Vol. 2, #27
July 16, 2005

Q: What is carbohydrate loading? - Layperson

A: Endurance athletes, e.g. marathon runners, rely on their stores of glycogen as a source of energy during competition. Carbohydrate loading is a legal method of boosting the amount of glycogen in the body prior to a competition.

Carbohydrate loading or Glycogen loading is a concept derived from research published in Sweden by Drs. Saltin and Hermanson in 1967. Their work lead to a deluge of muscle glycogen and endurance performance studies throughout the world. Using muscle biopsy techniques they studied athletes under a variety of dietary and athletic endurance protocols. It was shown that significant increases in muscle glycogen (the muscle storage form of glucose) could be achieved by increasing the carbohydrate intake over several hours to days before endurance performances.

In fact, these developed glycogen levels were optimum when the high carbohydrate diet was preceded by prolonged (over one hour) exhaustive exercise thus depleting the muscle glycogen stores.

This evidence and several other similar studies lead to the indiscriminate practice of "carbo-loading" prior to all kind of competitions. It also lead to a plethora of supplement products designed to "optimize" these glycogen stores.

The fallacy of such practices are based on:

  1. The assumption that muscle glycogen levels are the limiting factor to performance. This is not true unless the event is continuous at high intensity for over 1 hour. Most long triathlons and marathons (the two events where there may be some logic to some carbohydrate manipulation) are run at 65 - 75 per cent of aerobic capacity and even then, we still obtain 50 per cent or more of our energy from fat in these events.
  2. There is also an assumption that the higher the muscle glycogen the more strength one exhibits and the faster they can run. Neither is true. There is no evidence that a muscle "half or one quarter full" of glycogen is stronger ore faster. In fact Saltin reported, in conjunction with his original work, that speed differences over a one hour period were unaffected by glycogen levels.

Other issues or concern are:

  1. Every gram of glycogen stored obligates 3 grams of water. If an athlete glycogen loads to a maximum level they can be carrying an additional 4 to 6 pounds of water at the beginning of performance.
  2. Elevated carbohydrate intake increases the insulin production which increases fat storage, interferes with fat mobilization for energy, disturbs other hormonal balances, and increases blood pressure, blood cholesterol and triglyceride levels.
  3. "Carbo loading" can aggravate hypoglycemic responses during team sports and this can impair judgment, concentration and other performance measures.

As far back as 1939 it was shown by Christensen and Hansen of Sweden that a high carbohydrate meals immediately prior to a "strenuous" athletic competition impaired performance.

There is evidence that if blood glucose levels can be maintained, muscle glycogen can be spared and therefore lead to longer, high intensity performances during endurance events exceeding one hour. Maintain good muscle glycogen levels with 40 percent of calories coming from complex carbohydrates; observe good recovery practices and during long endurance performances consume a high quality sport drink to maintain the blood glucose levels.

Six days prior to a competition the process begins.

  • For the first three days the athlete consumes minimal carbohydrate and exercises so as to deplete the bodies glycogen stores
  • In the last three days the athlete consumes primarily carbohydrate and reduces the training load

At the end of day three the body will think that there is a problem with its glycogen stores and that it should store more glycogen than normal.

In the last three days, when the athlete consumes carbohydrate, the body will replenish the glycogen stores and hopefully top them up with a little bit extra.


In the first three days ensure adequate carbohydrates (about 60grm per day) are consumed to maintain the functioning of several important system in the body.

In the last three days the diet should be primarily carbohydrate, do not over eat, but you do need to consume adequate protein, minerals, vitamins and fluid. You will find that you will need to consume larger amounts of fluid than normal. Monitor your urine, it should be clear, to ensure you are drinking sufficient.

If you decide to try carbohydrate loading then try it in stages during your training. Start with depletion, low carbohydrate diet for one day, high carbohydrate diet. If no adverse effects then extend the period of the low carbohydrate diet to a maximum of three days. Keep a detailed log of what you do and what happens.

Carbohydrates are simple sugars or long chains of sugars which are linked together [starches]. Paradoxically, carbohydrates are the preferred fuel during exercise of high intensity but they are stored in extremely limited amounts in the body. This storage form of carbohydrate, called glycogen, is found primarily in muscles and liver. The glycogen in the muscle is used directly by the muscle which is being exercised. In other words, once its limited stores of glycogen are gone it cannot "borrow" from other resting muscles.
Depletion of glycogen by the working muscles leads to severely impaired exercise performance, which at its extreme is known as "hitting the wall". This makes obvious the need to

  1. increase glycogen stores prior to exercise, and
  2. supply carbohydrate during prolonged exercise.

How much carbohydrate is enough? We often express recommendations in terms of percentages of total calories. Even recreational athletes probably need to obtain 55-60% of their daily calories from carbohydrates. Most people can do this if they consume 3 grams of carbohydrate per pound of body weight. However, seriously training athletes probably require 4 grams of carbohydrate per pound body weight, or 60% of their calories from carbohydrate. For example, a 150 pound person who is cycling, say, 300 miles per week would require approximately 600 grams of carbohydrate daily. This carbohydrate would provide 2400 calories. Good examples of high carbohydrate foods are breads, cereals, grains, pasta, vegetables and fruits.

