Blood glucose fuels only 2 minutes of exercise ? ?

[Terra Nova]

I was flipping though a book on Sports Nutrition ( I know virtually ZERO about the biochemistry of exercise btw ) During a discussion on the topic of aerobic metabolism, the author includes a chart of " Theoretical Average Energy Stores " citing the amount of calories and exercise time ( in minutes ) each potential energy source might provide. The chart values are laid out as........

Liver Glycogen...................... .......306 calories..................16 minutes of exercise.

Muscle Glycogen...................... ....1,529 calories.................80 minutes of exercise

Blood glucose....................... ........38 calories.....................2 minutes of exercise

Fat........................... ...................92, 787 calories.............4,856 minutes of exercise

My questions are about blood glucose & the 2 minutes of exercise they cite above.

1. Do these values suggest that if, for example, I got up one morning and ( assuming my blood sugars were stable ) if did a fasted session of 30 minutes of moderate intensity cardio that the glucose in my blood would only provide about 2 minutes of glucose energy to fuel the cardio ? The vast majority of fuel during the 30 minutes is therefore provided by a combination of fat and glycogen I assume ? .

2. Assuming my blood sugar was bit low in the morning, if I had a light morning snack of 100 - 150 calories about 45 minutes +/- before my 30 minutes of moderate intensity cardio ...to boost it a bit....,theoretically speaking, I assume I could then stil derive much more than just 2 minutes of glucose to fuel the cardio due to having a light pre-workout snack ? In other words, the pre-work snack serves to not only stabilize blood sugar but to ' spare ' the burning of fat and glycogen to some extent as well ?

While I think I understand the role of blood glucose to sustain brain function and various other 'non exercise ' needs and the role of the fat stores and glycogen stores to fuel exercise, I don't think I have a solid grasp of the extent to which blood glucose fuels exercise.

Anyone's thoughts toward shedding some light on this for me would be greatly appreciated.

Thanks !

[RobLL]

If you have a normal glucose function you will almost never have low blood sugars some mornings. The various other energy sources will kick in as needed. There likely are some exceptions to this but as a rule a person with a healthy metabolism can coast for a long time on their current stores.

[Trevor M.]

Originally Posted by Terra Nova

I was flipping though a book on Sports Nutrition ( I know virtually ZERO about the biochemistry of exercise btw ) During a discussion on the topic of aerobic metabolism, the author includes a chart of " Theoretical Average Energy Stores " citing the amount of calories and exercise time ( in minutes ) each potential energy source might provide. The chart values are laid out as........

Liver Glycogen...................... .......306 calories..................16 minutes of exercise.

Muscle Glycogen...................... ....1,529 calories.................80 minutes of exercise

Blood glucose....................... ........38 calories.....................2 minutes of exercise

Fat........................... ...................92, 787 calories.............4,856 minutes of exercise

My questions are about blood glucose & the 2 minutes of exercise they cite above.

1. Do these values suggest that if, for example, I got up one morning and ( assuming my blood sugars were stable ) if did a fasted session of 30 minutes of moderate intensity cardio that the glucose in my blood would only provide about 2 minutes of glucose energy to fuel the cardio ? The vast majority of fuel during the 30 minutes is therefore provided by a combination of fat and glycogen I assume ? .

2. Assuming my blood sugar was bit low in the morning, if I had a light morning snack of 100 - 150 calories about 45 minutes +/- before my 30 minutes of moderate intensity cardio ...to boost it a bit....,theoretically speaking, I assume I could then stil derive much more than just 2 minutes of glucose to fuel the cardio due to having a light pre-workout snack ? In other words, the pre-work snack serves to not only stabilize blood sugar but to ' spare ' the burning of fat and glycogen to some extent as well ?

While I think I understand the role of blood glucose to sustain brain function and various other 'non exercise ' needs and the role of the fat stores and glycogen stores to fuel exercise, I don't think I have a solid grasp of the extent to which blood glucose fuels exercise.

Anyone's thoughts toward shedding some light on this for me would be greatly appreciated.

Thanks !

1. Do these values suggest that if, for example, I got up one morning and ( assuming my blood sugars were stable ) if did a fasted session of 30 minutes of moderate intensity cardio that the glucose in my blood would only provide about 2 minutes of glucose energy to fuel the cardio ? The vast majority of fuel during the 30 minutes is therefore provided by a combination of fat and glycogen I assume ? .

Moderate intensity assumes cellular respiration aerobic metabolism. The substrate utilization for ATP formation would be split between both fat and glucose molecules(mainly from glycogenlysis reactions). The overall ratio of fat to glucose molecules used depends on the overall relative intensity and duration of the given exercise.

In which case, this means that so long as liver and muscle glycogen stores remain sufficiently stocked. Blood glucose levels should remain somewhat elevated, but, stable throughout such a short duration.

