How to Calculate Carbohydrate Intake After Your Overnight Fast

It’s long run Sunday (or Saturday), and your alarm wakes you. How can it be time to get out of bed already?!

Are you one of those runners who just rolls out of bed, puts on your shoes, and wanders out the door half asleep?

If so, you might be curious whether you’re going to run out of stored carbohydrates earlier in your run because you have heard all kinds of horror stories about running on a completely empty stomach.

After all, it’s widely recommended that you fuel up in the early hours of the morning before you race a marathon. Did you read our post on How to Fuel Your Body for Any Workout?

Energy Requirements

How much energy do you actually burn during your overnight fast? And if you do end up running before breakfast, is your run going to be negatively impacted because your carbohydrate “gas tank” isn’t full?

The easiest way to figure out the first question is to do some simple math.

If you ate dinner at 6pm and went to bed at 10pm, we could safely assume your carbohydrate reserves were fully topped off when you fell asleep. If you slept for eight hours, got up and went for your long run starting at 6:30 am, it’s just a matter of totaling up how much energy you expended while asleep, and figuring out what proportion of this energy comes from carbohydrates.

The energy you expend while you sleep is dependent on your height, weight, and age, but for a typical male runner—5’10, 150 pounds, and 35 years old—you’re expected to burn about 68 Calories per hour while at rest.¹

About 30% of this energy is from carbs, so now it’s a straightforward math problem: 30% of 8.5 hours times 68 Calories per hour, or about 175 Calories’ worth of carbohydrates.²

If you are running at an easy to moderate pace, this is about three to three and a half miles’ worth of energy.

Now, there’s going to be some individual variation in these figures, which is why they’re only an approximation. Women, for example, have a lower resting metabolic rate, so women burn less energy overnight. Unfortunately, as you get older, your metabolic rate drops too.

In most cases, though, it’s safe to assume you will burn through between two and four miles’ worth of carbs overnight.

If this is going to be a problem— if your running group has planned a 23-mile long run in the morning—you might want to grab a carb-rich snack before you head out the door.

Can your body pace itself?

Multiple scientific studies have found that the body does not alter its consumption of carbohydrates during exercise based on how much carbohydrate energy you have in reserve: if you’re running the same speed, your carbohydrate usage will be the same whether or not you’ve had breakfast before your run.^{3, 4, 5}

In this sense at least, your body does not “pace” itself. And, contrary to earlier research which suggested that training in a state of low carbohydrate availability, the latest and most comprehensive research on well-trained athletes shows has not uncovered any clear advantages (at least in events lasting for about two hours) to “training low,” i.e. doing some or all of your workouts after intentionally keeping your body’s carbohydrate stores low.⁶ Did you read our post on Can Limiting Your Carbohydrate Intake During Training lead to Better Performance?

You do not need a scientific study to tell you that your performance is going to be significantly impacted if you run out of carbohydrates during a long run or in your next marathon.

Conclusion

The good news is that your overnight fast doesn’t incur too big of a penalty to your carbohydrate stores. You can easily make up the deficit with a light snack that you can eat before you head out for your long run or make your way to the start line.

In our earlier example, our runner lost around 175 Calories of carbohydrates during his overnight fast. This can be easily restored with a banana and a piece of toast, a small bowl of cereal, or a granola bar—each of these totals around 150-200 Calories, most of them from carbohydrates. This is good if you’ve got a finicky stomach!

As long as your carbohydrate stores are topped off before you go to bed, you only need a light snack in the morning to be ready to go.