.: Ski XCOttawa.ca :: Skiing in Ottawa and Gatineau Park

Sheila's Nutrition Digest Vol. 3 – Beating the Bonk!
By:  Sheila Kealey   (2005/02/15)

Sheila's Nutrition Digest Series: In this new series, XC Ottawa member Sheila Kealey will help athletes choose the best foods for performance and overall health. Sheila has a Masters in Public Health and works in the field of nutritional epidemiology as a Research Associate with the University of California, San Diego.

"A man jumped out of the woods and hit me on the head with a frying pan!" my friend Marc exclaimed when I asked him how his Keskinada 50k ski race went. He sounded serious. He had bonked . . . and badly! While his vivid description is unique, it's always the picture that comes to mind when I think of bonking.

"Hitting the wall" or "running out of gas" is how other endurance athletes describe the overly fatigued muscles and disoriented sensation that leads to a dramatic drop in performance. A cross-country skier's dynamic glide turns into a sorry shuffle, and fast snow now feels like molasses; a runner's stride becomes a walk; and any blip of a hill looks like Mount Everest. What exactly is going on when your muscles and brain scream "no more"? Is this a situation you can avoid?


Extreme fatigue that severely impairs performance is likely a result of depleting muscle glycogen stores, liver glycogen stores, dehydration, or a combination of these. Muscle glycogen is the primary fuel for endurance athletes. Although you also burn small amounts of fatty acids and amino acids, glycogen is the major player.

What makes glycogen? Your body breaks down carbohydrates from the foods that you eat into glucose. Your liver extracts a small amount of this blood glucose and stores it as glycogen, while a larger amount of glucose is carried into your muscle cells and stored as glycogen.

  • You have a limited supply of stored glycogen in your muscles, and once you've used it all up, exhaustion sets in quickly. Without glycogen, your muscle cells have difficulty contracting, so at first you may compensate by using other muscle groups. You may have observed some of these glycogen-depleted individuals out on a race course or crossing the finish line with less than perfect form.
  • Your liver releases its stored glycogen as glucose into the bloodstream to fuel your brain and nervous system. When exercise depletes your muscle glycogen, your hungry muscles begin to draw more glucose from your blood, which leads to low blood glucose levels. At this point, your liver may not be able to maintain adequate blood glucose levels, making you feel tired, lightheaded, uncoordinated, and unable to concentrate. This low blood sugar induced mental fatigue can lead to a perception of muscular fatigue, even when muscle glycogen is not fully depleted. Your brain processes all kinds of information during an intense athletic event - your blood pressure, hydration status, stress hormones, etc. If enough of these are out of whack, it may just decide to slow or even shut your body down as a protective mechanism.
  • In addition to muscle and liver glycogen depletion, dehydration can impair your performance. During exercise, you are constantly losing water through sweating and evaporation. What some athletes interpret at low blood sugar levels or low energy stores may actually be fatigue from dehydration.


Thankfully, there is plenty you can do to avoid the dreaded BONK!

Months Before the Event

Proper Training. Your muscles can increase their capacity to store glycogen through training. Research suggests that a trained individual may store about 20 to 50 percent more glycogen than an untrained individual.

The Week Before the Event

What you do the week before a long race can dramatically influence the amount of glycogen you'll have in your muscles on the start line. With reduced training and a high carbohydrate diet, you can maximize your glycogen stores.

Training. You can help conserve your stores of glycogen by reducing your training volume the week before a big event. If this isn't a key race or your training schedule doesn't permit this (for example, cycling stage races, or National or World cross country ski championships with week long racing ending with the distance event), try to follow good nutrition strategies to ensure optimal glycogen storage (see Sheila's Nutrition Digest Vol. 1 - Refueling for Recovery.)

Diet. You can "superload" your muscles with glycogen by carbohydrate loading. The original carbo-loading protocol discovered in the late 1960's featured a difficult glycogen depletion phase through intense exercise and a low carbohydrate diet followed by a loading phase of rest and a high carbohydrate diet. Subsequent research suggests that the depletion phase may not be necessary: you can maximize your muscles' glycogen storing capacity by tapering your training and increasing the amount of carbohydrates you eat in the 3 or 4 days leading up to your event. Experts recommend getting about 60 to 70 percent of your calories from carbohydrates. To do this, eat a low-fat diet that emphasizes foods like whole grain cereals, breads and pasta, vegetables, and fruits and limit protein. Sweets are o.k. too, as long as they're not high in fat.

  • Don't stuff yourself - just try to substitute some fat and protein with carbohydrates. Overeating, especially the night before your event, won't lead to increased glycogen storage and may leave you feeling uncomfortable.
  • Drink extra fluids like water, juice, or a sports drink.
  • Don't be alarmed if you gain weight - it's likely just water. You store about 3 grams of water for each gram of glycogen. The added water may even help prevent dehydration during your event.
  • Most of the carbo-loading studies have been done with male athletes. Recent studies suggest that carbohydrate loading may be less effective in women, but more research is needed.

Race Day

  • Breakfast. A high-carbohydrate breakfast including fluids is important to prevent hunger, dehydration, and maintain your blood sugar levels. Cross-country ski racers tend to favor a large bowl of oatmeal. Individuals vary considerably with what they can eat before competing. It's a good idea to practice your pre-event meal during training and not experiment with new foods on race day. Drink a water bottle full (500ml) of sports drink in the 1 to 2 hours before the event.

  •  During the Event. Drinking and eating carbohydrate-containing foods and fluids delays fatigue by keeping blood glucose levels elevated, maintaining central nervous system function, preventing dehydration, and supplying fuel for the working muscles. Start consuming a sports drink and/or energy gels with water early in the event: waiting until you feel thirsty or hungry may be too late. If possible, consume small amounts frequently (about every 15 minutes) for best absorption. The XC Ottawa Racing Team uses eload™ sports drink for training and racing.

  • How Much? The amount you need to consume depends on your size, the length and intensity of the event, the climate, and what your digestive system can tolerate. A general recommendation is to consume 30-60 g carbohydrates per hour. For reference, 500 ml of a sports drink or most energy gels have about 30 g carbohydrates, but check the labels to be sure how many carbohydrates you're getting. Sports drinks formulated for optimal absorption during exercise usually have a carbohydrate concentration of 5-7% (30g carbs/500ml) and are a good option since they supply you with fluids, fuel, and electrolytes. If you are consuming gels don't forget to hydrate as well. It's a good idea to practice eating and drinking during training to find out what works for you.

If you practice these strategies, you'll be sure to keep the "man with the frying pan" at bay, and you've set the stage for a great performance.

Interesting Reading. . .
Interested in supporting XC Ottawa or advertising on our site? Email: info@xcottawa.ca.
© Copyright 2001-2006, www.xcottawa.ca. All Rights Reserved. Contact us before re-publishing anything seen here.