Carbs & Performance: Should You 'Go Low'?


Going Low - Adjusting carbohydrate intake for athletic performance

There is so much confusion around carbohydrates right now for active people and athletes - from lower carb to keto, and train-low race-high strategies. But do any of them actually work? Should you ‘go low’? Read on to learn more about the evidence.

your nutrition should be tailored to meet your body’s needs and your training goals
— Lindsay Van der Meer

Training + Nutrition = Results

In the same way that your training type, volume, and intensity varies from day to day or when training for a specific race or event, your nutrition should be tailored to meet your body’s needs and your training goals. After all, you can’t do the same thing everyday and expect different results.

In fact, the result of training (your fitness and performance) is due to both how you train and how you fuel your workouts and recovery. Though many people notice the obvious results of training such as faster times or possibly changes to the number on the scale or size of clothing, training can lead to adaptations inside the body as well, such as how efficient your body is at using fuel and what kind of fuel it uses.

Carbohydrate Basics

Carbohydrates or carbs are your body’s preferred source of energy; they’re found in starchy foods such as bread, cereals, and grains, but also in fruits, vegetables, beans, lentils, and dairy products. The storage form of carbohydrates is called glycogen. The majority of your body’s glycogen is found in the muscles (80%) with the remainder in the liver. When you exercise, especially at higher intensities, your body prefers to use your glycogen to fuel your workout.

However, unlike shifting gears in a vehicle, your body rarely uses just one fuel source for energy at a time. Instead it is often using carbohydrates predominantly but also often uses fat as a secondary fuel source in smaller amounts.

Historically, athletes and active people were advised to follow a high carbohydrate diet all the time to support their training and performance goals. New research published during the last 5 years is shedding more light on the complex interactions between fuel and performance and the role that varying carbohydrate intake can offer beneficial adaptations to training.

Strategies that train your muscles to use fat as an energy source may make them better/more efficient at using carbohydrate when it is available and less reliant on ingesting carbohydrate while exercising.

There are several variations of modifying carbohydrate intake that may be applicable to endurance training specifically. These include:

  • Training twice a day (eg. AM run, PM ride) - carbohydrate intake is limited between sessions

  • Training fasted (eg. early AM exercise on an empty stomach) - muscle glycogen is normal, but liver glycogen is low

  • Carbohydrate restricted recovery (eg. limited carbohydrate for several hours after glycogen depleting exercise such as a long run or ride)

  • Sleep low (eg. evening training and minimal recovery food before sleep) - both muscle and liver glycogen levels drop

  • Limiting carbohydrates during training (eg. not using carbohydrate foods during prolonged 90 min + exercise)

  • High Fat Low Carb (HFLC) or Ketogenic Diets (eg. using ketones for energy)

The intention of all of these variations is to train your metabolism to be more flexible: to make your muscles more efficient at using carbohydrate when it is available and to be able to rely on fat when there isn’t any carbohydrate. The exception to this is a ketogenic diet, where the intention is to use fat as a fuel source all the time.

Research is early and ongoing in this area and has mostly focused on cellular changes, with very few studies examining performance. Of those that have examined performance, results are not consistent. Sample sizes for most studies are very low (n = 5-10 individuals) and short in duration (< 3 weeks). On the whole, the quality of the research is not yet good enough to draw conclusions that apply to most endurance athletes and active people.

Training after an overnight fast and sleeping low after an evening workout in particular have shown some promising results, but studies are not consistent. Ketogenic diets have only been shown to have performance benefits on performance in multi-day ultra-endurance events. In most research, athletes following ketogenic diets had reduced adaptation to training and ability to exercise at high intensities when compared to athletes following diets that included carbohydrates.

So, which one should I choose?

The best available evidence to date still recommends consuming carbohydrate rich foods in preparation for, during and after strenuous exercise to ensure that glycogen stores are maximized for exercise and repleted afterwards.

If you prefer to limit carbohydrate foods, it’s important to know when and how to do this to make sure you are fuelling well for your training and not compromising your recovery.

If you have questions about modifying your carbohydrate intake and fuelling optimally for sport, consult a Registered Dietitian for personalized information.


  1. Jeukendrup, A. E. (2017). Periodized nutrition for athletes. Sports Med, 47(Suppl 1): S51-S63.

  2. Burke, L., & Hawley, J., (2015). Nutritional strategies to enhance fat oxidation during aerobic exercise. In Burke, L. & Deakin, V. (5th Eds), Clinical sports nutrition (pp 463 - 492). Sydney, NSW: McGraw-Hill.