People often think that low-intensity exercise, in which your body taps into fat metabolism, is the most efficient way to lose body fat. Some cardio machine displays even claim that you’re in the fat-burning zone when your heart rate is low and that you exit the zone once your heart rate rises. Physiologically, the second part of that statement isn’t true.
The science of metabolism:
Every second of the day, your body metabolizes, or breaks down, carb and fat stores to give you the energy you need to perform.
During low-intensity workouts like steady-state cardio, your body realizes the demand for energy isn’t urgent, so it taps into fat metabolism. The fats in your body (which contain vastly more energy than carbs) exist as triglycerides that float through your bloodstream and hang out within adipose or fat tissue. Because fat offers a slow-and-steady stream of energy, you’ll feel like you can sustain that effort for a long time, says Alex Zimmerman, CSCS, Los Angeles-based director of Equinox’s Tier X program.
Once you reach your ventilatory threshold around 78 percent of your maximum heart rate, you’re working at a moderate intensity. That’s when you switch gears and start burning carbs in the form of blood sugar and glycogen because they’re instantly available, he says.
The key to getting lean:
Fat metabolism is the most efficient way to produce energy, allowing you to go and go and go without burning through a ton of calories. But efficiency is the enemy of fat loss. To lose body fat, you want to churn through calories—calories from carbs, calories from fat, calories from anything—and you get there through carb metabolism. “The more energy-demanding the work, the more likely you are to lose body fat,” Zimmerman explains.
How to optimize fat loss:
To lose fat, the bulk of your sessions should be high intensity like running and cycling sprints, he says. Heavy lifting and circuit-style strength training also use carbs for energy. They’re important for fat loss for another reason: They build muscle, which increases the number of calories your body naturally burns on a daily basis.
The ideal divide between high- and low-intensity exercise depends on each person’s fitness levels, training history, and more. As a general rule, when trying to lose fat, use low-intensity cardiovascular exercise to warm up and cool down from high-intensity, circuit-style, or heavy strength workouts as well as a form of active recovery on rest days.
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