Why you shouldn’t follow a low-carb diet like keto or carnivore if you’re working out regularly

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Including carbs in your diet is essential for performing high intensity exercise. Getty/Alexander Spatari
  • Performing high intensity exercise on a low-carb diet can lead to various negative consequences.
  • These include “exercise flu,” limited performance, and muscle loss through gluconeogenesis.
  • This is when the body breaks down muscle and converts it to energy due to lack of carbs.
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From keto to carnivore, paleo to Atkins, there are plenty of diets that require eating very low carb.

And while many of these diets are popular among fitness fans, cutting carbs out of your diet is actually detrimental for people who work out a lot.

According to Dr. Mike Molloy, a nutrition coach who works with Olympians and CrossFit athletes and is the founder of nutrition coaching business M2 Performance Nutrition, your body needs carbohydrates to perform at high intensity, so if you don’t eat any, you may suffer from what’s known as “exercise flu” and lose muscle too.

Your exercise performance is limited without carbs

“Exercise choice and modality really does have a big impact on what type of fuel you should be using in your body,” Molloy told Insider.

He said that if you’re doing intense workouts like CrossFit MetCons (metabolic conditioning), Barry’s Bootcamp, Orange Theory Fitness, F45, or any other HIIT (high intensity interval training), this is high intensity anaerobic exercise and it requires carbohydrates as fuel.


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If there are no carbs in your system, your performance will hit a ceiling, your capacity will be limited, and there’s only so hard your body can work.

“You’re setting a regulator on how hard you can really push. Your body’s going to just stall,” Molloy said.

You may suffer from ‘exercise flu’

Molloy said he can often tell when someone has cut out carbs because at the end of an intense workout, they’re suffering from what’s known as “exercise flu.”

Mike Molloy, Founder of M2 Performance 18
Dr Mike Molloy is is the founder of nutrition coaching business M2 Performance Nutrition. Dr Mike Molloy

“If a person is eating unintentionally or intentionally low carb, at the end of a really high intensity workout, they can end up in a pretty nasty place where they just feel totally out of it, their body aches or their head hurts, and they just want to lay on the ground and not move for 20 minutes,” Molloy said.

He continued: “Laying on your back covered in a pile of sweat might feel good in the moment, it might feel like you worked hard, but it is not a sign of an effective workout by any stretch of the imagination.”

If you don’t eat carbs, your body will break down muscle

There are certain parts of the body, such as the brain, that only run on carbs, so if you don’t have any in your diet, your body will make them, and it does that by breaking down the protein in the muscle and turning it into carbohydrate.

This process is called gluconeogenesis, and it’s something that occurs if someone is consistently eating very low-carb.

If body recomposition is your goal (losing fat while holding on to — or even building — muscle), you definitely don’t want to be cutting carbs, because if you don’t eat them, your body will make glucose by breaking down your muscle.


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“It could be counterproductive, especially if you’re in a calorie deficit,” Molloy said.

Dropping your calories too low can lead to muscle loss too

Fasted cardio (doing cardio in the morning before eating anything) can also increase the risk of gluconeogenesis, but Molloy says that you should be OK if you eat afterward, or you ate late enough the night before. He also stresses that fasting tends to work better for men, whereas in women there can be more negative consequences such as increased risk of amenorrhea (losing your period).

“In general, we don’t recommend fasted training for high intensity exercise simply because again, if you have fuel in your body, it’s going to do better during that workout,” Molloy said.

What you also don’t want to do is drop your calories too low, because this can lead to gluconeogenesis too.

Molloy uses the example of someone who needs 2,000 calories a day to maintain their body as it is: if they drop their calories to, say, 1,200 — whether low carb or not — their body will lose muscle.

“That’s not enough in general,” Molloy said. “So your body will run this process to make more carbohydrates. It’s definitely a problem.”