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?
To set the record straight, I called up my doctor. Not only has he been on the ketogenic meal plan for over a decade—way before it was trendy—but he also keeps on top of all the latest scientific studies related to the eating plan. He’s even done his own personal deep dive into how exactly ketosis affects working out. Ready to find out what he has to say?
What your body depends on for fuel
“If you have a ‘normal meal plan,’ 70–80 percent of what your body uses for fuel is going to come from glucose, which is in carbs,” my doctor says. “This means fat accounts for only 20 percent of the calories you burn.” He explains that because your energy level changes, the way you feel during certain exercises will also change.
“If someone is used to high-intensity exercise, more used to yoga, the transition will be harder,” my doctor said. So, like the oatmeal you eat for breakfast, your body gets used to burning sugar first when you participate in short bursts of intense exercise, like a spin class or sprinting. Now that you’re on the low-carb ketogenic diet plan, you don’t have sugar stores for energy and you’ll feel more tired than usual.
However, during activities such as barre, pilates or jogging (where the heart rate is not high), your body needs 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 use of the nutrients you consume as fuel. Cardio enthusiasts can still understand this; it didn’t just involve going out for 45 minutes. (Some fitness classes, like OrangeTheory, for example, have been known to follow this model.) Other exercises to focus on: running, floor exercise, and strength training.
As with all exercise and diet, my doctor says the most important thing is to pay attention to what you’re doing. Your body will tell you a lot about what works and what doesn’t; You just need to listen.