The Myth of the “Insulin or Sugar Crash” during exercise…

28 Mar

I have been discussing this topic recently with a teammate (thanks for the mind fodder Jason!). There is an idea out there that people need to consume “slow burning” sugar (or carbohydrate) during exercise or else your body will experience a “sugar crash” and you will die… (ok-that last part is not really what people think)…but if nothing else, your race will be sabotaged by “quick burning”, “insulin spiking” types of sugar.

I want to tell you all that this IS NOT TRUE and, in fact, THE OPPOSITE IS TRUE about what you should eat during exercise!!!

First, I want to define the difference between a “simple” and “fast” vs. “complex” and “slow” sugar. “Simple” and “fast” do not mean the same thing. Simple means that the sugar is a monosaccharide or disaccharide- ok what the hell does that mean?  It means that the sugar is made of one or two units of something.  For instance, fructose is just one block of fructose and glucose is just one block of glucose.  Sucrose is a disaccharide (or made up of 2 units of something). Sucrose is made up of a block of glucose and a block of fructose stuck together.  According to the National Institute of health, a “complex” sugar is one that is made up of at least three “blocks” of something. (

Maltodextrin is by definition a “complex” carbohydrate because it is a polymer and made up of many “blocks” of glucose pieces stuck together (In turn, starch is a bunch of maltodextrin pieces stuck together and is also, obviously, complex)  Here is why I have to define these terms though- Maltodextrin is a COMPLEX carbohydrate that acts like a “FAST BURNING” sugar…ok that is just crazy but it is true…

So I obviously have to prove it.  How quickly a sugar “burns” or is digested, absorbed and has an effect on your bloodsugar is described through the glycemic index (the higher the score, the more quickly the sugar is digested, absorbed and affects blood sugar).  This index is subject to a lot of influencing factors, including whether or not you are consuming fat, protein and fiber with your carbohydrates- but putting that aside and just looking at the sugars- we get an idea of how quickly these sugars act. Here is a very rough (but hopefully ball park accurate list- it is tough to get this information because the glycemic index is almost always represented in real foods for practical reasons):

Glucose 85-111
Maltodextrin 105
Honey 32-87
Sucrose 58-65
Lactose 46
Fructose 12-25

So you see now that maltrodextrin is a “complex” sugar that “burns fast” and fructose is a “simple” sugar that burns “slow” (However there are other problems with fructose in the way it is processed in the body that go beyond the scope of this article that don’t really make it glycemic index friendly in large quantities- but this is a separate issue).

So getting back to what to consume during exercise.

During rest, you can experience a “spike” in blood sugar and a subsequent rise in insulin that causes a “crash” or mild hypoglycemia (low blood sugar) a bit later.  This is true, DURING REST. See graph below:


BUT…this graph is NOT TRUE when you are exercising or racing!!!!  This is the myth that a lot of people carry around that is insane in the membrane.  We have evolved (if you believe in that type of thing) to fight tigers and ligers and dinosaurs (right?) and that is why we have historically exercised.  To do this our body produces an adrenaline response when we exercise, similar to a “fight or flight” response.  When we exercise today our body thinks we need to fight something or hunt something so it is smart and increases our heart rate for the battle, shuts down our digestive system and shunts blood to muscles, adjusts our pupils so we can see better – and lots of other stuff too- but it also decreases insulin production out of our pancreases so we have consistently high bloodsugar levels for the battle with the dinosaur.   And this is why you will not have an “insulin crash” during exercise!!!  (this is also why you need to worry about sparing your glycogen stores during exercise- because that glycogen is getting broken down to keep blood sugar up but that is a separate discussion for another article as well)

So why does this matter- it matters because sugars that are higher on the glycemic index typically absorb quicker than those lower on the index.  Gastric emptying times (how fast the sugar gets out of your stomach) are shorter.  And this matters A LOT during moderate to hard exercise.  Remember how I said when you are fighting the dinosaur that your body shunts blood from your digestive system to your muscles for the battle??  That is also what happens when you exercise.  So gastric emptying is a BIG DEAL!! It is an even bigger deal if you are racing or exercising hard-  I am sure everyone who reads this has experienced that truth in all its vomiting glory at one point in their lives during a hard interval or race.

Therefore, high glycemic sugar is where it is at for exercising and immediately after exercising (for quick absorption recovery purposes- beyond the scope of this discussion).  Hammer Nutrition is  super researchy with their products and they have found that maltodextrin has the fastest gastric emptying time. Hammer gels use maltodextrin almost exclusively (a little fruit juice too) as their carbohydrate source and that is why I use them – most nutrition companies do not use maltodextrin at this time because it is a more expensive ingredient to use than alternative sugar sources.

Conventional wisdom is currently that one should eat low glycemic index foods during rest and prior to exercise and high glycemic (preferably maltodextrin for during) during and immediately following exercise.

Now if someone tells you to eat whole wheat bread or some crazy sh#t like that during exercise to avoid the “sugar crash” you can call BS on them and know exactly why they are whack….

7 Responses to “The Myth of the “Insulin or Sugar Crash” during exercise…”

  1. Andrew March 28, 2012 at 6:26 pm #

    “Now if someone tells you to eat whole wheat bread or some crazy sh#t like that during exercise to avoid the ‘sugar crash’…”

    That sounds like something Tre would say…

    • nkelleher March 28, 2012 at 7:33 pm #

      Hahaha…oh man andrew- you have no idea how much I can appreciate that comment…

      I know I havent met you but I check out your blog quite a bit. Thanks for stopping by mine! How your training is going well!!

  2. Rachel March 28, 2012 at 8:26 pm #

    Thanks Nicole- good stuff!

    • nkelleher March 29, 2012 at 7:49 pm #

      Hey rachel!! hope you are doing great!! Really hope I get to austin this year and would love to catch up with you…hope you guys are having fun- it always looks like you are- which makes me smile =))

  3. Mark March 29, 2012 at 6:06 pm #

    It’d be cool to get a glycemic response graph during exercise (versus at rest) to illustrate the difference…man…what a nerdy analyst comment…

    • nkelleher March 29, 2012 at 7:45 pm #

      HAHA!! so funny…literally 7 words into reading this comment I knew it was you =) I’ll look for that and see what I find..kick ass tomorrow!!

  4. sarah April 17, 2012 at 7:40 pm #

    awesome! keep the good science coming! Would love a post with your recommendations for triathlon articles to read – especially articles geared towards those looking to make the leap to Pro… 🙂

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