Can You Work Out While Eating a Keto (CarbX) Diet?

One major advantage of a ketogenic (very low-carb) diet like CarbX is that you can lose a significant amount of weight while doing little (or no) exercise. But what about people who want to eat a low-carb diet and still incorporate exercise? Can you still work out while eating on a CarbX diet?

A little about muscle fuel

First, it’s important to understand that any activity that gets your heart-rate up, or causes your muscles to exert themselves, requires glucose. Glucose is the primary source of energy for muscles (think of each muscle cell having a tiny glucose fuel tank that needs to be kept full in order to achieve optimal performance from that cell. The fuel tank is also known as a “Glycogen Store”).

During a workout, as glycogen stores are depleted, the muscle begins to fatigue, your heart-rate slows down, etc. In fact, if you begin a fresh workout on a half-empty glycogen tank, you might not even know it, but your performance will suffer both in terms of your ability to work, and the ability of your heart-rate to really get going.

As a quick reminder, carbs are the most efficient form of glucose (thus, eating carbs is the fastest way to re-fill your glycogen stores). But, your body can also convert fat to glucose through a process known as gluconeogensis. It’s not a quick process, but it explains how the body is able to fuel muscle cells even if you’re not eating carbs.

To understand all of this better, my article on Glycogen is worth a read before continuing.

What happens if you work out on a half-empty tank?

I mention in the article above that if your glycogen stores are low and you attempt to do vigorous exercise, you’ll feel fatigued. But this isn’t the whole picture. You might feel fatigued and not even know it. For example, if you’re fully cardio-adapted (i.e. capable of long-term cardio performance), and you can’t get your heart-rate into the high end of the aerobic zone and keep it there for 30 to 60+ minutes, that’s a potential signal that you are improperly fueled.

What does that mean? If you work out while eating a very-low carb diet and try to do aerobic activity, or weight lifting, you’ll find that you simply can’t handle the workload that you could if you were eating carbs. But, you won’t really know that you could have done more work, you’ll simply get worn out sooner. This will feel normal to you (like you’re pushing your limits, just like any athlete!)—but the glycogen shortage is fooling you and you’re not performing as well as you could otherwise.

Becoming “Fat Adapted”

Your ability to do workouts on a ketogenic diet will also depend on how “fat adapted” you’ve become. During the first few weeks of switching to a keto diet, it’s going to be pretty crappy. You’ll get tired fast because your body isn’t very good at converting fat to glucose. However, the more time you spend on keto, the better your body becomes at this.

What about athletes who do Keto?

The professional athlete who eats a strict ketogenic diet is actually a bit of a myth.

There are some fringe cases where you hear about a professional athlete doing keto. Don’t be fooled. If you peel back on the onion on what they actually eat, they are rotating between a very low-carb diet 2-3 days a week, and a very high carb diet the other days of the week. In other words, they aren’t following a strict keto protocol. There is SOME research to indicate that this kind of carb-cycling might be a better eating style for some performance athletes, but the science on this is still very fringe.

This is the same fundamental concept I talk about in my carb cycling article. It’s not the best weight-loss strategy if you’re eating keto (although it does work), but it IS a decent strategy for athletes trying to extract enough energy from a kind-of-sort-of ketogenic diet.

The Claims

Proponents of the keto approach for athletes (there are a few), claim that one of the primary benefits is that the “bonk” is no longer a problem (meaning, they don’t run out of fuel for their muscle cells, because in theory, they have a never-ending supply of body-fat to convert to glucose).  While this might be true, (there are no real studies to support this conclusively), athletes who eat purely ketogenic 100 percent of the time also typically can’t get their heartrate into the aerobic or anaerobic zones for extended periods of time where glycogen burns quickest. So, while they may not “bonk,” they’re also not performing at capacity.  Put another way, it may be that they don’t run out of fuel simply because they aren’t able to burn much fuel in the first place.

A tiny, tiny, TINY segment of the population may actually be able to become so fat adapted (over several years), that they can actually replicate the performance results of eating carbs without eating them at all. However, these are “claims” from people that have never been observed or verified under lab conditions. And trust me, if better performance was even remotely possible with a ketogenic diet, you’d see the entire NCAA and NFL changing the diets of their players. The money follows performance, and as of now, the people with the money aren’t convinced. (Exceptions being the kind notes above, where low-carb days are being rotated in with high-carbs days).

For the rest of us, if performance matters, carbs are the only fuel source we can use to get a serious workout.

So can I exercise while on keto, or not?

You absolutely can do hard-core strength training or cardio work out while eating a ketogenic diet. In fact, there are whole subsets of the keto community built around teaching people how to squeeze the best possible performance from a ketogenic diet while maintaining high levels of physical activity. In fact, once you become fat-adapted, you’ll feel great!

But, if you’re a serious endurance or a performance athlete, concerned about metrics, performance, numbers, or getting the BEST possible performance out of your body, keto is like putting the performance-handcuffs on. As a general rule, it will prevent you from getting your heart-rate up and it will prevent you from doing the same level of strength training that would otherwise be possible.

  • Can you still work out while eating a ketogenic diet? Yes!
  • Will I be tired, and sweat, and get quality exercise? Yes!
  • Does it help with fat burn? Absolutely!
  • Can I run a marathon? Not likely. But maybe, very slowly.
  • Will I be performing at optimal levels?  Not even close!