Runners know they need to carb-up to get the energy necessary to push their muscles to get in those miles. But if you're following a low-carb diet, your muscles don't have the carbs to burn and may turn to fat instead. While there haven't been studies on runners specifically, your body does burn fat for fuel when you exercise on a low-carb diet. Due to risks of injury and fatigue when following a low-carb diet as a runner, it's important to consult your doctor to discuss safety and concerns.
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Energy Sources for Running
Many people choose to engage in aerobic exercise, such as running, as a way to burn fat. However, as an endurance activity, your body prefers to burn off glycogen stores -- stored carbs or sugar -- first. Glycogen is easier to convert into energy than fat. But, after you've been running for about 30 minutes, your body uses up much of its glycogen stores and switches over to fat and protein for fuel. And it isn't until you've been running for about 40 minutes that your body is burning all fat. Someone on a low-carb diet may already have depleted glycogen stores and might tap into fat energy stores at different times.
Running and Fat Burning on a Low-Carb Diet
Your body may prefer the use of carbs to fuel the first 30 minutes of your run, but if carbs aren't available, your body burns fat for fuel. A 2014 study published in the Journal of the American College of Nutrition compared the effects of a low-carb diet and a low-fat diet on exercise in a group of overweight men and women. While the study doesn't indicate whether the groups were running, the researchers found that the group following the low-carb diet burned more fat than the low-fat group and didn't have issues with fatigue when pushed toward intense aerobic exercise.
However, while a similar study published in 2009 in Obesity also found that a low-carb diet increased fat burning during aerobic exercise, the researchers noted that energy and muscle strength was lower in the low-carb group.
Things to Consider
You may want to wait to start or resume running until you've been following a low-carb diet for a couple of weeks, according to a 2007 article published in the American Journal of Clinical Nutrition. Those first few weeks your body is adjusting to a state of ketosis -- where it burns fat for fuel -- which may affect your running performance. To prevent loss of muscle and strength, aim for 0.7 grams of protein per pound of body weight, or 112 grams for a 160-pound person. If you're following a low-carb diet and feeling tired during your run, you may need to up your carbs or talk to your doctor about alternative diets or workouts.
Pre- and Post-Run Snacks
Even on a low-carb diet, you need to properly fuel your body before and after your run to keep muscles strong and improve performance. Eat a snack one to three hours before to ensure your muscles are properly fueled. Good low-carb options include two hard-boiled eggs or rolled no-carb deli turkey. If you have a few carbs available, have a pre-run snack of 24 roasted almonds with 2 grams of net carbs or 1/2 cup of whole-milk ricotta cheese and 1/2 cup of sliced cucumbers with 6 grams of net carbs.
Fuel up within 20 minutes after your run to repair and build muscles. Try a cup of mixed greens topped with grilled chicken or slices of roast beef wrapped around fresh-cooked green beans. Each of these post-workout snacks has about 2 to 3 grams of net carbs.