Is carbohydrate (glycogen) loading ACTUALLY effective?

For all the runners out there!!!

Quick Review:

Carbohydrates (CHO) act as the body’s primary source of energy during exercise. Energy derived from CHO’s comes from glucose and the breakdown of liver and muscle glycogen, providing fuel for the central nervous system (CNS) and working muscles.

The Importance of CHO’s for Endurance Athletes:

Optimal nutrition is essential for athletes to achieve their full potential. Athletic performance is limited by CHO availability, thus the quantity, variety, and timing of foods consumed has a large impact on an athlete’s performance. Adequate CHO intake is important for endurance athletes for numerous reasons such as it maintains high levels of CHO oxidation, enhances the CNS, prevents hypoglycemia, and maximizes skeletal muscle glycogen stores, which can later be used for fuel during exercise.

The diet of a physically active individual should consist of mainly CHO’s, equating to about 55-60% of their total caloric intake. Most sources of CHO’s consumed should come from unprocessed, fibre-rich, whole grains and fruits and vegetables.

During long durations of exercise, glycogen stores are almost entirely depleted. High-intensity, aerobic exercise for 1-hour in duration decreases liver glycogen by ≃ 55%, while a 2-hour strenuous workout nearly completely depletes the glycogen content of the liver and exercised muscles. Adequate levels of CHO’s must be consumed to replenish liver and muscle glycogen stores before and after training sessions. A low- CHO diet compromises both glycogen reserves and athletic performance. Inadequate CHO intake will leave endurance athletes feeling tired and fatigued, thus it is vital that individuals consume sufficient amounts of CHO-rich foods, at every meal when training. Additionally, consuming adequate amounts of CHO’s will spare proteins, and preserve tissue protein by minimizing protein breakdown for energy, resulting in gain of muscle mass.

What is CHO (glycogen) loading?

CHO loading is a strategy used by endurance athletes that enables individuals to maximize glycogen stores in both the muscles and liver. After CHO’s are consumed, they are broken down into glucose. Excess glucose is converted to glycogen and stored in the skeletal muscles or liver for later usage, or to assist as fuel during exercise. Glycogen can later be turned back into glucose when needed for energy.

CHO loading works to maximize one’s glycogen stores. Skeletal muscles provide the body with the majority of glycogen storage, equating to about 500 grams, while the liver provides about 100 grams of glycogen storage. CHO loading is done by reducing CHO intake and eating meals high in fat and protein, while simultaneously heavily exercising to exhaust glycogen stores. This is followed by reducing exercise duration and intensity and reversing to a high CHO diet prior to the days leading up to a long run or race. This process maximizes glycogen stores, enabling muscle glycogen stores to increase to 2-4 times their typical level. Typically, each 100 grams (g) of muscle contains 1.7 g of glycogen, CHO loading increases these stores to approximately 5g of glycogen per 100g of skeletal muscle.

CHO loading is often only recommended for athletes participating in endurance events of 1 hour long or more in duration. There is minimal benefit of CHO loading in low-moderate intensity or short-term exercise as these events do not require heavy demands on the body’s glycogen supply. Some activities beneficial for CHO loading include: half marathons and marathons, triathlons, distance swimming, cross-country skiing, soccer, cycling time trails, and distance canoeing.  While some activities not suitable for CHO loading include: basketball, < 10 km runs, most swimming events, walking and hiking, weight-lifting, and most track and field events.

There are three main types of CHO loading:

  1. Classic Carb Loading
  2. Modified Carb Loading
  3. Rapid Carb Loading
    1. Classic Carb Loading
  • Reduce muscle glycogen content with prolonged exercise about 6 days prior to competition
  • Maintain a low-CHO diet (about 60-100 g/day) for 3 days to further deplete glycogen stores – glycogen depletion enhances formation of glycogen-storing enzymes in the muscle fiber
  • 3 days prior to long run or race, transition to a high-CHO diet (400-700g/day) and maintain up until pre-competition meal
Two- Stage Dietary Plan to Increase Muscle Glycogen Storage
Stage 1 – Depletion Day 1: Perform demanding exercise to deplete muscle glycogen stores

Day 2, 3, &4: Low-CHO intake (high % fat and pro intake in diet)

Stage 2 – CHO Loading Day 5, 6, &7: High-CHO intake (regular % of pro intake in diet)
Competition Day Abide by a high-CHO pre-competition meal
  1. Modified Carb Loading
  • 6-day procedure that does not require prior exercise to exhaustion
  • Exercise at ~75% of VO2 max (the maximum rate of oxygen consumption measured during exercise) for 1.5 hours, 6 days prior to long run or race
  • On consecutive days, gradually taper or reduce exercise duration
  • Continue to gradually reduce exercise duration, while increasing CHO intake for the 3 days leading up the the long run or competition (similar to the classic CHO loading method)
  • This method reduces some of the negative effects associated with the classic CHO loading procedure

Days 1-3: CHO supply about 50% of total calories.

Days 4-6: CHO increases to 70% of total energy intake.

  1. Rapid Carb Loading – 1-day procedure
  • Engage in 3 minutes of intense exercise
  • 24 hours prior to competition, ingest foods containing a high glycemic index (10g/kg body weight)
  • This method results in ~ an 80% increase in glycogen storage (similar to other 2 day and 6 day regimes)

Negative Aspects of CHO loading:

  • Only beneficial for prolonged, intense aerobic activities lasting more than 60 minutes in duration
  • Potential negative implications: muscle tightness and swelling, loss of flexibility, weight gain before competition (due to the addition of 2.7g of water with each gram of glycogen stores)
  • Potential to negatively affect individuals with muscle enzyme deficiencies

Goals of Post-Exercise Nutrition

  1. Replenish electrolytes and fluid lost in sweat
  2. Restore glycogen stores
  3. Consume protein (amino acids) to aid in muscle repair and synthesis

So… is CHO loading actually effective?


CHO loading is a very effective method for endurance athletes! CHO loading prevents athletes from “hitting a wall” which often leads athletes to feel a sudden loss of energy or fatigue while participating in endurance events. CHO loading works effectively to maximize skeletal muscle glycogen stores, which are used to constantly replenish glucose circulating in the blood and as a main source of fuel during exercise.  To ensure optimal performance, ingesting CHO’s one to four hours prior to and post endurance exercise is critical as glycogen- synthesizing enzymes are most active during this time, thereby increasing CHO availability during exercise. CHO loading is a well researched, common, and simple method used by endurance athletes that greatly enhances performance.

What is better than another excuse to eat more carbs?!?

Happy CHO Loading!!

Anna x


With help from my favourite Professor at the University of Adelaide, Dr. Nichola Thompson.


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[7] Thompson, N. (2018). Carbohydrate Metabolism.