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Fuel Or Fast? When Less Carbs Can Be More

  • By Paul Moore
  • Published July 26, 2011

Carb loading is a staple piece of advice pedalled to every triathlete. But is it actually accurate? Recent research suggests that it might be anything but beneficial.

Written by: Gregory Cox, R.D.
Advice from sports nutrition professionals over the past 40 years has remained essentially the same: “Load up” before exercise; consume adequate fluid and fuel during exercise; and attend to post-exercise nutritional recovery as soon as possible. But could straying from that typical plan lead to a better race-day performance?

In the ’60s, exercise physiologists first discovered that higher pre-exercise muscle glycogen stores allowed subjects to exercise longer. In the past five years, however, a stream of research has investigated the effects of decreasing fuel (namely carbohydrate) availability before and during training to see if it has an effect on how the muscles adapt to increased training loads, frequency and intensity.

In response to regular endurance training, the muscles’ ability to burn fat is improved. In turn, this spares muscle glycogen (stored carbohydrate), a valuable fuel for high-intensity efforts. In a recent report from Belgium, researchers compared untrained subjects training in either a fed or fasted state over a six-week training block. They found that the fasted, low-carbohydrate subjects showed greater improvements in fat burning. In addition, the fasted subjects did not suffer from the exercise-induced drop in blood glucose compared to those who always trained under fed conditions.

Does this mean you should always fast for all workouts? Definitely not. But there are specific workouts when your body will benefit most from fasting.

When to Fast
For key sessions, such as a 40K time trial on the bike or an intense interval un session, you should stick with high-carbohydrate availability. This doesn’t mean “carbo-loading”; it simply means having carbohydrates in the system — eat a snack before starting or consume a sports drink or gel during, as you would while racing. Without this available carbohydrate you won’t be able to achieve high-intensity, quality work, meaning you will struggle to meet interval times and will simply under-perform. And ultimately if you are not able to train with intensity and achieve training PBs then you will not see improvements on race day.

For endurance sessions, i.e. swims or runs over an hour and rides over 90 minutes, you have more of a choice. If your workout goal is to push the pace for a long duration, fuel it. If it’s more about “getting in the miles,” try fasting it. Training with low levels of available carbohydrate will force your body to become more efficient at burning fat for energy as well as enhance capacity for storing muscle glycogen once you do get to replenish carbohydrate again.

How to Fast
Next time you head out for an easy- paced 16K run or 50K cycle, try it without a pre-exercise snack, or change the timing so it follows another session where you haven’t had adequate time to fully recover. Exercising before breakfast is an easy way of doing such depleted training—and a method you probably already employ occasionally. Additionally, when you train heavily and more than once a day, some sessions may naturally take place when you are depleted.

Gregory Cox is a registered dietitian and has a master’s degree in sports nutrition.

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Paul Moore

Paul Moore

Paul Moore is the Online Editor for Triathlete Europe. When not glued to a computer he can be found writing books - most recently Ultra Performance: The Psychology of Endurance Sports and The World's Toughest Endurance Challenges. Both are available on Amazon. Paul has also written Ultimate Triathlon: A complete training guide for long-distance triathletes which is also available on Amazon.