When you’ve been trained to think of fat as “bad” for most of your life, starting the ketogenic meal plan, which focuses on healthy fats, can take a little getting used to. After all, it’s the very thing you’ve been told to “burn off” during your workouts. Does eating more of it mean you have to work out even harder? Or maybe you’ve heard that the fat-focused meal plan is directly connected to staying within the fat-burning zone. Is there any truth to that?
I called my doctor to give direct information. Not only has he been working on the ketogenic diet plan for over a decade (long before it became popular), but he also has all the scientific research on the new diet. He even took it upon himself to understand how ketosis affects exercise. Are you ready to learn what to say?
What Your Body Relies on for Energy
“If you have a ‘normal meal plan,’ 70-80% of the energy your body uses will come from the sugar found in carbohydrates,” said the Doctor. “That means fat is only making up 20 percent of what you’re burning.” And because your fuel source is changing, he explains, how you feel during certain types of workouts will likely change, too.
“If someone is used to doing high-intensity workouts, they’re going to have a harder transition than if someone is used to doing more yoga,” my doctor says. This is because when you work out super intensely for short periods of time—like a spin class or series of running sprints—your body is used to burning that aforementioned glucose first, such as the oatmeal you had for breakfast. Now that you’re on a low-carb ketogenic meal plan, you don’t have that glucose store to pull from for energy, and you’re likely to feel more fatigued than you usually do.
But for activities like barre, Pilates, or light jogging—where your heart rate isn’t as high—your body relies on fat for energy. It makes sense: Fat stays in the body longer than carbs, which is why the body can depend on it for a longer, more steady workout.
“You can burn fat for more intense workouts, but it takes time to build up to that,” my doctor says, adding that it’s tricky to know exactly when muscles start using fat versus glucose. His standard guideline is this: “If you’re working out at a level four or below on a scale of 1–10, your body will burn fat. But if you’re working out at a five to seven, or above, on a consistent basis, it’s going to take time for you to feel comfortable working out at that level in ketosis because that’s something most people need glucose to do.”
Where the “fat burning zone” comes in
Does this mean you have to stop your weekly spinning dates to maximize your keto efforts? (Hey, if you’re giving up grain bowls and pasta, you want it to be worth it, right?) Not exactly, according to my doctor, but chances are good that you aren’t going to have the energy to power through at the same intensity as you could when you were eating carbs—at least for a few weeks or even months. “This is difficult biochemistry because we don’t know exactly which muscles are used to adapt,” he said.
At the same time, exercise that keeps you in your fat-burning zone (i.e., exercise with a heart rate between 60 and 75) will increase your body’s energy use. Eat whatever you use for fuel. Cardio enthusiasts may still consider this; it didn’t just involve going out for 45 minutes. (Some fitness classes, such as OrangeTheory, are known to follow this model.) Other exercises to focus on: running, floor classes, and strength training.
As with exercise and diet, my doctor says the most important thing is to pay attention to how you feel. Your body will tell you a lot about what works and what doesn’t; You just need to listen.