It's been too long since I've written anything practical! I'm going to try to explain how the runner's body uses different sugars and then in a following post, the different types of sweeteners and sugars one can buy and what they contain. Parts of this will be very technical, but I'll try to minimize them. [They'll be bracketed like this.]
Exercise energetics in a nutshell: Resting muscles burn fat; burning fat requires oxygen. Exercising muscles prefer to burn sugar to burning fat and, when contracting faster than you can supply them with oxygen, burn only sugar. Running a lot of long slow miles burns fat (and this is often the only reason a person will run), but unless one ingests fewer calories than are burned, the fat gets replaced. Training by running at a faster pace causes the body to adapt by becoming more efficient and thus burning more fat and less sugar at a given pace. Training at an exhaustingly fast pace causes the body to adapt by storing more sugar and by shifting some of the energetic needs of the muscles to the liver.
The body has an absolute requirement for this sugar; your red blood cells can use nothing else and your brain has to have it as well. If you don't have any in your system, your body will make it out of whatever is available. It's the basic currency of sugars in the human body. A typical 2000 calorie diet will contain about 200 grams of glucose.
If you ingest a large amount of glucose, the sugar will have three different fates. First, whatever immediate energy needs the body has will be taken care of by burning this sugar. Second, some of it will be stored in the form of glycogen in muscles and liver for future use. Third, whatever is left over will be turned into fat; this fat can not be turned back into sugar, so one needs to keep supplying the body with a source of glucose.
The same typical 2000 calorie diet will contain about 100 grams of the sugar fructose. This sugar is utilized differently than glucose. Muscles do not have the required machinery to use fructose for energy, so it has to be used elsewhere or converted into glucose elsewhere before the muscles can use it.
In the liver (under normal conditions), fructose cannot be easily turned into glucose. [It gets converted to fructose-1-phosphate, then to glyceraldehyde and dihydroxyacetone phosphate. The glyceraldehyde gets phosphorylated and can then recombine with the dihydroxyacetone phosphate to form fructose 1,6 bisphosphate, which can in turn become: fructose-6-phosphate, glucose-6-phosphate, glucose-1-phosphate, UDP-glucose, then glycogen. Whew!] Instead, it gets broken into smaller parts and used for energy. These smaller parts can be exported to muscle cells for use there.
The majority of fructose ends up going directly to fat cells, which cannot make glucose or glycogen out of it, either. It is used to make more fat (that's what fat cells do). This fat can then be exported and used by muscles, if needed.
There are dozens of different sugars, but most occur in very tiny quantities. There are two that occur in large enough quantities to warrant mention: galactose and ribose. These sugars are available in forms one can buy, so it's worth looking to see if there's a biochemical "shortcut" one can exploit for energy needs.
Galactose can be converted into glucose in the liver and then get exported or turned into glycogen. The only dietary source of this sugar comes from the breakdown of another sugar, lactose, found in milk. Adults have a widely varying amount of the enzyme used for breaking lactose into galactose and glucose; that which cannot be broken down goes into the intestine, where bacteria feed off of it (causing gas) or it gets excreted (often as diarrhea).
Ribose is used as part of DNA in every cell, so it's everywhere, but in very small quantities, compared to glucose and fructose. The amount of ribose one ingests is negligible, though it has recently become available as a dietary supplement, so it is theoretically possible one could use it for energy. If one were to ingest a large amount of ribose, the body has a system (the pentose phosphate shunt) that allows it to be used. It's very complicated.
[ Under aerobic situations, 3 ribose-5-phosphates become 2 fructose-6-phosphates and 2 glyceraldehyde-3-phosphates. Then, 4 fructose-6-phosphates plus 2 glyceraldehyde-3-phosphates plus water become 5 glucose-6-phosphates and one free phosphate. Then glucose-6-phosphate plus 2 NADP plus water become ribose-5-phosphate, 2 NADPH, 2 hydrogen atoms and carbon dioxide. The sum total of these reactions is glucose becoming carbon dioxide.
Under anaerobic conditions, ribose becomes ribose-5-phosphate, which becomes fructose-6-phosphate and glyceraldehyde-3-phosphate. Fructose -6-phosphate becomes fructose 1,6-bisphosphate, which then splits into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. Dihydroxyacetone phosphate becomes glyceraldehyde 3-phosphate. All this glyceraldehyde 3-phosphate gets turned into pyruvate, which can be used by muscles to make lactate, which then gets shuttled back to the liver, which can reuse it to make more glucose.]
Not that you needed to know that! Trust me, there will be useful information forthcoming.
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