 Each time you exercise muscle glycogen becomes depleted to some extent. By providing high carbohydrate intake every day, it more likely that you will restore the carbohydrate which has been used, thereby allowing for another hard bout of training the following day.

Although we typically think of endurance athletes as having high carbohydrate needs during exercise, other sports such as soccer have been shown to significantly drain stored glycogen. For example, studies have shown that muscle glycogen was depleted to less than a quarter of its preexercise levels after one soccer game. Most of this loss occurred during the first half of the game [Karisson]. Furthermore, supplying carbohydrate during events such as soccer games may help to spare muscle glycogen and increase performance, particularly during the second half.

This raises the question of timing of carbohydrate intake. If the carbohydrate is to be taken during exercise it should probably be in beverage form. Beverages may be more quickly absorbed than solids and present less potential for stomach upset. A sports-type drink that has a concentration of 6-8% carbohydrate is likely to be easily absorbed during exercise. Most people can tolerate 1/2 cup to 1 cup of liquid every 20 minutes. This tolerance depends upon the individual and the type of exercise performed. Jostling sports like running are associated with more complaints of gastric distress after drinking than are sports such as cycling.

To avoid hypoglycemia or low blood sugar during exercise, carbohydrate should probably not be consumed within 1 hour of the start of exercise. The best pre game strategy is to eat a light meal which contains 100 or so grams of carbohydrate 3-4 hours prior to exercise.

 In addition, one of the best times to provide carbohydrate to the body is immediately after a workout. Immediately after exercise the muscle is most avid to restore the glycogen it has used during exercise. Perhaps the best way to restore glycogen is to keep a drink which contains carbohydrate in your gym bag, and drink it prior to leaving the locker room or before you hit the shower at home. Alternatively, you can eat a high carbohydrate food, such as bread, bagels, pretzels, or fruit. The goal is to consume at least 50 grams shortly after exercise.

Athletes will sometimes eat a pasta dinners the night before competition and believe that they have "carbo loaded". As you will see in a moment carbo loading is far more difficult to acheive than simply eating one meal high in carbohydrates.

[Carbo-Loading Chart] Occasionally it may be prudent to supersaturate the muscle cells with glycogen. This is done by "carbohydrate loading" and is of value if you plan to compete in an event which will last for at least 90 minutes and which will lead to exhaustion or near exhaustion. Carbo loading actually entails a weeks worth of preparation: beginning a week prior to the event exercise is cut by 50% every second day, thus sparing the depletion of glycogen. This will also allow for complete rest the day or two prior to competing. With four days to go the diet is increased to approximately 70% carbohydrate. For most people this would mean eating about 4 grams of carbohydrate per pound body weight.

According to this formula, a 150 pound person would therefore be required to eat 600 grams of carbohydrate per day during the loading period. This is a lot of carbohydrate!

[Carbohydrate Chart] 

The bad news

Some problems associated with the classic carbohydrate-loading diet include increased blood cholesterol and urea nitrogen levels, which may cause problems for people susceptible to heart disease, diabetes, or kidney disease. The glycogen depletion stage may cause vitamin and mineral depletion, ketosis, the loss of lean tissue, and a reduction in training capability leading to a negative effect on performance.

Other potential side effects are:

  • Muscle stiffness
  • Diarrhea
  • Chest Pain
  • Depression
  • Lethargy
  • Weight gain. It's easy to gain weight when you're carbohydrate loading because carbs help your body store extra water. Expect to gain 2 to 4 pounds if you do it properly.
  • Digestive discomfort. You may need to avoid or limit some high-fiber foods one or two days before your event, depending on your individual tolerance. Beans, bran and broccoli can cause gassy cramps, bloating and loose stools when you're loading up on carbohydrates.
  • Blood sugar changes. Carbohydrate loading can affect your blood sugar levels if you have diabetes. So talk with your doctor or a registered dietitian about blood sugar management before you get started 

Following a less stringent, modified carbohydrate-loading diet can eliminate potential problems with the classic carbohydrate-loading diet. The modified carbohydrate-loading plan is followed for six days prior to competition. It requires the athlete to consume a 50% carbohydrate diet for the first three days and then increase to a 70% carbohydrate diet (or 4.5 grams per pound of body weight) for the last three days before competition. The athlete begins training at a high aerobic intensity; then training time is gradually reduced on successive days.

Several studies show that trained runners can load their muscles maximally with glycogen just by cutting back on their workouts for three days and eating a little extra food. Exhaustive exercise three or fewer days prior to competition will not increase endurance beyond that. Carbohydrate loading does not increase endurance unless you are a highly-conditioned athlete. Eating extra carbohydrates will not store extra muscle glycogen unless the enzymes in muscles are primed by regular hard exercise. Carbohydrate loading in non-competitive athletes stores extra fat.

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DISCLAIMER:  The information in this column, is NOT intended to diagnose and/or treat any health related issues and is provided solely for informational purposes only. Consult the appropriate healthcare professional before making any changes to your healthcare regime. Even what may seem like simple changes in the diet for example, can interact with, and alter, the efficiency of medications and/or the body's response to the medications. Many herbs and supplements exert powerful medicinal effects. Neither the author, nor the website designers, assume any responsibility for the reader's use or misuse of this information.

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