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Differences in Glycemic Index of Cereal is Due to Glucose Removal

Schenk S et. al. Different glycemic indexes of breakfast cereals are not due to glucose entry into blood but to glucose removal by tissue.  Am J Clin Nutr. (2003) 78(4):742-8.

BACKGROUND: The glycemic index (GI) of a food is thought to directly reflect the rate of digestion and entry of glucose into the systemic circulation. The blood glucose concentration, however, represents a balance of both the entry and the removal of glucose into and from the blood, respectively. Such direct quantification of the postprandial glucose curve with respect to interpreting the GI is lacking in the literature. OBJECTIVE: We compared the plasma glucose kinetics of low- and high-GI breakfast cereals. DESIGN: On 2 occasions, plasma insulin concentrations and plasma glucose kinetics (by constant-rate infusion of [6,6-(2)H(2)]glucose) were measured in 6 healthy males for 180 min after they fasted overnight and then consumed an amount of corn flakes (CF) or bran cereal (BC) containing 50 g available carbohydrate. RESULTS: The GI of CF was more than twice that of BC (131.5 +/- 33.0 compared with 54.5 +/- 7.2; P < 0.05), despite no significant differences in the rate of appearance of glucose into the plasma during the 180-min period. Postprandial hyperinsulinemia occurred earlier with BC than with CF, resulting in a 76% higher plasma insulin concentration at 20 min (20.4 +/- 4.5 compared with 11.6 +/- 2.1 micro U/mL; P < 0.05). This was associated with a 31% higher rate of disappearance of glucose with BC than with CF during the 30-60-min period (28.7 +/- 3.1 compared with 21.9 +/- 3.1 micro mol. kg(-)(1). min(-)(1); P < 0.05). CONCLUSION: The lower GI of BC than of CF was not due to a lower rate of appearance of glucose but instead to an earlier postprandial hyperinsulinemia and an earlier increase in the rate of disappearance of glucose, which attenuated the increase in the plasma glucose concentration.

My comments: This is another older paper that I wanted to talk about since it ties in somewhat with the feedback on milk below.   In way of introduction, I should probably define glycemic index (GI) for readers who aren’t familiar with it.
The GI is used to rate carbohydrates by examining the blood glucose response to 50 grams of digestible carbohydrates.  After fasting, subjects are first given some reference food; this used to be glucose but researchers now use white bread.  The blood glucose response to white bread is defined as 100.  Then, the test food is given and the blood glucose response is measured and compared to that of the test food.  A food that shows 60% of the blood glucose response to white bread is given a GI of 60.

It has commonly been assumed that GI and insulin response are related and bodybuilders and athletes commonly use GI to determine which foods are or are not acceptable to eat (especially on a fat loss diet).  Low GI foods are usually assumed to digest slowly and it is the slow rate of glucose into the bloodstream which causes the low GI.

A massive number of foods have been tested for GI although there is still much debate as to the validity of GI in meal planning.  GI can vary significantly by food and how it is prepared, as well as between individuals.  Also, GI is measured for 50 gram quantities of foods (that’s 50 grams of digestible carbohydrates).  But this can be misleading; for example, carrots are very high on the GI scale but few people would eat 50 grams of digestible carbohydrates worth of carrots in a sitting.  To counter this, some researchers have proposed a measure called the glycemic load (GL) which is the total amount of digestible carbohydrate multiplied by the GI.  This at least recognizes that, in the real world, carbohydrate intake varies.  GL can be lowered by either picking lower GI foods or by eating less total carbohydrate, or some combination of the two.

Additionally, GI tends to be affected by other nutrients (protein, fat and fiber) although not always in the way you’d think (and not all research finds a significant impact of protein and fat).  For example, protein tends to lower the GI of carbohydrates but insulin levels often increase when you add protein to carbs.

For reference, the most complete site on the web for information about GI is Rick Mendosa’s Site.

Which brings us to the above study.  As mentioned above, bodybuilders and athletes usually assume that a low GI means a low insulin response but the study above draws that conclusion into question.  Rather it found that the low GI food showed a lower blood glucose response because it generated a higher early insulin response (clearing blood glucose out of the bloodstream) at the 30 minute mark (by 60 minutes, both foods showed similar insulin levels).  Quoting directly from the paper “Bran cereal has a low GI because a more rapid insulin-mediated increase in tissue glucose uptake attenuates the increase in blood glucose concentration, despite a similar rate of glucose entry into the blood.”

That is to say, both foods released glucose into the bloodstream at similar rates, but the bran cereal showed faster uptake due to a higher initial insulin spike, which lowered the overall GI response.

The researchers also noted that the bran cereal contained more protein than the corn flakes and this is probably what caused the higher insulin response (and lower blood glucose) which ties into my comments above.

Somehow, I don’t think bodybuilders would argue that combining low GI carbs with protein is bad for fat loss, yet here we have an (as of yet unreplicated paper) showing that the initial insulin response is higher; essentially, the higher initial insulin response caused the lower GI in this case.  Yet most bodybuilders also believe that high insulin is detrimental to fat loss.  Here we have a study that I think questions that idea. At the very least, the small initial insulin spike certainly wouldn’t appear to be hurting things, it’s likely that sustained insulin levels would be more problematic by limiting the ability to mobilize fat for fuel.

Then again, at least one study found that spiking insulin (high GI condition) resulted in a larger rebound in blood fatty acid levels (after blood glucose crashed) compared to keeping insulin low but stable (low GI condition) so maybe there’s more to this picture than we yet realize.


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