Confused about carbohydrate useage when doing long events/training.

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2006-09-03 6:24 PM

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Subject: Confused about carbohydrate useage when doing long events/training.
A nutrition article in the latest Inside Triathlon magazine states that we can only use about 350 calories from carbohydrates per hour. Any energy requirement more than that means the body will start burning fat, which is many time more efficient in supplying energy to the muscles than are carbohydrates. So, if you are exercising at a level of burning 500 calories per hour (probably typical for a long, slow distance run) after only 45 minutes you are going to start burning fat and not carbs. This is good because you can keep going many hours justy burning fat. You don't need to eat any energy bars or Gu. This article also implies that carb loading prior to a long, low intensity exercise or event (say running a 4+ hour marathon) may actually interfere with the crossover from carb burning to fat burning and cause a rapid drop in energy until the body can make the transition. So, to me, this flies in the face of carb loading prior to a long distance event. Anyone else see this article or have any input on the subject?

2006-09-04 9:17 AM
in reply to: #529985

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Subject: RE: Confused about carbohydrate useage when doing long events/training.

I haven't read the article (though it's laying around here somewhere), but I do have a few observations.

1.  You're using both carbs and fat for energy from the time you start.  There's no switch that gets thrown to make you go from one to the other.  Depending on the intensity of what you're doing, you may use one more predominantly than the other at a given time, but they're both active systems when fuel is available.

2.  I have no idea where they get the 350 cal/hr from carbs.  If it's available and the demand is there, carbs will be used as fuel.  There's no physiologic upper limit on that based on anything I've ever seen.  There may be issues getting it from your digestive tract to your bloodstream, but that's a whole separate issue.

3.  You are far more efficient at burning available simple carbs than you are at burning fat.  Giving your body a constant supply of that in the form of food, gels, whatever ensures that that easily used fuel source is constantly topped off.  You're not going to stop burning fat for fuel because of that.

4.  You might be overthinking things. 

2006-09-04 5:58 PM
in reply to: #529985

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Subject: RE: Confused about carbohydrate useage when doing long events/training.
I probably am overthinking this, I usually do! Thanks for the reply. What you have said certainly makes sense. After you read the article, please let me know what you think.
I am doing a 26 mile run each week and I am trying to fine tune my nutrition as much as possible.
2006-09-05 5:40 AM
in reply to: #529985

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Subject: RE: Confused about carbohydrate useage when doing long events/training.

I haven't read that specific article but from reading other stuff my understanding is that your however many energy bars/gels/drinks you consume your body is only able to utilize about 350 calories from these carbs per hour.  Basically there is a maximum rate at which your body can get the carbs from your stomach to your muscles.  So asuming you keep eating, even after hours of exercise you will still be burning some carbs - just no more than 350 per hour.  Also even if you are exercising at a level where you are burning less than 350 calories per hour you will still be using both fat and carbs as an energy source.

If you tried to run a 4h maraton with no carb suppliment during the race it is fair to say that you would experiance the bonk big time.

I'd also like to add that the figure of 350 is only an average.  This is going to depend on your size, weight, level of fitness etc.

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