Toolbox: Metabolic Fatigue
Every cyclist loves to eat, and half of the fun of cycling is in having a built-in excuse to eat in large quantities. What we put into our bodies before and during our rides, however, can have a direct impact on our performance and exercise capacity. The topic of sports nutrition is immense, but we’ll start with a primer on fuel utilization during exercise and the importance of carbohydrates.
One of the things I really focus on with my exercise physiology class is the incredible efficiency of the body, with efficiency defined very broadly. With respect to the metabolic and fuel delivery system, the human is an omnivore that can eat and digest almost anything and convert it into fuel for exercise. Compare that to the koala bear and its reliance on eucalyptus leaves, or my older son’s reliance on meat and aversion to any vegetables or fruits!
But just because we can process a wide range of foodstuffs doesn’t give us an excuse to rely on Pop Tarts and Twinkies for our diet. It remains important to eat a wide range of quality, nutrient-dense foods that provide vitamins and minerals in addition to calories. But we will leave that for another day…
Carping about Carbs
The mass public has been accused of having the attention span of a flea and the instant gratification needs of an infant. One only needs to scan through the pages of any lifestyle magazine to see the wide range of products promising instant weight loss or instant fitness, most without the need to do more than popping a pill or a six-minute exercise program.
The cycling world is no different, unfortunately. The fundamental basics of building fitness (building base, progressive increase in workload/intensity, balancing training stress and recovery) have been laid out and known for quite some time now, yet we remain bombarded with “Climb like a helium balloon in two weeks” training programs or ads to pimp our ride down to the 6.8 kg limit rather than reducing the spare tire around our middle.
The worst example of this constant need to tinker, in my mind, is the over-complication of sports nutrition and specifically the attack on carbohydrates as the source of all that is evil. The high carbohydrate diet of endurance athletes (typically 55-60% carbohydrates with about 20-30% fats and 15-20% protein) have been besieged by the periodic arrivals of low carb diets, most with an approximately 40-30-30% mix of carbohydrates, fats, and proteins. While these various diets often have laudable goals of reducing insulin levels, increasing consumption of nutrient dense foods like fruits and vegetables, and minimizing consumption of saturated fats, the underlying assumption is that carbohydrates are something to be wary of.
This study on the effects of a high carbohydrate diet (65-15-20 carbs, lipid, protein; purple bars) compared to a high lipid (21-62-17%, respectively; green bars) demonstrate the importance of carbohydrates on high-intensity aerobic exercise performance. Following 7 weeks on the diet coupled with training, time to exhaustion increased greatly in both diets, but much more so with a high carbohydrate diet. With the high lipid diet switching to a high carb diet in the 8th week, time to exhaustion continued to increase, demonstrating that the results after seven weeks were not due to subjects hitting their performance limits. From McArdle Katch & Katch, Exercise Physiology: Energy, Nutrition & Human Performance, 6th Edition, 2006.
Carbohydrates and Exercise
Yes, these ideas may be very useful for the largely sedentary public or for individuals who exercise moderately, but carbohydrates remain the primary currency of endurance sports and the ability to exercise hard day after day. Riders in the Grand Tours can easily expend up to 9000 kCal daily, and this is similar to those from other male athletes (e.g., cross-country skiers) during periods of intense training. Not only is a heavy emphasis on carbohydrates in the diet essential for restoring glycogen day after day, it is also critical for maintaining energy throughout the course of each stage.
Remember the last time you bonked big-time on a ride and crawled home? That’s not because you have run out of fat stores in your body, it’s because you’ve depleted your glycogen stores, and also because you haven’t eaten enough carbohydrates, with the net result that you don’t have ready access to a rapidly metabolized macronutrient. There’s still lots of fat in the system, but the body is not able to process it as quickly for energy, forcing you to greatly reduce your speed and start dreaming of chocolate chip cookies. The other component is that your central nervous system (i.e., brain) can only metabolize carbohydrates for energy except for extreme situations like starvation, so hypoglycemia also helps to explain the extreme irritability and psychological fatigue associated with bonking.
Hypoglycemia in action with prolonged exercise. The body generally doesn’t like to be too much below 4 mM in plasma glucose. However, with continued intense exercise, the carbohydrates stored in the body as muscle and liver glycogen become depleted. The end result is that you’re literally running on fumes, reliant on the carbohydrates you’re ingesting to keep you from bonking. Here, the placebo (red line) drink does nothing to stop the continued drop in blood glucose. In contrast, a glucose polymer drink gets absorbed rapidly into the system, resulting in a rise in blood glucose and a happy, non-bonking cyclist. From McArdle Katch & Katch, Exercise Physiology: Energy, Nutrition & Human Performance, 6th Edition, 2006.
On the Road
Before I get inundated with emails, let me reiterate again that sports nutrition is indeed much more complicated than I am making it out to be here. But the point I’m trying to get across is the big picture and the forest. The type of carbohydrates and the types of fats and proteins is indeed important (remember, I wrote above that Twinkies are not the ideal sports supplement!). But the bigger message is that carbohydrates are not bad, just like fats are not bad. And for endurance athletes training hard day after day, an adequate carbohydrate intake is essential to restock the body’s glycogen stores. Without that and without an adequate supply of carbohydrates during exercise, you will not be able to train and recover to your full capacity, ultimately limiting your fitness progression.
In future Toolbox articles, I’ll probe in more detail issues related to carbohydrate feedings prior to, during, and in recovery from exercise. Keep riding and eating (well)!
Stephen Cheung is an Associate Professor of Kinesiology at Dalhousie University, with a research specialization in the effects of thermal stress on human physiology and performance. He can be reached for comments at [email protected].