Revisiting the Glycemic Index

Last month the British Journal of Nutrition published a study, Towards understanding of glycaemic index and glycaemic load in habitual diet: associations with measures of glycaemia in the Insulin Resistance Atherosclerosis Study, that concluded "The present results call into question the utility of GI and GL to reflect glycaemic response to food adequately, when used in the context of usual diet."

Today, an article about the study is found in The State that opens with "It’s trendy, it’s diet-y and it’s hard to understand. So naturally, the glycemic index has inspired all kinds of “expert” commentary and plenty of confusion among consumers. But a USC researcher says the glycemic index is so flawed, it’s pretty much useless for people trying to lose weight."

So, just what did this study investigate and what were the findings? More importantly, are the findings useful?

As I read through the full-text of the study, something immediately popped-out - the calculations for the food values were based on faulty assignment of glycemic index values. For example, cheese was arbitrarily assigned the same value as milk. Milk in the United States has a glycemic index value of 40 - low GI. Cheese hasn't been assigned a value, and you'd be hard pressed to find someone who would consider cheese as having a GI of 40 due to its protein and fat content, which makes it nearly impossible to test.

The reason it is difficult to test cheese is that consuming enough to reach the "test-load" intake - that is, eating 50g of digestible carbohydrate to reach the level to test - means you'd have to find enough people able to actually eat 8.6-pounds of cheese! So, that alone makes the data suspicious in my mind - assigning a value to an untested food, bad science!

But, that wasn't the only issue I had with the design of this study. The researchers used data from Food Frequency Questionaires (FFQ) not designed to measure glycemic index and/or glycemic load. Whether the assigned values are correct or not (I think they were inaccurate), those considered to be consuming a "low glycemic index" diet were in fact still eating a high level of carbohydrate at baseline and after five years.

One red flag that makes this an easy conclusion is the low amount of fiber in the diets - at baseline 16g on average and at the five-year evaluation 15g on average. This level was in the context of a high intake of carbohydrate - at baseline the average intake was 235g of carbohydrate, at five years it was 222g of carbohydrate.In looking at the assigned values from low to high for glycemic index - each level was HIGH, therefore the data was really only evaluating the effect of a high GI diet with a higher GI diet with an even higher GI diet, not a low GI against a high GI diet.

That's an important distiction here - and it is further supported by looking at the Glycemic Load (GL) levels assigned and assessed.

It is believed that GL is much more important at the end of the day than the GI of any one food or an entire day's GI measure. That's because the GI is not based on serving size of a food, but rather consumption of a specific level of digestible carbohydrate - 50g - in one sitting. With some foods a very small portion reaches this 50g intake, for others, like the cheese example above, the portion is so enormous it makes testing the food impossible and unnecessary. GL, on the other hand, is a calculation based on actual serving size of a particular food. For example, carrots have a high GI, but because you'd have to eat such a large portion to consume 50g of digestible carbohydrate, the actual serving size of a portion is measured and carrots are ranked as having a low GL.

I was happy to see that the research team tried to evaluate both the GI and GL because it provides support to my contention that few were following a low GI diet since only a small number actually fell into the low GL category - of the five groupings from low to high for GL, only one was calculated to consume less than the GL target of 100 to be considered consuming a low GI/GL diet. All the other groups ranked well above that 100 GL mark, so they were all consuming a higher GL diet. So again, we find comparison between high GL with high GL for the vast majority of those evaluated and for the one group that did seem to consume below the target GL level of 100, there is question with that due to the arbitrary assignment of GI values for untested foods as mentioned above.

My own opinion about the glycemic index is that it is very complicated and not really a very useful tool for the average dieter trying to lose weight. There is a lot of picking and choosing based on GI or GL numbers instead of nutrient-density. That said though, if one does follow it correctly - strive to maintain a low glycemic load each day by picking nutrient-dense selections - you're much better off than just eating a calorie restricted diet consuming whatever with only calories as your focus.

I personally think there are easier ways to plan what you're going to eat each day - simple basics - like real whole foods as the foundation of each day with lots of non-starchy vegetables, limit sugars and skip the processed junk food. Odds are high that if you're doing that, you're consuming a low GI diet anyway!

Here's an idea for researchers serious about investigating the efficacy of a low GI/GL diet - deisgn a study specifically to measure two or more groups consuming different levels over a period of time. Give the low GI/GL group specific foods allowed with instuctions on how to be compliant with the diet, the next group same thing, but higher value for GI/GL allowed and then have a control group eating whatever they want. Follow them for five years and then get back to us with REAL DATA!