Doug Cook RD
Nutrition Demystified. Health Exemplified
Nutrition advice that makes sense. Separating hype from reality.

How Do You Confuse Someone With Diabetes? Just Tell Them To Use The Glycemic Index


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I’ve been counselling people with diabetes for nearly 14 years and I continue to encounter a lot of confusion and misinformation about how best to manage diabetes with diet. Concepts like the glycemic index (GI) and glycemic load (GL) are no exception. People are routinely told to eat foods with a lower a GI but what does that practically mean and is healthy eating as easy as assigning a number?


Simply put, the GI is a measure (or metric) of a food’s ability [specifically any carbohydrate in that food] to raise blood sugar: in theory, the higher the GI, the greater the impact. The GI of a food is ranked either has high [>70], medium [56-69] or low [<55]. The GI was originally created for research purposes, where the goal was to try and determine how a food, eaten under special research conditions, would impact a person’s blood sugar level. It furthered our understanding of carbohydrate metabolism but in my opinion, it should never have become ‘mainstream’.


As a strategy for healthy eating, many people with diabetes have been advised to avoid, or limit, foods with a high GI but can it really be that simple? The GI of parsnips (a nutritious vegetable) has a GI of 80 – 92 [high] but the GI of table sugar [no nutritional value] is 58 [medium]. Likewise most chocolate bars have a medium GI [56-69].

I hear all the time how people should avoid soft drinks because the sugar will cause a rapid increase in blood sugar followed by a crash, yet gram for gram, soft drinks have less of an impact on blood sugar than parsnips if you rely on the GI of those foods alone. Using the GI in this context, to me, is a great example of nutritional reductionism, it’s easy to see how less healthy foods could be chosen over healthier options; what’s going on?

The GI of food, or meal, is affected by many factors such as the total amount of carbohydrate (or portion sizes), type(s) of sugar, the time of day, the presence of an acid like vinegar, the amount of sodium in the meal, the presence of fat, protein, and fibre – ugh, it’s enough to drive you crazy. This is why a chocolate bar may have a medium GI; the presence of fat, protein, and fibre in the nuts will slow down the absorption of the carbohydrate from the added sugar whereas the parsnip is nearly all carbohydrate.

The GI fails to take into account a few key considerations, making its use as a simple guide for blood sugar control limiting in my opinion.


The GI was originally created for research purposes but knowing that people eat meals and typically not just single foods, and also considering that they don’t sit down to 1 kg of parsnips in one sitting [the amount the poor subject had to eat to test the GI of parsnips], researchers knew the GI alone had limitations. What emerged was the Glycemic Load (GL), which not only considers the GI of a food but the portion size as well and therefore the amount of carbohydrate, in a typical serving. Because a ½ cup serving of parsnips (GI of 80-92) only has about 10 g of carbohydrate, the GL drops to about 5 [low]. Contrast this to can of cola with a GI of 58 [medium] but a GL of 15 [medium] thanks to all the added sugar – the choice is obvious. The GL ranges from <10 [low], 11 – 19 [medium], and >20 [high].


The problem with applying the GI and the GL to real life is that people eat foods and meals that are not as straightforward as referring to a list. Portion sizes, whether one serves oneself, or served by others [like in a restaurant or at friend’s place] will vary from day to day as will the accompanying amount of protein, fat and fibre; the real impact of blood sugar will vary and the reality is, the only sure way to know the impact of a food/meal on blood sugar is to keep a food log and to test blood sugar 2 hours after the first bite – a little daunting at first but proves to provide invaluable information [known as the ‘2 hour post-prandial blood sugar’].


As an educator I do my best to deliver direct, easy to understand and apply messages with an appropriate dose of context. I personally don’t like the GI or the GL because in practice I’ve never found it to work well because of the limitations already mentioned. People eat food and I find it easier to talk about strategies that are appropriate to an individual’s situation that help to keep blood sugar levels in a healthy range with tips like the following.

  1. ‘Lower carbohydrate’ is not a ‘bad’ phrase or idea and is a valid approach to manage blood sugar.
  2. Limit or avoid fruit juice (whole fruit is better because it has fibre).
  3. Limit the amount of starch-rich carbohydrate foods like pasta, rice, bread, corn, potato, sweet potato, and yes pulses [chickpeas, lentils, peas & beans]. These foods easily increase blood sugar, even whole-grain versions
  4. Choose more non/low-starch vegetables like broccoli, cauliflower, kale, celery, radish, cabbage, asparagus etc. or those that ‘grow above the ground’.
  5. Include fibre-rich foods at each meal.
  6. Don’t be afraid of fat, it’s an important source of calories as you reduce carbohydrates. Use olive, avocado, macadamia nut, & virgin coconut oil.
  7. Include a good portion size [4-6oz/114-170g] of protein and each meal.
  8. Avoid sources of trans fat.
  9. Increase your intake of omega-3 fats from fish & seafood, or supplements if needed [fish or algae]
  10. Limit foods with added sugars [don’t be fooled by ‘natural’ sugars]
  11. Limit the amount of refined/processed fats, especially omega-6 rich grain/seed/vegetable oils.