CycleOps Power recently received a question for Dr. Lim in regard to his Calorie Calculator...

Dr. Lim:

The premise of the article seems to be that so long as one can accurately assess carbohydrate expenditure for a given work output/duration, one can put it back through fuel taken on board.

My understanding is that the body principally stores CHO in two ways: as glucose in the blood and glycogen in the muscles and liver predominantly.

From what I have read, the consensus on the split here is about 25% glucose and 75% glycogen.

I have also often read that to all intents and purposes, glycogen cannot be 'topped up' while on the go (riding, racing, training etc) as the conversion process from ingested CHO into muscle glycogen is an involved one that takes time, hence the need to 'carb-load' several days out from a long race. I am led to understand that what is topped up is glucose as this can readily be affected, as any diabetic will agree.

So, my central question is: Is glycogen indeed the lion's share of our stored carbohydrates and can it, or can it not be affected in the short term via drinks/gels/bars etc? If I am wrong and it can be added to, then why is it important to have a wide aerobic base for endurance events as fat utilization would not then be as important?

-Peter B.

Peter,

Thanks for your question. First, the body principally stores carbohydrate in the body as muscle and liver glycogen. In contrast, the amount of carbohydrate or glucose content in blood is quite small. Assuming a normal blood glucose of 100 mg per deciliter of blood (100 ml per deciliter) and a person with a 5 liter blood volume (5000 ml), then at rest, there is only about 5000 mg, 5 grams, or about 20 Kcals (about a small pack of sugar) worth of glucose or carbohydrate circulating in the blood. In contrast, a 70 kg endurance athlete may store anywhere from 1500 to 3000 Kcals worth of carbohydrate as glycogen in muscle and liver. Thus, stored glycogen (endogenous) and the food we eat (exogenous) while we exercise are not only our primary forms of energy for working muscle, they are also the main way we maintain our blood sugar — a variable that is tightly guarded and vital for nervous system and brain function.

While it is true that muscle glycogen cannot be topped off while exercising, especially high intensity exercise, the food we ingest can help “spare” muscle glycogen. That said, it isn’t blood glucose that is necessarily being topped off, it is the maintenance of one’s blood glucose level through eating that allows carbohydrate, fat, and proteins to be transported to working muscle and used as an exogenous source of energy. In effect, blood is a vital conduit of fuel. As a crude example, if a cyclist burns 1000 Kcals in an hour, eats nothing, and assuming the intensity is high enough that carbohydrate is the primary fuel source, then 1000 Kcals of stored glycogen will be used to maintain that exercise intensity. If on the other hand 500 Kcals of carbohydrate is ingested and is able to get transported across the small intestine during that hour, then only 500 Kcals of stored glycogen will be consumed. While the math isn’t quite that simple, suffice it to say that our intent with feeding our athletes and in adapting them to handle a large quantity of food while they race is to tip the scales on glycogen sparing. The reason is that once, muscle and liver glycogen are gone, it becomes next to impossible to maintain blood glucose, a high exercise intensity, and important metabolic processes important for utilizing other fuel sources like fat and protein.

So to answer your question, glycogen is indeed the lion's share of our stored carbohydrate, and it can be affected during exercise in the short-term by drinks, gels, and bars. However, the amount of “exogenous” carbohydrate that can be transported before one hurls, is limited. At best, I find that our guys can uptake only about 500 kcals an hour. So over a 5 hr, 5000 Kjoule day, they are just making it to the end with both exogenous and endogenous fuel. Thus it is extremely important that fat is also used to help spare glycogen. Because the use of glycogen is dependent on the relative intensity of exercise as one gets relatively fitter, more fat is burned at a given absolute exercise intensity and more glycogen will be spared allowing for top end energy reserves for that late-in-the-day attack and win.

Allen