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Glycemic Index vs. Glycemic Load: A History and Comparison

Updated on June 13, 2012
Parsnips have an incredibly high GI (97) -- but does that mean you shouldn't eat them?
Parsnips have an incredibly high GI (97) -- but does that mean you shouldn't eat them?

If you've been paying attention to Western diet crazes in the past decade, you'll probably be familiar with the term "Glycemic Index." Carbohydrates, scientists argue, are not all created equal. Bread, cereal, tortillas -- all of these have a high Glycemic indices (70, 88, 78) when compared to other food items like barley (25), milk (20) or chocolate (30). But what precisely does the Glycemic Index (GI) indicate? And how does that differ from another term you might have heard, "Glycemic Load" (GL)?

What is the "Glycemic Index?"

The Glycemic Index is a scale that rates the effect that certain foods have on blood sugar levels when compared to pure glucose. A "0" indicates the food causes no blood glucose response; a "100" indicates a food produces the same effect that pure glucose would. The higher the GI rating of a food the greater the blood sugar spike--but why is this considered bad? If you are a diabetic the implications of blood sugar spikes are obvious. For the rest of the population, however, over-consumption of high GI foods has been linked to obesity and heart disease, hence the explosion of diets touting "low glycemic index carbs" as the answer to weight loss struggles. And while it is true that most people outside the realm of elite athletics do not require vast amounts of carbs with easily processed sugar, using the GI as a metric of a food's acceptability as a carb is problematic.

Problems with the Glycemic Index

The issue with using the GI is that it compares equal quantities of carbohydrate (50g) among foods, and then measures the blood's response. This sounds well and good, until you take into consideration typical serving size. Take watermelon and milk chocolate, for example. Milk chocolate only has a GI of 43, while watermelon has a GI of 72. By conventional GI wisdom, the chocolate might seem a more appropriate diet choice than the watermelon. Common sense tells us this is incorrect -- and common sense is right here. Serving size, which the GI doesn't account for, makes a big difference here. Where you only have to eat 3 oz of chocolate to get 50 grams of carbs, you would have to eat 1.5 lbs of watermelon to consume the equivalent amount of carbohydrates!

What is "Glycemic Load?"

To correct for this difference in serving size, the idea of the "Glycemic Load" was developed. Conceived by Harvard scientists in 1997, "Glycemic Load" is defined as the GI of a good multiplied by the carbohydrate content in a typical serving. Although 'typical serving' remains somewhat a subjective measurement, the creation of the GL allows for much more logical comparisons between food choices to be made. Let's look at how a food's GL might differ from it's GI:

Glycemic Index (GI)
Glyecmic Load (GL)
Boiled Beets
Graham Crackers
Pineapple -- a smart choice!
Pineapple -- a smart choice!

For some carbs, like Graham crackers and Cornflakes, the GI is a fairly good indicator of the GL. But for others? By all GI standards, parsnips, pineapples, and beets should be off-limits. Yet when you look at their GLs, it is apparent that they are excellent food choices that will not produce massive blood sugar spikes. Thus, when evaluating foods based on their glycemic responses, a food's GL is a much more reliable indicator of what [not] to eat!


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