Carbo-loading. You can but should you?

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It’s a term that is familiar to athletes from many sports, and especially in the world of running. Yet the term “carbo-loading” is probably one of the most misunderstood nutrition strategies for marathon runners and other endurance athletes.  


For all ‘The Office’ fans out there…Carbo-loading Michael Scott style

With the Boston Marathon coming up in a little over a week, athletes may be beginning to ramp up their intake of carbohydrates to ensure they start the race with a full tank. And rightfully so. Your body can only store enough fuel or glycogen (storage form of carbohydrate) to sustain around 90 minutes of exercise. Beyond this, without taking in any fuel (gels, blocks, sports drinks), energy levels begin to drop and fatigue sets in. Boosting muscle and liver glycogen stores therefore can help to reduce fatigue and boost performance.

Early carbo-loading strategies includes a 3-4 day depletion phase where athletes undergo hard training and keep their carbohydrate intake low, followed by another 3-4 days of easy training and carbohydrate loading. With this strategy, it was found that muscle glycogen bounced back much more than just eating carbohydrate every day, it was “supercompensated”.

Fortunately, a few years later, it was discovered that a more moderate approach to carbo-loading could be adopted. The glycogen depletion phase is no longer necessary and provides no additional benefit to carbohydrate loading. Thank goodness, right? Light activity in the 2-3 days before your race (hello taper!!) along with a higher carbohydrate intake will have the same beneficial effect on performance.
This all sounds great, right? So why the ‘but should you?’ in the title? Well this is where it gets interesting!!

Studies have shown the the rate at which we break down glycogen to use as fuel during exercise is proportional to the amount of glycogen present in the muscle. Simply put, if you have extremely high glycogen stores you will break them down faster than when you have normal or high glycogen stores. So an hour or two into exercise, glycogen stores will be comparable whether you started with extremely high or just high glycogen stores.
So rather than aiming for extremely high glycogen stores before the start of the race, it’s enough to just ensure that they are sufficient.

As mentioned above, this can be accomplished in the 2-3 days before your race by eating carbohydrate rich foods while reducing training. But with decreased training comes decreased energy expenditure. So your higher carbohydrate intake should not be the result of eating more. Instead, it should be achieved by emphasizing carbohydrate sources and reducing fat intake.

With a lower energy expenditure, aiming for a carbohydrate intake of 5-7g/kg per day is enough, in most cases, to ensure that glycogen stores are sufficient. The type of carbohydrate consumed has little effect and both solid and liquid carbohydrates have the same effects. If you are an athlete that experiences GI issues, you should choose your carbohydrate sources a little more carefully and may benefit from a lower fibre intake.

Speaking to a registered sports dietitian is a great way to to find the perfect carbo-loading plan for you!

Writer: Stephanie MacNeill (RD)

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