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Ask the Diet Doctor: The Truth About Carb Loading

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Q: Will carb loading before a marathon really improve my performance?

A: The week before a race, many distance runners taper their training while increasing carbohydrate intake (up to 60-70 percent of total calories two to three days before). The goal is to store up as much energy (glycogen) in the muscles as possible to extend the time to fatigue, prevent “hitting a wall” or “bonking,” and improve race performance. Unfortunately, carb loading only seems to deliver on some of those promises. While carb loading does super saturate your muscle glycogen stores, this doesn't always translate into improved performance, especially for women. Here’s why:

Hormonal Differences Between Men and Women
One of the lesser-known effects of estrogen, the primary female sex hormone, is its ability to change where the body gets its fuel. More specifically, estrogen causes women to use fat as the primary fuel source. This phenomenon has been further confirmed by studies in which scientists give men estrogen and then observe that muscle glycogen (stored carbs) is spared during exercise, meaning that fat is used for fuel instead. Since estrogen causes females to preferentially use fat to fuel their efforts, drastically increasing carbohydrate intake to force your body to use carbohydrates as fuel doesn't seem like the best strategy (as a general rule, fighting your physiology is never a good idea).

Women Don’t Respond to Carb Loading as Well as Men
One study published in the Journal of Applied Physiology found that when female runners increased their carbohydrates intake from 55 to 75 percent of total calories (which is a lot), they did not experience any increase in muscle glycogen and they saw a 5 percent improvement in performance time. On the other hand, the men in the study experienced a 41 percent increase in muscle glycogen and a 45 percent improvement in performance time.

The Bottom Line on Carb Loading Before a Marathon
I don't recommend that you load carbohydrates prior to your race. In addition to having a minor (if any) effect on your performance, drastically increasing carbohydrates often leaves people feeling full and bloated. Instead, keep your diet the same (assuming it’s typically healthy), eat a high-carbohydrate meal the night before the race, and focus on what you personally need to do to feel your best on race day.

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