Carbohydrate loading for endurance – still a good practice?

Having a basic understanding of how carbohydrates are used during exercise can take your training and performance to the next level, and carb loading is a piece of the puzzle

Brittany Johnson

By Brittany Johnson
March 2nd, 2022 | 6 min read

Carbohydrates – to load or not to load?

Back in high school, my water polo team hosted spaghetti nights, where the team would load up on carbohydrates the night before a big game. Maybe you’ve had spaghetti nights of your own, or perhaps heard of carbohydrate loading somewhere along the way, but is it still the best practice? 
Having a basic understanding of how carbohydrates are used during exercise can take your training and performance to the next level, and carb loading is a piece of the puzzle. Let’s start by defining carbohydrates, then discuss how the body uses carbohydrates during exercise and explore what the research says about carb loading for performance.


Types of Carbohydrates

Carbohydrates are found in grains (e.g., oatmeal, farro, barley, quinoa), fruit (e.g., whole fruits and fruit juice), milk, beans, starchy vegetables (e.g., potatoes, peas, and corn), sweets (e.g., cakes, cookies, candy), and sugary beverages.

There are two main types of carbohydrates when we think practically: simple and complex. Simple carbohydrates are foods lacking fibre, such as fruit juice and sweets. Complex carbohydrates contain fibre, such as oatmeal and whole fruit. Carbohydrates are rated on a scale called the glycemic index (GI), which represents how quickly a carbohydrate-containing food increases blood sugar levels after ingestion. Low glycemic foods include green vegetables, most fruits, beans and whole grains. High glycemic foods include white rice, white breads, and white potatoes. For health benefits, such as reducing the risk of Type 2 Diabetes, our diets should contain mostly complex/low GI carbohydrates. However, in some cases, such as playing sports or exercising, choosing simple carbohydrates/high GI foods is more optimal. We will discuss that in the next section.

What’s the science on carb loading (aka – eating that plate of pasta)?

The concept of carb loading aims to increase the stored muscle glycogen in an effort to prolong endurance and/or improve performance. Low glycogen training has been shown to negatively impact exercise intensity in both anaerobic (doesn’t need oxygen to produce energy, e.g. resistance training) and aerobic (needs oxygen to produce energy, e.g. endurance training) exercise. Maximising glycogen stores can reduce fatigue during both anaerobic and aerobic exercise. However, most of the promising research shows results in aerobic, endurance-based activities.

Carb loading for short-duration activity
Very few studies have investigated the impact of carb loading on short-duration anaerobic performance. A study in male basketball players found no difference in peak power after seven days of carb loading following a four-week low carbohydrate diet. While carbohydrate loading didn’t produce higher peak power, the results from the study have implications for the importance of adequate glucose availability. After following the four-week low carbohydrate diet, the players’ performance decreased. However, after seven days of carbohydrate loading and replenishing glycogen storage, peak power returned to baseline. This means carbohydrate loading following a four-week low carb diet is effective at recovering baseline anaerobic power, emphasising the importance of glucose availability for optimal performance. Another research study looked at the impact of carb loading on jump squat power and found no improvement. So, we can conclude carb loading is not necessary for anaerobic, short-duration exercise but the amount of glucose available at the start of an activity is an important factor.

Carb loading for long-duration activity
The effects of carbohydrate loading on long-duration endurance exercise have garnered much more attention in the research world. However, the results differ depending on the event distance/duration and population studied. It’s been found that after an 80-minute rugby game, glycogen stores are no different in players who carb-load for 36-hours before the game to players who don’t. In other research based on individual running race times, carbohydrate loading failed to improve times for 10km and 25km treadmill runs. However, carbohydrate loading prior to a 30km cross-country run and a 30km treadmill run limited fatigue in well-trained athletes. Therefore, carbohydrate loading extends the time to glycogen depletion in running events longer than 30 km, ultimately allowing athletes to maintain race speed for a longer duration and improve race times. It appears endurance activities greater than 90 minutes support the practice of carbohydrate loading as an effective strategy to improve performance and exercise capacity.

Gender differences
There appears to be some discrepancies between women and men using glycogen stores during activity. Women tend to have lower resting muscle glycogen concentrations, which can impact the benefits of carbohydrate loading. Research has shown large differences in these benefits when comparing males and females. When both sexes followed a high carbohydrate diet (~75% of calories from carbohydrate foods), cycling performance increased by 45% in males and only 5% in females. Further, males managed to increase their glycogen stores by 41%, while there were no changes among females.

Knowing this, it may be beneficial for women to increase total calorie and carbohydrate ingestion during the loading phase to maximise glycogen stores.

Other considerations

An important factor to consider for carbohydrate utilisation during endurance exercise is the glycogen availability beforehand. Skeletal muscle has the ability to increase the amount of glycogen stores through training and diet. However, this does not happen overnight and needs 24-48 hours of high carbohydrate intake to occur.

While carb loading tends to favour performance benefits for endurance events greater than 90 minutes, it is just one dietary manipulation method in the tool kit for athletes. Athletes should plan their carbohydrate intake based on sport-specific and goal-specific outcomes. Overall carbohydrate intake is important pre-, post-, and possibly during (events lasting longer than 60 minutes) physical activity.

Another benefit of adequate glycogen availability for endurance exercise is improved skeletal muscle repair and recovery. Higher glycogen stores improve post-exercise muscle recovery. So, not only will that extra serving of spaghetti help you tick off that bucket list item of running a marathon, but it will aid in your recovery afterward, too – and you’ll need all the help you can get!

Summing up the pros and cons of carbs

The current research doesn’t have a clear answer, but promising results appear to benefit endurance exercise greater than 30km distances. Different results are likely contributed to small sample sizes, sport-specification, and population (e.g. highly trained athletes/males). Further, protocols for carbohydrate loading differ in research and practice. In most practices from our sport teams growing up, carb loading took place one night before the event, whereas research protocols vary, with seven days of carb loading and tapering training sessions (gradually reducing the amount of training) for 24-48 hours leading up to the event. Maybe we should have a week of spaghetti nights instead of one night before to increase glycogen concentrations more effectively! More seriously, conclusions drawn from the research imply carb loading is only one piece of the dietary puzzle that can advance performance for endurance athletes.

So, what are the practical applications for increasing glycogen storage? 
The recommended dose for higher carbohydrate intake is 8-12 grams per kilogram of body weight. For example, an athlete who weighs 85kg should consume around 680-850 grams of carbohydrates. Consuming high GI foods appears to promote an increase in glycogen stores in athletes. The most important factor for carbohydrate loading is meeting the overall daily individual carbohydrate needs.

Carb loading timing strategies to increase the amount of glycogen stores include: 

  1.     Exercise resulting in exhaustion followed by several days of high carbohydrate intake 
  2.     An exercise taper combined with several days of increased carbohydrate intake
  3.     A 24-48-hour high carbohydrate intake for trained athletes

While carb loading can be an effective strategy to improve your endurance performance, a holistic approach to carbohydrate availability is most important.

Key takeaways

  • Consume adequate carbohydrates before, during and after exercise 
  • Plan to consume 8-10 g/kg of carbohydrates for at least 24-48 hours before an event to maximise glycogen storage
  • Meet your overall daily carbohydrate needs for sport-specific performance
  • Women should increase total calories and carbohydrate intake during the loading phase
  • Brittany Johnson

    Brittany Johnson

    Brittany is a Scientific Affairs Manager at GNC (General Nutrition Centers), reviewing scientific evidence supporting sport nutrition supplements. As a certified specialist in sports dietetics, Johnson’s primary focus is understanding how to best fuel athletes for optimising health and enhancing performance.

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