In this article we will take a look at some of the more salient points regarding the body's use of glucose and fructose, a continuation of a previous discussion. The fundamental guiding principle of this discussion should be the fact that glucose is the energy currency of brain and muscle cells, and fructose is only usable by the liver cells for conversion to glucose or lipids.
1. High blood glucose levels, say following a meal, signal for insulin release. Insulin in turn signals cells to take up glucose, and also serves as a signal to stop using fat as an energy source.
2. For sugars to be utilized by a cell they must first be phosphorylated, which prevents their diffusion back across cellular membranes. The phosphate group comes from the reaction
Sugar + ATP -> Sugar-P + ADP
3. Interestingly, muscle cells do not contain the enzyme glucose-6-phosphatase, which is present only in the liver cells, and is responsible for converting phosphorylated glucose back into glucose so that it can leave the cell membrane. Thus, once glucose is in a muscle cell it is there for good. I think this explains why we get dizzy and lose coordination as exhaustion set in - the brain cells have little glycogen reserves and take most of their glucose directly from the blood. The muscle cells, having no choice but to utilize the glucose that gets to them, are in direct competition with the brain for glucose. The brain's only defense then is to signal to the muscles to stop moving, i.e. the "heavy-legs" feeling. The only way out of this is to provide more glucose to the brain, which means either cutting back on the muscles' usage of glucose, or introducing more glucose to the blood stream.
4. Glucose is stored as long polymers called glycogen, which is a ready-to-use energy storage configuaration. Catabolism of glycogen into glucose is stimualted by epinephrine (addrenaline) and glucagon. Glucagon is released by the pancreas when low blood glucose levels are detected and signals the liver to start releasing glucose from its glycogen stores to feed the muscles and brain. Ephinephrine is regulated by the central nervous system and with norepinephrine (a psychoactive hormone) causes the body the "fight or flight" response.
1. Fructose is not readily absorbed by the small intestine, compared to glucose and sucrose (sucrose=glucose+fructose). However, sucrose can be absorbed fairly quickly in the small intestine, close to the rate of glucose. Also, uptake of fructose, occuring only in the liver cells, is not regulated by insulin.
2. Fructose will readily be converted into glycogen in the liver if liver glycogen stores are low, at rates that exceed that of the glucose to glycogen conversion. However, if liver glycogen stores are normal, then fructose will be converted into lipids (fats).
3. Fruktokinase is the enzyme required for the metabolism of fructose which, interestingly, is only found in liver cells and sperm cells. This makes me think that the fertility of men may somehow be related to the abundance of fructose in the diet. For a gestation period of 9 months, a summer conception really offers about the best chance of survival for a child, being born in the spring and having 9 months under its belt by the time of its first winter.
4. Very small amounts of fructose motivate the activation of glucokinase, which facilitiates the phosphorylation of glucose for production of glycogen. Remember, the fructose is only active in the liver, so fructose is motivating the formation of glycogen in the liver. This means that glucose release to the muscle and brain cells will be impaired by the presence of fructose.
[Wrench edit: See my thoughts about fructose and post-race feeding in the comments section.]
1. Pre-race: According to Keith, at the TransRockies races there was about 2 hours between breakfast and go time. That's a bit short for a heavier breakfast. Probably the best thing here is a good cup of coffee, a solid pile of carbs and some fruit.
2. In-race: We want to be supplying our muscles with a steady source of glucose, but not so much that the insulin causes the fat metabolism to decrease significantly. Ingestion of fructose, causing the liver to retain its glucose as well as being not directly usable by the muscles, makes little sense. Probably the best mixture would be something like 25% glucose, 50% maltodextrin (glucose-glucose dimer), and 25% starch. This combination should ensure that glucose is readily delivered to the muscles (glucose), that the insulin levels are not increased dramatically(maltodextrin), and provides some long-term energy to buffer the drop in blood sugar levels due to the irregularity of eating (starch).
3. Post-race: Immediately following the race we need to replenish our glycogen stores first in the muscle cells, a post race snack high in glucose and absent of fructose is best. Higher levels of insulin, in addition to signaling the uptake of glucose, have the added benefit of stimulating DNA replication, amino acid uptake, potassium uptake, and protein synthesis. Once the muscle cells have a good store of glucose to work with, say after about an hour, we should ingest some fructose in combination with the glucose to motivate the liver to start the storage of glucose in the form of glycogen for tomorrow's race. All of this should be be followed by a well rounded meal and plenty of rest.
Ranking of the available gel products
Lowest Fructose Concentrations:
Luna Moons: brown rice syrup (glucose, maltose, complex carbs), cane juice
Hammer Gel: maltodextrin, fruit juice concentrates
Cliff Shot Gel: brown rice syrup, fruit juice concentrates
Moderate Fructose Concentrations:
GU: maltodextrin, fructose
Carb BOOM: maltodextrin, fructose
Accel Gel: maltodextrin + high fructose corn syrup
Power Gel: maltodextrin + fructose
Highest Fructose Concentrations:
Enervit: Italian company that can't be bothered to supply nutritional information. Their gels contain fish products.
Image credit: www1.br.cc.va.us/biology/lab/chemistry/procedure.htm