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Carbohydrate Manipulation For Better Performance

Use this nutrition strategy to boost physical and mental fitness when preparing for your next marathon.

Carbohydrate manipulation across your training cycle is one of the simplest nutritional strategies to boost physical and mental fitness. For more than 20 years, I’ve used this strategy on myself, as well as with many of the athletes I coach, and have had great success.

In a nutshell, you supplement with carbohydrate before and during long runs early in your training cycle. Then, you wean yourself off carbohydrates for long runs in the middle portion of your training cycle. Finally, you sprinkle in some low-carb runs in the final few weeks of training, but balance these with long runs that practice your race-day carbohydrate fueling. The end result is that you get the performance benefits from using carbohydrate before and during long runs—higher quality long runs—but also get the positive physical and mental benefits from low-carbohydrate training, including exposure to race-like suffering, greater fat burning and carbohydrate sparing.

Training Goals

When training for longer races like a half marathon or marathon, there are three main goals:

1. Increase the proportion of energy at race pace that comes from fat as opposed to carbohydrate. Race-pace running requires energy from both fats and carbohydrates but carbohydrate stores are limited. So, if at race pace you can get a higher percentage of your energy from fat, you’ll spare those precious carbohydrate stores.

2. Increase the total store of carbohydrate available. Think of this as having a larger fuel tank. You’ve probably read that carbohydrate stores typically last 90-120 minutes before they get low, so if you have a larger tank and burn the fuel at a slower rate, you’ll have more fuel at end of the race. No more hitting the wall for you!

3. Get acquainted with the “pain” at the end of long races. Racing your best half marathon or marathon is going to be mentally challenging. It will require more mental fortitude than you are often used to, so while training for the race, you actually want to experience the level of fatigue—physical and mental—where the brain is screaming at you to stop. You want to have some experiences of running in extremis.

You’ve probably read about the central governor theory whereby the brain, if it feels threatened by your rapidly depleting energy stores or tired muscles, will consciously send greater and greater sensations of fatigue and can actually cut the power to the muscles and slow you down. This leads to the “wall” or “bonking” that runners fear. Given that, some of your training must expose your brain to these feelings so it will feel less threatened on race day.

Manipulating your carbohydrate intake across your training plan can help you optimize each of the adaptations.

Case Study

Lynn, an athlete I coached, was about to embark on an 18-week marathon training plan. She had three marathons under her belt—so she knew she could finish—but was now looking to finish faster and qualify for Boston. As I built her plan, I broke it into three, six-week phases for how we would modulate carbohydrate intake to boost her fitness.

Phase 1

In Phase 1, she ate carbohydrates before (breakfast, slow-release carbohydrates) and during (sports drinks, gels) so that she could complete her long runs feeling fresh mentally with only the usual muscular fatigue from running so long. Her long run increased for three weeks, then we took a shorter long run in week four for recovery. Then, she finished off Phase 1 with two more high-carb long runs.

The goal of Phase 1 was to help her finish those runs feeling good. This would build her motivation and get her long runs building without undue fatigue.

Phase 2

With a few long runs under her belt, Lynn felt more confident in her fitness. At this point, she slowly eliminated her carbohydrate intake before and during her long runs. She was “training low” or training on low-carbohydrate stores in order to challenge her body to burn more fat for fuel (when carbohydrates aren’t plentiful, the body burns more fat) and stimulate her body to store more carbohydrates after the runs (increasing the fuel tank).

We took four long runs to eliminate the carbohydrates. First, she eliminated breakfast. In the second long run, the next weekend, not only was she building her distance but she also spread out her carbohydrate intake, taking carbohydrates only every hour instead of every 30 minutes during the run. In the third long run, she only took a gel in the last 30 minutes of the run—no carbohydrates before or during the run except that one gel.

In Week 4 of Phase 2, her long run was 45 minutes shorter than the previous week and was a perfect time to go carb-free. Lynn didn’t eat breakfast before the run and didn’t ingest any carbohydrates during the run—only water and electrolytes.

To her surprise, Lynn did fine, which is what most runners find if they wean themselves off carbs gradually like Lynn did. She was more tired toward the end of the run, but this was expected. I also added an extra recovery day after the long run just to make sure we respected this additional training stress.

For Weeks 5 and 6 in Phase 2, Lynn ran carb-free on her long runs. These were tougher because they were longer than the “down” long run of Week 4, but she quickly learned that these long runs are not about pace. They are simply about getting time on your feet.  If you get really tired, that’s good. You are forcing your body to burn more fat and you’ll store more carbohydrate for future runs. You’ll also get a hefty dose of the mental challenge that will be faced at the end of the race.

Phase 3

For the final phase of training, we had two goals:

1. Extend her carb-free long runs to her maximum duration (3-3.5 hours).

2. Practice her race-day nutrition strategy on the other long runs.

In practice, this meant that one week, she did a carb-free run where she just ran for time and didn’t worry about pace. On the other week, she carbed up before and during the long run, following the nutritional plan she’d implement on race day. This allowed her to dial in her marathon nutrition plan and she also improved the quality of the run by finishing fast the last few miles.

Powering to the Finish

On race day, Lynn ran a new PR and qualified for Boston. But, the most important takeaway for me was how strong she was over the last few miles. This is exactly what I experienced when I first used this strategy. It wasn’t that I could suddenly run faster than the pace my training indicated. It was just that I didn’t get the usual mental and physical fatigue at the end of the marathon, or at least to the extent I usually did.

Lynn had a new level of power over the last few miles. She had a larger carbohydrate store and used these stores more sparingly across the race. So, when the last few miles hit, she had this fuel available instead of having it running low.

And, because her fuel stores weren’t depleting rapidly, and because the carb-free long runs exposed her brain to the type of fatigue she would experience in the race, her brain didn’t perceive the pace to be a concern. She wasn’t going to run out of gas so her brain was happy and a happy brain is a brain willing to keep pushing.

The mental and physical fatigue make carb-free runs very tough. You will likely have “bad” long runs where you may bonk. This is precisely the training stress you’re after, as it conditions the mind to those feelings of severe fatigue and stimulates the body to adapt.


  • Not for first time runners. Use carbohydrates to help you finish.
  • Not for those with blood sugar issues (hypoglycemia).
  • Use common sense and gradually wean off of carbohydrates on long runs.
  • Plan for extra recovery (day of and an extra day after) after carb-free long runs
  • Have a rescue gel with you and a buddy with you.
  • Use carbs on race day. Carb-free is a training strategy.