Carb Loading

Contributed by Dr. Steven Devor, Head MIT/FF Physiologist

A carbohydrate loading diet for endurance athletes can be a highly effective strategy to increase the amount of fuel (i.e., glycogen) stored in your skeletal muscles and liver in order to improve your running performance. Carbohydrate loading generally involves greatly increasing the amount of carbohydrates you eat a few days before a high intensity endurance athletic event. You also typically scale back your activity level during carbohydrate loading, what we refer to as the final taper phase the week of your event.

As I have discussed many times, any physical activity or exercise you do requires an adequate intake of dietary carbohydrates to provide your skeletal muscles with the necessary fuel to function at their peak capacity. For most recreational activity, your body utilizes its existing energy stores in the skeletal muscles and liver for fuel. But when you engage in long, intense endurance events (greater than 90 minutes – a half or full marathon), your body needs more carbohydrate than what is stored normally in your skeletal muscles and liver. The purpose of carbohydrate loading is to provide your tissues with the additional storage to complete an endurance event with less fatigue, and thereby hopefully improving your running performance.

It is worth mentioning that carbohydrate loading is most beneficial if you are an endurance athlete — such as a half or full marathon runner, distance swimmer, or distance cyclist — preparing for an event that will last 90 minutes or more. Other athletes generally do not require the additional glycogen that is able to be stored during carbohydrate loading. For events that are less than 90 minutes we have learned through research that it is enough to simply eat a diet that derives at least half or more of its calories from carbohydrates.

Your skeletal muscles (primary storage area) and liver (secondary storage area) normally store enough glycogen for approximately 90 minutes of endurance exercise. So if you exercise intensely for more than 90 minutes, your muscles may run out of glycogen. At that point, you may start to become fatigued in your skeletal muscles, and your performance will likely suffer.

With carbohydrate loading you are able to increase glycogen storage, what we term as “super-compensation”, in your skeletal muscles and liver. In a super-compensated state you will be advantaged to make it through longer endurance events without overwhelming muscle fatigue. However, you still will need to consume some energy sources during your event. So on race day stick your normal plan for hydration with a carbohydrate electrolyte beverage (e.g., Gatorade) and gel packets if that is what you typically practice.

There are traditionally two primary steps to properly carbohydrate load your skeletal muscles and liver. And these two steps are best followed the week your endurance event is going to happen.               

Step 1. Approximately seven days before your event, adjust your carbohydrate intake, if needed, so that it is no more than 50% to 55% of your total calories. Do this for two days. For the next two days decrease your carbohydrate intake down to no more than 40% of your daily total calories. Increase both protein and fat intake to compensate for the decrease in carbohydrate consumption. Continue your training with your normal race week tapering.

This four day dietary adjustment in your carbohydrate intake will deplete your skeletal muscle and liver carbohydrate storage, and make room for the loading phase that comes next.

Step 2. Three days before the event, increase your carbohydrate intake to approximately 70 percent of your daily calories. Do this for two days. The day before your race increase carbohydrate intake to approximately 75% of your daily calories. In order to make room for this additional carbohydrate intake, cut back on foods higher in fat to compensate for the extra carbohydrate rich foods. Once again, continue with your normal race week tapering

How much actual carbohydrate you will require depends entirely on your total daily calorie intake goal. For most endurance athletes we know that 6 to 9 grams of carbohydrate daily per kilogram (2.2 pounds) of body weight is correct.

There is the potential for mistakes with carbohydrate loading. We know that many endurance athletes that attempt to carbohydrate load fail to achieve their goal. Through the years I have observed and noted several common mistakes, which include:

1. Carbohydrate loading requires an exercise taper. Endurance athletes can find it difficult to back off training for 1 - 4 days before your race. Failing to rest will compromise carbohydrate loading;
2. Many endurance athletes fail to eat enough carbohydrate. Followng the simple guidelines above will help to get you where you need to be with regard to appropriate carbohydrate consumption;
3. In order to consume the necessary amount of carbohydrate, it is necessary to cut back to a cerain degree on high fiber foods, and intake concentrated sources of carbohydrate such as honey, jams and jellies, and fruits. Consuming too many high fiber carbohydrates may result in intestinal distress and result in a feeling of being quite bloated;
4. Carbohydrate loading will most likely cause your body weight to increase by as much as 3 pounds. The extra weight is due to the increase in muscle glycogen and water. For some endurance athletes, a fear of weight gain may prevent them from carbohydrate loading adequately. This slight increase in your overall body weight will not decrease your race performance; and,
5. Endurance athletes can sometimes use carbohydrate loading as an excuse to eat everything and anything in sight. However, we know that consuming too many high fat foods will make it difficult to consume sufficient carbohydrate. Stick to high carbohydrate, mostly low fiber and low fat foods while in the carbohydrate loading phase.

Stay well, and best wishes for your continued training success.

Dr. Devor