High-carb diets have long been synonymous with endurance sports, but there has been a growing trend of athletes and coaches questioning this dietary dogma and suggesting that a lower-carb, higher-fat approach to fueling endurance performance may be better.

There are several purported benefits to a lower-carb diet, such as increasing the body’s ability to burn fat while sparing muscle glycogen (stored fuel) and promoting additional training adaptations. In contrast, the scientific research has consistently supported the use of carbohydrates for improving performance as well as its role as the primary fuel for endurance athletes.

Listening to both sides of this discussion, nearly anybody would be confused. Well who’s right? It turns out that both sides might be. This is because nutrition for optimal performance may not always be the same as nutrition for getting the most out of your training sessions!

Put another way, you will almost always perform better when you are fueled up with carbs, but you may be able to elicit a greater training response when you train with a reduced carb intake in and around your workouts.

Let’s break this down a bit further …

Despite the favorable changes that can occur, fat-adaptation strategies typically fail to provide any benefit during high-intensity (i.e. race-pace) performance (1). As it became clear that low-carb diets were generally of little benefit to race-intensity performance, the next logical step many people took was to follow a “train-low, race-high” approach. This meant following a low-carb diet for training and then racing on a higher carb intake, usually after a day or two of carb loading. Unfortunately this didn’t work so well either as it turned out that high-fat diets rapidly down-regulate the enzymes involved in burning carbs (which are needed at high intensity), and even carb-loading before a race couldn’t fully restore high-intensity exercise capacity (2).

So what’s the answer?

“Fueling for the work required” is the term often used (3), and seems to be the most prudent and sensible approach. But before you can follow that advice, you need to understand what it actually means!

When we exercise at a low intensity we get a higher percentage of our energy from burning fat and as we increase exercise intensity we increase our reliance on carbohydrate. This is in part because carbs can be burned without oxygen (anaerobically), providing a faster and more efficient source of energy than fats.

You might now be able to surmise that a low-intensity, long-duration bike ride will rely more heavily on fat burning while a high-intensity interval workout will be fueled primarily by carbohydrates. This is important to understand because if you try to do a high-intensity workout with a depleted gas tank then your performance will suffer. On the other hand, if you’re doing a low-intensity ride then there is not a great need to carb load or take in a lot of gels, sports drinks, etc. In either case the adaptations made by your body may not be maximized after the workout.

Practical Application

At the most basic level, we can think about fueling differently for high and low intensity workouts. For the lower-intensity workouts, try doing them with a lower carbohydrate gas tank than you usually would, in order to stimulate the adaptations in your body.

There are several ways to exercise with low carbohydrate stores:

• Have a low-carb breakfast and only drink water during the workout (or water plus electrolytes)
• Workout after an overnight fast
• Minimize intake of sports drinks and gels during exercise
• Perform two training sessions in a day without full carb repletion between them
• “Sleep-low” — perform a high-intensity session in the evening and only consume protein and vegetables afterward, then wake up and do a fasted low-intensity workout (5). Be careful with this one as it can add more stress to your body and is not appropriate for certain training blocks.

At the same time there are many reasons to also include workouts where you are fueled up. If you have an interval workout or hard group ride, this is the time to fill up your gas tank the night before, and take in sports drinks, gels, etc. during the workout. You will be able go faster and improve your body’s ability to burn carbs at very high heart rates (which is actually a good thing, contrary to what some might say) (4).This also allows you to practice your race fueling and train your gut to be able to effectively absorb carbohydrates.

An endlessly fascinating subject, this is really just the tip of the iceberg! Further considerations for the advanced triathlete can include planning your carb intake two to three meals ahead of key workouts, doing two workouts per day and changing your carb intake between them (fat burning is increased during the second workout of a day, even more so than during a fasted workout (6), or doing high-intensity sessions with a low gas tank (7). All of these can be beneficial, but how to best integrate these sessions requires a more nuanced approach and some additional consideration.

So, fuel for the work required, and experience the rewards of both a low and a high carbohydrate diet in your next race.

Jeff Rothschild, MS, RD, CSSD is a registered dietitian with a master’s degree in nutritional science and a board certified specialist in sports dietetics. He works with triathletes to create individualized nutrition plans that allow them to train with confidence and race with certainty. You can work with Jeff at TriFit in Santa Monica, California, a USA Triathlon certified training center, as well as via Skype. Get in touch at eatsleep.fit.

The views expressed in this article are the opinion of the author and not necessarily the practices of USA Triathlon. Before starting any new diet or exercise program, you should check with your physician and/or coach.