Of course, a diabetic has an increased likely hood of having a hypoglycemic episode during exercise or a person who ate a very high GI meal before exercise, Or the person who started exercise with an already reduced glycogen storage.

2. Assuming my blood sugar was bit low in the morning, if I had a light morning snack of 100 - 150 calories about 45 minutes +/- before my 30 minutes of moderate intensity cardio ...to boost it a bit....,theoretically speaking, I assume I could then stil derive much more than just 2 minutes of glucose to fuel the cardio due to having a light pre-workout snack ? In other words, the pre-work snack serves to not only stabilize blood sugar but to ' spare ' the burning of fat and glycogen to some extent as well ?

Yes, Pre-exercise and during exercise snacks can reduce glycogenlysis reaction thus sparing glycogen and increasing exercise duration.

I've read many studies that show snacking during marathons and so on can extend the marathoners capacity to continue.
Of course, the breakdown of glycogen in such a situation far out paces the formation of it. So the benefit is limited.

However, in such a short workout time as you described. A mid exercise snack would be unnecessary.
The average person can carry a max of 1,400-2,000calories of glycogen. I highly doubt this could be depleted in 30 minutes by just about anyone.
muscular fatigue sets in before glycogen depletion occurs anyway so again it is unlikely.


I'm a bit in a hurry and kinda rushed through this. If you need to know something else or have something explained a tad more in detail feel free to ask.

[Terra Nova]

Originally Posted by Trevor M.

Moderate intensity assumes cellular respiration aerobic metabolism. The substrate utilization for ATP formation would be split between both fat and glucose molecules(mainly from glycogenlysis reactions). The overall ratio of fat to glucose molecules used depends on the overall relative intensity and duration of the given exercise.

In which case, this means that so long as liver and muscle glycogen stores remain sufficiently stocked. Blood glucose levels should remain somewhat elevated, but, stable throughout such a short duration.

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

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 ?

Originally Posted by Trevor M.

Of course, a diabetic has an increased likely hood of having a hypoglycemic episode during exercise or a person who ate a very high GI meal before exercise, Or the person who started exercise with an already reduced glycogen storage.

Again....makes sense.

Originally Posted by Trevor M.

Yes, Pre-exercise and during exercise snacks can reduce glycogenlysis reaction thus sparing glycogen and increasing exercise duration.

I've read many studies that show snacking during marathons and so on can extend the marathoners capacity to continue.
Of course, the breakdown of glycogen in such a situation far out paces the formation of it. So the benefit is limited.

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. 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 ?

Originally Posted by Trevor M.

However, in such a short workout time as you described. A mid exercise snack would be unnecessary.
The average person can carry a max of 1,400-2,000calories of glycogen. I highly doubt this could be depleted in 30 minutes by just about anyone.
muscular fatigue sets in before glycogen depletion occurs anyway so again it is unlikely.

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 ?

Originally Posted by Trevor M.;522313I'm a bit in a hurry and kinda rushed through this. If you need to know something else or have something explained a tad more in detail [I

feel free to ask[/i].

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

[eastcoastsurfer]

Originally Posted by Terra Nova

1. Do these values suggest that if, for example, I got up one morning and ( assuming my blood sugars were stable ) if did a fasted session of 30 minutes of moderate intensity cardio that the glucose in my blood would only provide about 2 minutes of glucose energy to fuel the cardio ? The vast majority of fuel during the 30 minutes is therefore provided by a combination of fat and glycogen I assume ? .

2. Assuming my blood sugar was bit low in the morning, if I had a light morning snack of 100 - 150 calories about 45 minutes +/- before my 30 minutes of moderate intensity cardio ...to boost it a bit....,theoretically speaking, I assume I could then stil derive much more than just 2 minutes of glucose to fuel the cardio due to having a light pre-workout snack ? In other words, the pre-work snack serves to not only stabilize blood sugar but to ' spare ' the burning of fat and glycogen to some extent as well ?

While I think I understand the role of blood glucose to sustain brain function and various other 'non exercise ' needs and the role of the fat stores and glycogen stores to fuel exercise, I don't think I have a solid grasp of the extent to which blood glucose fuels exercise.

Anyone's thoughts toward shedding some light on this for me would be greatly appreciated.

Thanks !

I'm no expert, but I did watch this show on Discovery a few weeks ago about the human body. One of the key take aways for me wast that it takes time for the body to switch over from one fuel to another. Experienced marathon runners and the like can switch over to using fats as fuel quickly and without much notice. People who are not experienced can take longer to switch over and may not be able to keep up the exercise while the switch is taking place. Just something else to think about

[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.

[EricLikesLifting]

I believe the timing thing was a way to show the relative amounts of glucose available in the body. Clearly, fat serves as a massive reservoir of glucose in comparison to the amount available in the blood. I don't believe the author was referring to depletion of blood glucose at all. If blood glucose gets depleted, you will have much larger issues than exercising to deal with.