Is The Glycemic Index Misleading? Yes, Here’s Why

By William Lagakos, Ph.D. / September 25, 2017

We all know that different foods affect our bodies in different ways, and that certain foods are better or worse for our health and weight. There are certain tools that people use in order to calculate why specific foods elicit a negative response – and can lead to weight gain – or vice versa, but these tools are not always 100% accurate. One of the most controversially used is called the glycemic index because while it does provide some good information, it can be very misleading in terms of how good, or bad, a food can be.

What Is The Glycemic Index?

The glycemic index (GI) is a method to assess how a certain food will impact your blood glucose levels.

To determine the GI of a food, the increase in blood glucose is measured after a person ingests an amount of that food that provides 50 grams of carbohydrates to the body. This increase is then compared to the increase in blood glucose that would occur after a person ingests 50 grams of pure glucose.

Foods that are high on the index include dates, baked potatoes, and popcorn, medium index values between 55 and 69 include sweet potatoes, wild rice, and pineapple, and low index values are below 55 include bananas, carrots, and peanuts. Here’s a Glycemic Index Chart if you would like to see more foods.

In general, carbohydrates increase the glycemic index, whereas fat and protein decrease it – however, exceptions to this abound and some of them are discussed below.

Why Is The Glycemic Index Important?

In determining the physiological impact of different foods on your metabolism, GI is an important factor, but it is not the best indicator of how foods really affects you.

In part, this is because GI refers to one single food item consumed by itself – the GI of mixed meals is usually much lower than that of any single carbohydrate-rich food, and we generally tend to eat more than a single item when we sit down to a meal.

It is also important to note that high glycemic foods with a high insulin index lead to fat storage, whereas low glycemic foods that have high insulin index (generally foods high in protein) will normally be stored in muscle tissue – and this has a very different effect on the body.

Is the Glycemic Index Misleading?

Yes, equivocally. There are numerous other indicators of diet quality which shed light on the shortcomings of GI as an assessment method:

Glycemic load (GL):

GL is a product of the GI and the amount of carbohydrates contained in 100 grams of the food. Recall the GI is the amount of food which contains 50 grams of carbohydrates – the difference is this: to get 50 grams of carbs from watermelon (to calculate GI), you need to eat almost 2.2 pounds of watermelon. Apparently somebody might have done this because we know the GI of watermelon is 72. BUT there are only 5 grams of carbs in 100 grams of watermelon. Thus, (72 * 5)/100 = 3.6. The GL of watermelon is 3.6, which is very low, despite its high GI. This is one of the flaws of GI: it doesn’t take into account carbohydrate density.

Insulin index:

This is determined by comparing the body’s insulin response to 239 kilocalories of a given food relative to the same amount of white bread. Since insulin is responsible for much of the negative effects of carbohydrates, some consider the insulin index superior to GI and GL in terms of its measurements on how food affects your body and your health.

As previously mentioned, foods with a high GL and high insulin index will inhibit fat loss and promote fat storage. However, some foods with a low GL also stimulate insulin secretion, but this is expected to favor muscle growth. For example, lean beef causes a greater insulin response than what would be predicted by its carbohydrate content, which is close to nil, and lean beef definitely favors muscle over fat. And while the fat added to baked goods would be expected to reduce the GI, the high overall level of easily digestible carbohydrates still leads to a sizeable insulin secretion.

Fructose index (FI):

The fructose index hasn’t received wide attention – yet – but offers another insight into the failing of GI & GL. Whereas carbohydrate-induced insulin secretion can be responsible for body fat accumulation, fructose doesn’t cause insulin secretion, but it may cause much more severe metabolic deterioration. Fructose, in excess, is thought to directly contribute to insulin resistance, the precursor of type 2 diabetes. Essentially, the insulin index measures insulin secretion, whereas the fructose index would assess how likely a food is to contribute to insulin resistance.

Thus, some researchers have suggested in addition to GL & II, a FI should be instituted to account for foods rich in free fructose, sugar, and high-fructose corn syrup. This index would be most applicable to fruits, baked goods & confectionaries, and soda & processed foods.

Below are some charts for further reference on the GI, GL, and FI:

So, while all three are tools you can use to calculate certain effects of a food, it’s wise not to put your trust in one of the measurements in order to decide whether or not a specific food is good for you. Focusing on eating whole, unprocessed foods is likely your best bet, not worrying about what an index implies is healthy.

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  • Marc Perry, CSCS, CPT says:

    Great job with this article, Bill. You clearly show how oftentimes nutrition charts and concepts, while helpful, can have some downsides. The truth is often found only when digging deeper.

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  • uncadonego says:

    "Focusing on eating whole, unprocessed foods is likely your best bet, not worrying about what an index implies is healthy."

    Thank you for ending this article on that note. :D

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  • Alexander says:

    Hi William, I understand that mixing protein and fat to carbohydrates will lower the GI of a meal. Does this mean that if we consume a whey protein shake followed immediately by whole meal bread, for example, the whey protein won't be digested rapidly and hence reduce its effectiveness (like post-workout)?

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    • Bill Lagakos says:

      Hi Alexander,
      If anything, I would expect that ingesting a piece of bread after drinking a whey protein shake will create a more anabolic environment due to 1) more calories and 2) greater insulin response.

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  • Brian Dunn says:

    I have borderline hypoglycemia. I was diagnosed with it many years ago and although it did manifest itself in certain ways in my previously sedentary life, it was easily managed. It's mild enough that I have never required any sort of medication for it, but it can manifest in certain ways at times. The BuiltLean program demands more me, though, and on less calories. How would I best tailor my dietary intake to provide best results on the BuiltLean program, especially in regard to the timing and exercise? What sort of fat/protein/carb ratio might be best for me?

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    • Marc Perry, CSCS, CPT says:

      @Brian Dunn - I'm super happy to hear you are getting started with the program, but your question about managing hypoglycemia is best a question for your doctor. He/she is in the best position to give you sound advice.

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