Carbohydrates and the Faulty GI Index – IDM II

By Jason Fung, MD

Canadians consume approximately 43 pounds of added sugar per person, per year.  That is 43 pounds of sugar that is added to our foods through processing (1).  This does not include naturally occurring sugars consumed from fruits, vegetables and dairy.  That is a whole lot of sugar and makes up 11% of our daily caloric intake (1).

Sugars are very simple chemical structures whose chemical names end in “-ose”.  Some common sugars are glucose, fructose and lactose.   The refined table sugar in your pantry is called sucrose.  Sucrose is composed of equal parts of glucose and fructose molecules.  When your doctor discusses your blood sugar level with you, he or she is referring to your blood glucose level.


There are two main classes of carbohydrates we are going to focus on: simple carbohydrates and complex carbohydrates.  Simple carbohydrates, like table sugar or highly refined wheat flour, consist of one of two of these simple sugar molecules bound together.  Complex sugars,  like fruits and vegetables, are composed of long chains of tens of thousands of these simple sugar molecules.  During the digestive process, these long chains of sugars are broken down into their simple sugar components, but it takes awhile to do so.  The longer it takes to digest a carbohydrate, the slower the absorption of their simple sugar components into the bloodstream.   

The glycemic index (GI index) essentially measures the rate at which different carbohydrates are digested and converted into blood glucose levels (2).  Researchers measured the rate of digestion of a solution of glucose alone, and assigned it the numerical value of 100.  Therefore, the GI index is a comparison of the rate of digestion of different carbohydrates to the response from drinking a solution of glucose alone.  The GI index probably has the most significant influence on the clinical management of diabetes.  Patients are told to eat more foods with a low GI index (less than 55), eat a moderate amount of foods with a medium GI index (56-59) and eat less of foods with a high GI index (70+) (3).


1.  It Ignores the Effect of Fructose and High Fructose Corn Syrup

The GI index completely ignores the effect of fructose and high fructose corn syrup (HFCS) (4).    Earlier we discussed how sucrose (refined table sugar) is equally composed of half fructose and half glucose molecules.  Fructose can only be metabolized in the liver.  As a result, fructose is transported directly to the liver via the small intestine, and has a very little immediate effect on blood glucose levels.  Therefore, only the glucose portion of a sucrose molecule is reflected in the GI index.  This is why the glycemic index of watermelon is 71 and a Snickers bar is only 51 (5).

2.  It is Always Based on 50 g of a Particular Carbohydrate

The GI index is always based on the consumption of 50 grams of a particular carbohydrate (2).  That can be a lot when talking about the consumption of certain carbohydrates, such as, a carrot.  In order to obtain the effect of consuming carrots found in the GI index, you would have to eat about a dozen, full-sized carrots in one sitting.  That is a lot of carrots!  Who eats that many carrots at once?  Also, the GI index fails to consider the amount of fibre, water, vitamins and minerals in the carrot.  There is actually very little sugar in a single carrot, and even less of you cook it (6).

3.  The Combination of Foods You Eat

Different foods can greatly impact the glycemic response of other foods if consumed together (6).  Fibre, which is an indigestible carbohydrate, can slow down the rate of digestion of the other foods you have consumed along with the fibre.  A grain like quinoa can caused large spikes in blood sugar levels, but when consumed with vegetables like broccoli, carrots, red peppers, the fibre in these vegetables acts as a buffer and slows down the digestion of the meal.  As a result, the carbohydrates from the quinoa will not cause as dramatic rises in blood sugar levels than it would if it was consumed on its own.

4.  It Does Not Take into Consideration Foods that have a Delayed Glycemic Effect

The GI index is only based on a timeframe of 3 hours.  However, certain food, such as alcohol sugars, have a delayed glycemic effect.  They impact blood glucose levels much later on after they are consumed (6).


Foods to Avoid Most of the Time (almost 0% of your daily dietary intake)

1.  Processed, Refined and Genetically Modified Carbohydrates

  • Products containing wheat or wheat flour, including: bread, bagels, breakfast cereals, pasta, crackers, and beer
  • Sugar and artificial sweeteners
  • Sugar or sweetened beverages, including: soda, diet sodas, and fruit juices
  • White rice 
  • Corn and soy containing products 
  • Cheese (processed cheese only)
  • Lunch meats
  • Margarine
  • Processed oils, including: vegetable and corn oils 

Foods to Eat Less Often (<10% of your daily dietary intake)

1.  Root Vegetables

  • Potatoes 
  • Yams
  • Beets

2.  Unprocessed Grains

  • Quinoa
  • Barley
  • Buckwheat
  • Spelt
  • Rye
  • Black rice  

3.  Bananas and Grapes 

Foods to Eat More Of (>90% of your dietary intake)

1.  Vegetables (grown above the ground) – high in fibre!
2.  Legumes and Lentils – high in fibre!
3.  Fruits – high in fibre when eaten with the skin!

  • Berries 
  • Cherries
  • Apples
  • Pears
  • Avacados – high in natural fat! 

4.  Nuts, Nut Butters and Seeds – high in natural fat!

  • Almonds, walnuts, pine nuts, and Brazil nuts are among the healthiest
  • Peanuts are actually legumes and not nuts, but they are a great source of natural fat and protein
  • Nut butters should be organic

5.  Meat, poultry and Fish – high in natural fat!
6.  Eggs – high in natural fat!
7.  Butter – high in natural fat!
8.  Unprocessed oils, including: coconut oil, olive oil and avocado oil – high in natural fat!

By The Fasting Method

For many health reasons, losing weight is important. It can improve your blood sugars, blood pressure and metabolic health, lowering your risk of heart disease, stroke and cancer. But it’s not easy. That’s where we can help.

Jason Fung, MD

By Jason Fung, MD

Jason Fung, M.D., is a Toronto-based nephrologist (kidney specialist) and a world leading expert in intermittent fasting and low-carb diets.

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