and absorb.

How could this be? Well, enter our friend insulin. Since insulin concentrations were 75% higher immediately after the low GI, all-bran meal, it stands to reason that the blood glucose cleared out much more quickly in this group. Therefore the reason the high GI corn flakes group had a larger rise in blood glucose is because they got a smaller insulin response and a slower clearance of the blood than the low-GI group.

So why the big insulin response with bran? Well, it could be the fact that the bran meal contained 15g of protein while the corn flakes meal contained only 1g of protein. Since protein promotes a large insulin response when combined with carbohydrate, the all bran cereal may have a low GI for the same reasons milk has a low GI — the protein content. In both types of low-GI meal, the low blood glucose response is probably due to the large insulin response and the subsequent rapid uptake into the tissues (Rd).

While this study adds a whole lot of confusion with respect to selecting appropriate carbohydrate choices (since it now looks like all bran might be worse than even corn flakes in terms of the insulin response), I believe that it does indicate that the insulin index might turn out to be a better determinant of carbohydrate quality than the glycemic index.

Now, we just need to convince some researchers to get us a comprehensive list of the insulin index of foods instead of the glycemic index. In the interim, don't freak out and draw all sorts of weird conclusions. Remember, the best determinant of whether a food is "good" or "bad" for your physique progress is how you look when it's regularly eaten.