Why you need to stop being scared of carbs

Of the three macronutrients, carbs provide the most readily available fast fuel source, as your body breaks them down into (most abundantly) glucose,  that’s quickly sent into your bloodstream and shuttled to your muscles, brain, organs, and so on. Any extra glucose that isn’t utilized right away is stored as glycogen in your muscles and liver, so it can be put to use later. 

For as much hype as there is around becoming “fat-adapted” and developing greater “metabolic flexibility,” carbs are still the fuel source that’s preferred by your entire body, especially for endurance athletes.  One of the reasons is that while stored fat offers a much larger potential energy reserve than carbs, it takes much longer to tap into this energy when you need fuel immediately during training or racing. While fat can be a valuable energy source for low-to-moderate intensity activities that have a lower energy demand, research has shown that when our bodies are working at the higher end of our VO2 Max, carbohydrates are the fuel source we need to have readily available.

The more miles you put in, and the faster you cover them, the more carbs you need. It’s the optimal fuel for high-intensity performance, providing sufficient energy to power muscle contraction and help you achieve and sustain your target pace. This first comes from glucose in your bloodstream that is available as soon as 15 minutes after you eat simple carbs, and then, from stored glycogen. Low carb intake = low glycogen stores = poor performance. 

Just because more miles and greater intensity should equal more carbs consumed, it doesn’t mean no miles = no carbs. There’s a common misconception among athletes that on days when their program only calls for an easy run or a real rest, they should dramatically cut down on their carb intake or eliminate it altogether. This neglects the fact that even at rest, your body typically gets about 20 percent of its fuel from glycogen, while your brain requires at least 130 grams of carbs per day to function optimally according to the National Academies’ Institute of Medicine, no matter what your training load is. Plus, you need to replenish the glycogen stores that you depleted on a heavy or moderate training day. In other words, stop cutting out carbs when you have a low mileage, low intensity, or off day planned.

Carbohydrates also help our bodies to absorb water and certain electrolytes during training and racing so that you don’t become dehydrated. Research shows that when we consume water + sodium + glucose together, absorption and hydration increases. A study found that a drink containing carbs and electrolytes helped participants finish a 21-kilometer time trial faster and with better visual motor capacity than those who used a placebo.

After training, most of the refueling advice has focused on getting enough protein into your body as quickly as possible to aid muscle repair. This is true, but, if you don’t have enough carbs in your system, this will be less effective, and muscle breakdown can still occur because your body will start using amino acids for energy instead of restoring and building muscle. Combining carbs with protein right after training in a meal, smoothie or shake will not only spare amino acids for repair, but also help you bounce back strong by replenishing the glycogen stores that you depleted during your workout.

We just explored some ways that carbs power performance and recovery. But how do you know when you’re not consuming enough? During your training, you might struggle to hit paces or find that you’re running out of steam earlier than usual. A paper published in Applied Physiology, Nutrition, and Metabolism found that during an intense cycling session, time-to-exhaustion was reduced for participants who were low on carbs and concluded that a low-carb diet “reduces both performance and total aerobic energy provision during supramaximal exercise.” Cutting back on carbs can also hamper your decision-making during and after exercise. 

Some of the side effects of not eating enough carbs – particularly when paired with an overall calorie deficit – can mimic the perils of overtraining, such as increased perceived exertion, excessive and persistent muscle soreness, and hormonal/menstrual cycle disruption. You could also feel like you’re low on energy, fatigued, and more emotionally volatile than usual. If you have trouble falling or staying asleep or wake up feeling like you need another couple of hours in bed, low-carb intake could be partly responsible, with researchers suggesting that this reduces the amount of time spent in REM sleep. 

Fortunately, if you find that you are underfueling, fixing it is easy: eat more carbs! You can do this by choosing foods that contain 70 to 95 percent carbs, including pasta, bread, rice, bananas, and bagels, instead of high fat foods such as french fries, donuts or chips. (Eating these occasionally is totally fine! But, doing it all the time would increase your caloric intake, as well as possibly increasing GI distress due to the higher fat content.) You can use high carb foods as standalone snacks on either side of your workouts, and also incorporate them into breakfast, lunch, and dinner. Then during your training, consider using goos, gels, tabs, sports drinks, and other sources of simple carbs to keep you going. 

 1. “Report Offers New Eating and Physical Activity Targets To Reduce Chronic Disease Risk,” National Academies of Sciences, Engineering, and Medicine, September 5, 2002, available online at https://www.nationalacademies.org/news/2002/09/report-offers-new-eating-and-physical-activity-targets-to-reduce-chronic-disease-risk.2. EM Wright and DD Loo, “Coupling Between Na+, Sugar, and Water Transport Across the Intestine,” Annals of the New York Academies of Sciences, 2000, available online at https://pubmed.ncbi.nlm.nih.gov/11193601.3. Z Gui et al, “Effect of Protein and Carbohydrate Solutions on Running Performance and Cognitive Function in Female Recreational Runners,” PLOS ONE, October 12, 2017, available online at https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0185982#. 4. Adriano E Lima-Silva et al, “Effects of a Low- or a High-Carbohydrate Diet on Performance, Energy System Contribution, and Metabolic Responses During Supramaximal Exercise,” Applied Physiology, Nutrition, and Metabolism, September 2013, available online at https://pubmed.ncbi.nlm.nih.gov/23905657.5. Ahmad Afaghi et al, “Acute Effects of the Very Low Carbohydrate Diet on Sleep Indices,” Nutritional Neuroscience, August 2008, available online at https://pubmed.ncbi.nlm.nih.gov/23419282.