Why Cutting Carbs Could Kill Your Exercise Performance

Why Cutting Carbs Could Kill Your Exercise Performance

Written by
February 16, 2016
Updated February 24, 2019

I’m already bracing myself for the inevitable backlash of self-proclaimed experts who will share their own anecdotal stories about how it’s possible to maximize performance (and burn more fat!) on a sustained, low-carb diet plan.

Bully for you and your experiences, I wish you well.

What I’m here to tell you is that for most athletes, most of the time, a very low carb diet is going to be a performance killer.

It All Starts With Goals

First I think it’s very important to talk about goals and how different goals make a big difference to an athlete’s most effective nutrition plan. Let’s look at Lebron James, for instance.

He made waves in 2014 when he leaned out significantly, dropping an estimated 20 or 30 pounds and chalking it up to a “no carb” diet.

Hey, if it’s good for Lebron, it should be good for the rest of us, right?

Maybe – if your goal is specifically to lean out and drop a few pounds. You see, Lebron followed his purported “no carb” diet (which is actually a misnomer) since fruits and veggies have carbs in them, but whatever during the off-season.

Not during pre-season or while in season, when back-to-back practices and games are more likely to zap glycogen stores and lead to impaired performance on a low carb diet.

Goals impact nutrition needs. Whether your goal is to lean out, or to get faster, or to get stronger or to go longer, you may have to adjust your nutrition plan to achieve said goal.

Pretending this isn’t true, or pretending there’s always one answer or one solution leads to an absurd amount of misinformation about performance nutrition.

Why Cutting Carbs Could Kill Your Exercise Performance

Understanding Carbohydrate’s Role in the Body

There are only three macronutrients – carbohydrates, proteins and fats, each of which carries out specific functions to preserve the structural and functional integrity of the body. In other words, they’re important. They’re all important.

Carbohydrates, specifically, carry out four roles in relation to exercise performance:

1. They’re an energy source. Particularly during high intensity exercise, energy (ATP) is derived from blood glucose and muscle glycogen.
2. They spare protein. When muscle glycogen is in short supply, glucose is derived from fat and, to some degree, amino acids (proteins). This can deplete amino acid stores and potentially lead to muscle catabolism.
3. They’re a metabolic primer. There’s a saying that “fat burns in a carbohydrate flame.” In other words, carbohydrate catabolism during exercise actually serves as a “primer” for fat oxidation.
4. They’re fuel for the nervous system. Carbs don’t just sit there in your body, boost insulin levels and turn into fat. No, in addition to acting as a fast form of fuel and a metabolic primer, they’re the primary fuel for your brain.

Of course, your body is incredibly smart. If, for whatever reason, you significantly reduce your carbohydrate intake, it will find other ways (namely, ketosis) to maintain blood glucose levels; but just because it’s possible, doesn’t mean it’s ideal – particularly for an athlete. This is particularly true for fast, high-intensity, repeated bouts of exercise.

Carbohydrates and Peak Performance

To reference the Lebron James off-season diet again, I want to highlight one of the primary differences between off-season performance goals and in-season performance goals: Intensity.

During the off-season, I’m sure James was still exercising. He was probably hitting the gym, maybe playing basketball a few times a week and possibly getting some extra cardio in.

What I’d almost guarantee you he wasn’t doing was back-to-back high intensity training sessions with high-load while competing against other athletes. He wasn’t trying to beat someone in a 100 meter sprint or improve his 5k time.

And this is a key differentiator when it comes the importance of carbohydrates and performance: Carbohydrates, specifically muscle glycogen, provide a very fast source of energy, making it possible for an athlete to perform at peak levels during intense exercise.

When it’s time to kick the intensity up a notch – for instance, sprint down the court or see continued gains during repeated training sessions at the gym – muscle glycogen is there to produce fast energy through the anaerobic energy pathway.

Why Cutting Carbs Could Kill Your Exercise Performance

Understanding Energy Pathways

Your body is constantly using a mix of energy substrates (fats, carbs and sometimes amino acids) for fuel. It rarely uses just fats or just carbs.

Because your body is highly efficient, it adjusts the ratio of fats and carbs used for energy production based on the intensity of the work you’re doing. For instance, if you’re sitting still reading this article, you’re probably burning mostly fats through the oxygen-requiring, slow-energy-producing, aerobic metabolism.

If you stand up and walk around, you’ll probably still be using mostly fats for fuel because your aerobic metabolism can still keep up with demands, but you’ll probably also start burning more carbs as your muscles respond to the new demand for action.

Then, if you head outside and take a sprint around the block, your body’s going to respond again – your aerobic system won’t be able to keep up on its own, so your fast-energy-producing, non-oxygen-requiring anaerobic metabolism will kick in.

It’s your anaerobic system that uses stores of muscle glycogen to create ATP. While running, your body continues using a combination of fat and carbs for fuel, but the ratio flip-flops and you’ll start burning more carbs than fats.

Why Consuming Carbs Is So Important

The human body has an almost endless supply of fat reserves that can contribute energy for aerobic metabolism for a long, long time.

The same can’t be said for carbohydrates. Carbs are stored as blood glucose and muscle and liver glycogen in relatively limited amounts. This means if you use up your glycogen stores during prolonged or intense exercise, you won’t have more stores to tap into, unless of course, you eat more carbs.

You may have heard of “the bonk” in endurance running. This, specifically, is the moment when glycogen stores deplete and the runner’s body has no choice but to revert to using fat as its primary source of energy production.

Does this mean the runner has to stop running completely? Of course not! What it means is that energy production slows considerably – aerobic metabolism, by nature, produces energy slowly – this results in a substantial reduction in the ability to respond quickly and effectively when intensity is required.

For athletes who need a final kick to push past the competition during a run, or who need to have the energy to train intensely day after day while improving performance, carbohydrates are the key to restocking muscle glycogen stores and producing fast-acting energy.

Low stores don’t mean you can’t exercise, they just mean your body won’t be able to react and respond as quickly, stunting your athletic performance.

Why Cutting Carbs Could Kill Your Exercise Performance

The Takeaway

If your goals as an athlete are to improve performance – to get stronger, faster and more powerful, to be able to respond quickly and effectively to outside stimulus – then maintaining muscle glycogen stores to fuel the need for fast energy production is an absolute necessity.

While exact demands vary from person-to-person, the general rule of thumb is that high-quality carbohydrates should account for roughly 50-60 percent of daily calorie intake.

Endurance athletes and mixed-sport athletes (for instance, basketball, lacrosse, soccer and boxing), typically burn through muscle glycogen more quickly than strength athletes, so an increased intake of carbohydrate may be beneficial to athletes competing in these sports.

If, however, your goals are to lean out, drop a few pounds, or participate in ultra-endurance sports (for instance, running events of more than 100 miles where using fat as a primary fuel source without the need for fast-energy-producing carbs might be more beneficial), a lower carb diet might help you achieve your goals.

Researchers are just now diving into this area of research, so time will tell whether there are appropriate ways for certain athletes to use low carb diets.

Laura Williams
Laura has a master’s degree in exercise science from the University of Mary Hardin-Baylor, and is a Certified Exercise Physiologist through the American College of Sports Medicine. She works as a full-time freelance fitness writer and has had bylines in publications such as Men’s Journal. As a recent widow, she believes strongly in the healing power of fitness. She can often be found surfing in Costa Rica, running in San Diego, or wrestling at home with her four dogs. Follow her on Instagram and Twitter, @lwilliams_exss.

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