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.
The prevailing formula for a long time on how much fat you’re going to burn was calories in minus calories out, based on your basal metabolic rate (BMR) and exercise efforts, explains strength and performance specialist Joel Seedman, Ph.D., owner of Advanced Human Performance in Atlanta. But with all the different biochemical reactions in the body, hormonal response, and endocrine function, there are an infinite number of factors that can affect how your body is storing and breaking down calories.