[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.