Pros and Cons of Training with Low-Carbohydrate Availability for Ultrarunning

Coaching is a push-and-pull between tradition and innovation, between “If it ain’t broke, don’t fix it!” and “Newer is better!”. During my coaching career I’ve been on both sides of that divide, always seeking innovation but wary that innovations that sound good don’t always lead to improved performance. Training when on a low carbohydrate (CHO) regiment is getting a lot of attention right now, so let’s take a look at what it means for you.

The idea behind “Train Low” is to deliberately go out for a training session when you have partially-depleted CHO stores, when you’re low on CHO energy. But there are questions athletes need answers to:

Why Train Low?

The reason most cited for this approach is to increase an athlete’s reliance on fat for fuel, thereby forcing the body to improve its ability to burn fat for fuel. If you can burn a higher percentage of fat at all intensity levels your CHO stores will last longer during training runs and competitions. That means you’d have more high-octane fuel (CHO) for high-intensity efforts when it counts. Sounds great!

Ultrarunners are particularly drawn to the concept of fat adaptation because the overall intensity level during ultramarathon competitions is relatively low. At lower intensity levels the percentage of energy coming from fat is higher, whereas CHO contribution to total energy output increases as intensity increases. It stands to reason, then, that improving your ability to oxidize fat for energy would be beneficial for long events at low intensity levels. And while that’s true, it’s important to put the potential benefits in perspective.

• Aerobic training increases fat oxidation: Just improving your cardiovascular fitness improves the amount and rate of fat oxidation during exercise. Training with low CHO availability may bump this up even more, but focusing on improving fitness will generally do you more good than focusing specifically on optimizing fat oxidation.
• CHO restriction can compromise training effectiveness: In an effort to manipulate CHO availability athletes are likely to experience diminished performance – at least for a period of time – during training. Athletes and coaches need to evaluate whether the potential benefit of increased fat oxidation will offset the losses incurred by a period of less effective training.
• Negative energy balance contributes to overtraining/under-recovery: The effort to restrict CHO can dramatically alter caloric intake, and many athletes encourage this in order to achieve some weight loss along with fat adaptation. Negative energy balance (more expenditure than intake) for a prolonged period of time is a major contributor to overtraining/under-recovery, which will do more damage to your season and performance than any amount of fat adaptation could enhance it.

By this point you probably think I’m totally against Train Low methods, but that’s not true. I think there’s a place for them, but that the vast majority of athletes have more to gain by just committing to the fundamentals of solid training, nutritious foods comprised of all macronutrients, and plenty of rest.

How to Train Low?

There are a lot of ways to reduce carbohydrate availability during exercise, but the two that are most common are starting a workout without replenished CHO stores or not consuming carbohydrate during prolonged workouts. The first scenario can be achieved by exercising first-thing in the morning (before breakfast) or training twice in a day without replenishing CHO between workouts.

What happens when you Train Low?

Your pace during interval workouts will be slower than during a similar interval workout with high CHO availability. When it comes to endurance runs you always slow down as the run gets longer. With low CHO availability you will slow down sooner and more substantially. Your perceived exertion will be higher. Essentially, the end of a 3hr low CHO run will feel like – and your muscles will be in a similar condition to – the end of a 5hr high CHO run.

This somewhat logical consequence of training with low CHO availability has led to the idea of Train Low/Race High and subsequently Train Low/Train High methods where you do lower-intensity endurance runs with low CHO and prepare to perform interval workouts or race with high CHO availability.

But does it work?

Yes and no. For instance, if you are going to Train Low studies (Akerstrom et al., 2009; De Bock et al., 2008) found that training without consuming CHO during workouts doesn’t enhance the adaptations we would normally see from those workouts. This suggests that a lack of CHO replenishment prior to a workout is a more effective way to Train Low than restricting CHO feedings during a workout.

The bigger problem is that while Train Low philosophy has positive impact at the cellular level it’s impact on real-world performance is less certain. In short, it improves performance for some athletes and not others, because in the real world you compete with your whole body and performance is also influenced by the cardiovascular, central nervous, and endocrine systems.

So, WHO is Train Low recommended for?

Intuitively, athletes think that the fittest riders are the ones that would benefit from new techniques like tweaking CHO availability. Pros who are already close to their maximum potential and are looking for tiny advantages can benefit from Train Low methods. In fact, they were doing it long before it was cool!

The other group that could benefit from Train Low is less obvious: time-crunched athletes. This group typically has 2-3 interval workouts a week and a longer weekend endurance ride. There may be some benefit – and no real harm – to completing this longer weekend run before breakfast (low CHO availability) for a few weeks. Just know you won’t feel great, your perceived exertion will be higher, and you should take food with you (remember, low CHO stores seem to be more important than low blood sugar). I still recommend completing weekday interval workouts with high CHO availability, however. Experiment with one portion of your overall training plan and stick with proven workouts for the rest of it.

I don’t recommend Train Low for runners on higher-volume training programs. It’s not because it won’t work, but rather because it introduces a lot of risk. If you already have a high workload there isn’t a lot of room for error and you haven’t reached the point where all the fundamental aspects of performance are already optimized (like the pros). Time-crunched athletes can get away with Train Low because their training workload and workout frequency are lower so they can better absorb and recover from mistakes. When ultrarunners on high-volume training programs make mistakes with how and when Train Low methods are utilized, it often does more damage and is harder to recover from.

Jason Koop
Author of “Training Essentials for Ultrarunning”