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Ask the Diet Doctor: Carb-Loading

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Q: Should I eat lots of carbohydrates before a half or full marathon?

A: Loading up on carbs before an endurance event is a popular strategy thought to boost performance. Since carbohydrate-loading temporarily increases the amount of sugar you can store in your muscles, the theory is that the more energy stored, the more you’ll be able to exercise. It's like, if you have a larger gas tank, you'll be able to drive farther, right? Two methods in particular—a traditional binge (three to six days before a race) and a 24-hour binge—have been shown to increase the amount of energy stored in your muscles. With a traditional binge, research shows you can almost double the amount of carbs stored in your muscles by manipulating your intake like this:

• Sunday through Tuesday: Consume 50 percent of your calories from carbohydrates
• Wednesday through Friday: Consume 70 percent of your calories from carbohydrates with 20 minutes of low intensity exercise (do not exercise Friday)
• Saturday: Race Day

The 24-hour approach can increase your muscle glycogen stores by 90 percent. (This involves eating 4.5 grams of carbohydrates per pound of your body weight the day before a race and opting for high glycemic sources of carbs—white rice, refined grains, sports drinks, and rice cakes.) Though this approach is research-validated, I don’t recommend it. Drastically increasing your carbohydrate intake can lead to bloating and digestive discomfort, thanks to higher-than-usual fiber intake. If you do follow this approach, add low-fiber and refined carbohydrates to your regimen to avoid over-consuming fiber. (Feel free to enjoy white rice, potatoes, and regular pasta.) Also, pair your carbs with protein and a drizzle of fat (olive oil, avocado oil, or butter) to slow the release of carbs from your stomach and help quell blood sugar spikes.

RELATED: Ask the Diet Doctor: The Low FODMAP Diet

The bottom line: On race day, you want to feel your best. And if carbohydrate-loading leaves you sluggish, there’s no way you’ll run your best. So before you boil an entire box of penne, ask yourself these three questions.

Are You Exercising for 90 Minutes or Less?
It takes your body around 90 minutes to use up a majority of its glycogen stores (if you're in great shape, this time frame can be longer, as your body will be more adept at using fat as fuel). Exercising less than 90 minutes? Carb-loading won’t do you any good—just as it doesn’t matter if you have a full or half tank of gas for a quick trip around the corner to run an errand.

Do You Have Access to Fuel While Exercising?
With the popularity and accessibility of sports drinks and gels, it's easy to fuel yourself during a race. This makes carbohydrate-loading irrelevant. If you can have a sports drink or gel every 60 to 90 minutes, you will constantly be refueling your muscles—thus not running the risk of using up stored energy.

Are You Eating Enough?
Research shows that men and women don’t always reap similar benefits from carbohydrate-loading. One study found that when both sexes underwent the same loading protocol, men experienced a 45 percent increase in muscle glycogen while women had no increase. These differences are thought to be due to hormonal differences—specifically estrogen-related. You may be able to overcome them (and reap the same benefits!), by increasing your total caloric intake by 30 to 35 percent during the loading period. If you currently consume 1,700 calories a day, you’d need to get to 2,200 calories a day during loading days.

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