Carbohydrate Loading: Does it Really Work?
Yes, it really works.
Would you rather start a cross country road trip with your gas tank at full or with the needle nearing empty? Since your muscles, liver and blood hold a limited amount of carbohydrate, “topping off your tank” with carbs prior to race day will increase muscle fuel and water content. Fat is another essential fuel source used during a marathon, but as you approach the second hour of running, carbs will eventually run out, fat stores will not. Starting the race with more carbs on board and equally important, consuming enough during the race will prevent the needle from hitting empty. If you are hoping to improve your race time and push your pace, carbs become the preferred fuel source as fat metabolism is slower and can’t keep up with the needs of intense bouts of activity (speed, hills, passing, and keeping a challenging pace). Also, keep in mind that during a marathon you are also fueling your brain and other vital organs and their preferred fuel is glucose (aka carbohydrate)
As a marathon runner and registered dietitian for the past 20 years I have seen carb loading guidelines change and evolve. I think the very best recipe needs to be customized to each individual, so this means practicing during your long runs to find what works best for you. If your training eases up a day or two before your long training runs, practice increasing your portions of easily digested carbs like cereal, bread, rice, potato, pasta, skinless fruit. This does not mean gorging just simply increase the portion and add some additional carb based snacks and fluids (juice). You may notice a weight gain of 2-4 pounds; this is a sign that your muscles are holding more glycogen and water. The morning of your long run practice exactly what you plan to eat on race morning. This should be mostly carbohydrate, such as toast with jam, cereal with milk, or a fruit and yogurt based smoothie. During your run start taking in carbs after 60-90 minutes at a rate of 30-60 grams/hour (1 gel has 25 gram carbs) so this could be one gel/hour consumed with water. You can also get carbs from sport drinks (15 grams carb/8 oz). All 3 of these factors matter (carbs 1-3 days before, breakfast the day of, and getting enough carbs every hour during your race or long training run).
Carb loading options:
Most people who try carb loading don’t eat enough to reach ‘fully loaded’ because it is quite a bit more food than you may be used to eating. One way to make this easier is to cut back on fats and protein during these 2-3 days before a race to allow more room for carbs. Have carbs in every meal and snack as well as in fluids (juice or sports drinks). Stick with carb based foods you typically eat, but increase the portions. Choose lower fiber options, as too much fiber before a race can cause stomach upset. Also avoid experimenting with new foods.
The accidental loader:
If you taper your running the week or two before your marathon, the carb based foods you normally eat will be stored instead of burned. In the 27 marathons I have run, I have found the accidentally loading option almost as effective as the fully loading option. Carb loading might be less effective for women, but most research has been conducted on men. This is why experimenting is the best way to see what really works for you.
Some people choose not to carb load and can still perform well provided they get the right fuel before and during the race. If you have no difficultly consuming 30-60 grams carb/hour during a marathon, you may do fine using the non-loading option. If however you have a sensitive tummy and find this difficult, then carb loading is a probably a wise choice.
Jacque Maldonado, MS, RD, CDE
Jacque has a B.S. and M.S. in Nutrition and is an avid endurance athlete having finished several full Ironman® Triathlons and over 25 marathons including qualifying for Boston. She is also a USA Triathlon coach Level 1.