[Trevor M.]

Originally Posted by Terra Nova

Thanks for the detailed reply Trevor ...really appreciate it.

haha - No problem man.

The ratio, intensity and adequate energy stores issues you cited make perfect sense.

On this issue of blood glucose and the ' 2 minutes ' . In this example I gave, of where I might do a fasted session of 30 minutes of moderate intensity cardio, do you any any idea what the bio-chemical / physiological rationale is for the author to make the claim that the glucose in my blood would only provide about 2 minutes of glucose energy to fuel the fasted cardio ?

Any thoughts as to why it isn't longer than 2 minutes - for example, maybe 5 - 10 minutes ?

I believe he is speaking in terms of blood glucose ONLY. Speaking as if the only glucose present in the body was that - which is normally floating in the blood stream.

If this is the case than I guess that would make sense?

However, seeing as how this isn't the case with any normal human being. I don't see much point in even mentioning it.

Analogy
Think of a car who's gas tank is empty but gas lines have some fuel left in them.

I believe this is what he is saying.

I don't understand his estimation of time. I mean, if the exercise consisted of walking. Walking really fast or like a snail would vastly change the rate at which that amount of fuel would last. I don't get how he comes to such a generalization.

Again....makes sense.

Haha - I'm glad I was making sense.

Interesting.

So, going back to my session of 30 minutes of moderate intensity cardio...and assume for argument's sake it burns 250 calories over the 30 minutes. And lets say I did have that pre-workout snack of about 150 calories ( mostly carbs ) about an hour before ....and it's digested before I train.

My general understanding based on what you've said is that my body will fuel that cardio ' primarily ' with energy sources derived from some combination fat and glycogen.

Right on, of course protein contributes to it all, but usually to a very small extent.

Remember though, your body doesn't "directly" use fat or carbs or protein to power anything. Its the "conversion" of these macros through many different biochemical reactions into ATP thats powers motion, among other things.

Would those 150 calories I have before cardio be reflected primarily as a bump in blood glucose ? And would my body preferentially access this excess blood glucose first before it taps ' stored ' energy sources such as fat and glycogen ?

This question is somewhat hard for me to answer...There are just so many variables to really guess what the body would do here...

So, is it safe to say, assuming stored energy source are topped up, that a small pre workout snack's primary purpose before something like a 30 minute moderate intensity cardio session is to simply ensure blood sugar levels are / remain stable and the pre workout snack should not be considered as a significant source of fuel for the cardio session itself ?

Yeah, for the most part. In this scenario, the pre-workout snack might serve more to help replenish glycogen stores as they begin to dwindle.

Though, it all depends on timing. If its been too long since the PWOS (pre workout snack) was ingested. Than its likely the body has already responded to the meal by converting many of those carbs into lipids. Assuming glycogen is near or fully taped off.

A normal healthy person should maintain a fairly normal Blood glucose profile after the ingestion of those carbs. Assuming they weren't High GI, of course.

The body simply doesn't let "excess" glucose rest in its circulatory system. Doing so can cause arterial damage, and increased free radical production.

Will do ...thanks again for all your help in trying to help me sort out this issue in my mind.

Yup, no problem.