Optimizing Glycogen Storage: How Effective is ‘Carbo Loading’?
Glycogen is the body’s storage form of carbohydrate and is the body’s favorite source of energy upon commencement of exercise. These ‘released’ carbohydrates provide fuel and hydration. The athlete who can dip into plentiful supplies of glycogen quickly has a distinct advantage.
Glycogen is stored in the liver and muscles. While the liver has the largest single deposit of glycogen (~30%), the muscles together contain the bulk of our glycogen stores (~70%). The liver uses its glycogen to maintain glucose homeostasis in our blood; the muscles use their glycogen solely to fuel the muscles. Dietary carbohydrate whether eaten in excess or not, will always contribute to glycogen production. Unlike us, the body always puts something in savings whatever the conditions.
Carbohydrate loading or ‘supercompensation’ protocol was first reported by Bergstrom, et al. in 1967.
Depletion Phase: 7 days prior to competition, an extremely low carbohydrate diet is combined with exhaustive exercise for 3.5 days
Repletion Phase: 3.5 days prior to competition, a high carbohydrate diet (~8g/kg BW/d) is combined with very light or no exercise
The idea is that by depleting the body it will supercompensate and maximum glycogen storage will be achieved. Unfortunately, the adverse effects of this protocol far outweigh the benefits. During the depletion phase, athletes report irritability, fatigue, inability to complete exercise and increased injuries. During the repletion phase, athletes complain of gastrointestinal distress, bowel inconsistencies and increased water storage creating a ‘heavy, waterlogged’ feeling. Not exactly the way a runner wants to feel the week before a race.
THE MODIFIED PROTOCOL
Sherman et al. in 1981 reported success with a modified version of the original carbo loading regimen that eliminates most of the adverse effects.
Depletion Phase: 6 days prior to competition, carbs are restricted to 5g/kg BW/d, the minimum amount recommended for endurance athletes in training, and 90 minutes of hard training (70% VO2max) on day 1 followed by 2 days of hard training for 40 minutes
Repletion Phase: 3 days prior to competition, carbohydrate intake is 10g/kg BW/d and 20 minutes of exercise for 2 days with a day of rest the day before the event
THE BOTTOM LINE
Overall, most endurance athletes find that eating a high carbohydrate diet year round keeps glycogen stores at high concentrations and allows for consistent training and competition. For many, altering the diet one week prior to a big race can cause anxiety and defeats the purpose of months of training. Carbohydrate replacement should begin 40 minutes into the long run no matter what protocol you used in training.
Kathleen Deegan, PhD, MS, RD
Sports Nutritionist, California State University, Sacramento
SRA Fueling Specialist
Coming next time…
The Truth about ‘Training Low’
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