Race Day Fuel!

Q: My question is about carbo loading for endurance races like a marathon, or the Birkie:  With the development of electrolyte-replenishing sports drinks like Gatorade and Powerade, and concentrated carbohydrates in the form of gel-paks, like GU, is it still necessary or important to carbo-load in preparation for the event?  It seems like I might just be packing on extra pounds that I’ll have to get rid of later.
And, a related question:  I noticed that the GU packet I used for my race this weekend actually contained 20 mg of caffeine.  If I am ingesting 6 of them for a four hour marathon (or perhaps more for the Birkie), that’s like drinking four Mountain Dews during the race, which I would never think of doing, because of the diuretic effects of the caffeine.  I have heard of some positive benefits of a little caffeine on race day, but this seems like too much of a good thing.  Am I inadvertently dehydrating myself by using a carbohydrate gel chock full of caffeine?

Answered by Dr. Abby Larson:


Thanks for the questions. To answer your first question: Carbo-loading once or twice a year for an important ski marathon event almost certainly won’t cause you to gain weight. With that being said, if you plan on racing a marathon each weekend you may want to rethink your carbo-loading strategy. Generally in the days leading up to an important race we decrease training volume which essentially spares muscle glycogen and allows us to use what carbohydrates we are consuming as a means to further build up those muscle and liver glycogen stores.  Although it is commonly thought that it is necessary to stuff yourself with carbs on the days leading up to your big event, I wouldn’t recommend this as it can lead to bloating and GI upset. About three days prior to race day,

I would suggest decreasing the volume of usual training and eating the same number of calories as usual but about 70% of those calories should come in the form of carbohydrate (preferably from whole grains and not refined sugars). By tweaking the composition of your diet and maintaining caloric intake you will avoid bloating, weight gain, and possible GI upset but will have adequate carbohydrate to allow for full glycogen repletion. The problem with relying on exogenous sources of glucose (GU and sports drinks) DURING the event is that your muscles will still preferentially be using stored muscle glycogen for energy. This is because it is more efficient to use glycogen for energy production compared to circulating blood glucose. Once you use all of your glycogen stores in a given muscle you will begin to rely more heavily on glucose in the blood but since that process is less efficient it means you will be less efficient. Blood glucose can’t be used to resynthesize muscle and liver glycogen during exercise, this process can only happen at rest.

The real purpose of sport drinks and gels during exercise is to maintain blood glucose thereby postponing the ever-dreadful “bonk”. This is different than being glycogen depleted in a specific muscle group (such as the quadriceps).  When you “bonk” it usually affects your central nervous system and you become a bit “loopy”, gels and sports drinks will delay this, but when you deplete your glycogen stores in a muscle, gels and sport drinks will be of little help and that muscle will no longer contract and relax as quickly or powerfully. To summarize, fill the tank with high octane fuel before the race, don’t overflow the tank because it won’t do any good, and try to top off the tank while you are racing…..

The amount caffeine that you would ingest in 6 gels is about 120 mg (if each contained 20 mg) – that’s about what you would find in 8 oz of coffee. I know a lot of athletes, myself included, that can’t even get their race number on without at least 16 oz of coffee. The diuretic effect that this amount of caffeine has is negligible, even for the unhabituated caffeine consumer. I wouldn’t be concerned about the amount of caffeine in gels from a hydration standpoint, as most research indicates that caffeine isn’t a very potent diuretic, but I would be concerned about stomach upset. The combination of highly concentrated carbohydrate and caffeine can cause GI upset in some individuals. Caffeine is a proven ergogenic aid for short and long duration events. The ergogenic benefits stem from its stimulation of the central nervous system. Urban sport legend touts caffeine as being able to enhance fatty acid utilization and demonizes it for being a diuretic – both of which are unsupported by the vast majority of research. This being said, if the combination of carbohydrate and caffeine doesn’t cause stomach upset then I wouldn’t hesitate to use caffeinated gels liberally on race day.


– Dr. Abigail Larson  Central Washington University, 2006 Nordic Olympian


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