Glucose and the Endurance Athlete
Glucose is a simple sugar or monosaccharide that is an extremely important form of sugar for your body. All other carbohydrates and forms of sugar need to be broken down into glucose before your muscles and brain can use them.
Muscles and other tissues can use fat and protein as an energy source but glucose is a much more efficient form of energy. For your brain and red blood cells, glucose is the only source of energy they can use.
During digestion, starches and sugars are broken down into glucose. Under normal conditions, the body cannot use all of the glucose produced during digestion.
To store the excess glucose your body turns it into glycogen.
Glycogen is a complex molecule made up of multiple glucose molecules linked together in long chains and hydrated with three or four molecules of water, as seen in this diagram.
The liver is an important storage site for glycogen. The liver can hold as much as 6% of it’s weight in glycogen. A well conditioned athlete may be able to store up to 8% of their liver’s weight in glycogen A healthy adult liver weights between 1.2 and 1.5KG and can hold about 100 grams of glycogen or 400 calories.
Muscles can hold up to 2% of their weight in glycogen or 350 to 500 grams. This provides 1,400 to 2,000 calories.
When energy is needed right away, glycogen stored in the muscles is converted to glucose right in the muscle. This allows for an instant source of energy. Most muscles lack the enzyme required to release converted glycogen into the blood stream, so all glucose is used in those muscles.
To replenish the muscles, glucose can be obtained through digestion or from the liver. Glycogen stored in the liver is converted to glucose and released into the blood stream for your body to use.
Glycogen in the liver is converted to glucose by gluconeogenesis when the blood glucose concentration is low. Glucose may also be produced from non-carbohydrate precursors, such as pyruvate, amino acids and glycerol, by gluconeogenesis.
But it is gluconeogenesis of glycogen that maintains blood glucose concentrations over extended periods of time, and especially for endurance athletes.
The endocrine pancreas
The pancreas has both endocrine and exocrine functions. The endocrine tissue is grouped together in the islets of Langerhans and consists of four different cell types each with its own function. Alpha cells produce glucagon. Beta cells produce proinsulin. Proinsulin is the inactive form of insulin that is converted to insulin in the circulation. Delta cells produce somatostatin. F or PP cells produce pancreatic polypeptide.
This information is from www.caninsulin.com, but I think it explains things clearly. If you click on the “Glucose Metabolism” header you can read their full article which includes some easy to understand diagrams on how the processes work. This stuff can get very technical.
Glucose and the Endurance athlete
Most runners use some form of gel, chew, beans or beverage to replenish their carbohydrates while performing an endurance activity. The supplement business has exploded over the past few years. Nestle owns the PowerBar, PowerGel product lines. Jelly Belly makes Sport Beans. Honey Stinger has a line of gels based on honey. Hammer Nutrition has the Hammer Gel line of gels.
Some have caffeine. Some have various electrolytes in various quantities. Everyone has the best combination to help you achieve at your highest level. Like most things, each athlete should try different products to see what their system tolerates best and is the most effective at delivery energy to their bodies.
All of these products contain sugar in various forms. Cane sugar, evaporated cane juice, invert sugar – all sucrose. Other ingredients include maltodextrin, glucose syrup from wheat, fructose.
I’ve used many of these products while running marathons or distances of at least 12 miles. There is general agreement that these supplements are not needed if you are running or working out for less than an hour.
For my recent Sunday Long run of 17.8 miles I tried something new. I’ve heard from other runners that glucose tablets work really well at restoring energy levels. I went to a local drug store and purchased a bottle of glucose tablets. Since I’ve never used these, I spoke with a pharmacist.
I told her why I was interested in these and how I planned to use them. She seemed a little chagrined. These tablets are normally used by diabetics when they need to boost their blood sugar levels. Here I was in my Boston Marathon jacket, the picture of health.
She told me most people take four at a time. They hit the system pretty quick since they are basically pure glucose. She warned me to be careful of my blood sugar levels but then said I probably do not need to worry about that if I’m out running.
This stuff is nothing to play with. If you sat around and ate these like sweet tarts your blood sugar would spike. The Glycemic Index of these tablets is 100, the same as table sugar. Diabetics can eat some candy to increase their blood sugar levels but glucose tablets are more efficient.
Repeated spiking of your blood glucose levels through over consumption of carbohydrates can eventually wear out your beta cells. Once they are gone or overwhelmed you have problems. These tablets are not candy.
Someone please correct me if I have this wrong.
During a marathon I often take four or more gels. Like many people, I experience “gastric distress” during a long run or marathon. Maybe it is all of that bouncing around that makes things move through the digestive process rapidly. Maybe it is what I am eating while I am running.
The Boston Marathon is about three weeks out. Now is the time to experiment with food and supplements. I had a 17.8 mile long run on Sunday and this seemed like the perfect time to try something new.
I took four tablets and put them in a sandwich bag. About four miles into our run I took the first one. It chewed very easily and dissolved in my mouth quickly. It wasn’t chalky or anything like that.
Within a few minutes my running felt less labored than it had a few minutes before. This is how my body reacts to commercial gels also. Not bad I thought.
At the first two water stops I only had a cup of water. No Gatorade, food or gels. At the third stop I had a mix of water and Gatorade and continued with that mix at the last two stops. I ended up only taking three of the tablets.
My stomach seemed to do better than normal. 17.8 miles is a long run and it’s not unusual for runners to look for a McDonald’s or Dunkin Donuts to visit on a run of this distance. I never had that urge.
I managed to get through all 17.8 miles without needing to use the facilities until I got home an hour or so later. Pretty normal.
Drawing conclusions from a single data point is pointless. I have the Eastern States 20 Miler next weekend and one more club SLR. I’ll do the experiment again and see how I feel.
The one draw back that I see to using these tablets is that they are just glucose. No electrolytes. That could be a problem. My marathons usually take over four hours, so I need to replace my electrolytes. Perhaps I could use these tablets early in the race to delay any adverse intestinal reactions to gels and Gatorade.
I could also save one or two for that last mile or so. That quick boost of energy could be just what I need to finish strong.
Run well my friends!
Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 21, Glycogen Metabolism. Available from: https://www.ncbi.nlm.nih.gov/books/NBK21190/