Heart Rate Training & Fat Loss Cardio

Sam Stephens

January 2020

What is Respiratory Exchange Ratio (RER), how does this value relate to fat loss, and why is it so important for heart rate specific cardiovascular training?

If you’re trying to lose a few pounds, a smart cardio program should be one of your top exercise priorities along with a great dietary strategy. To most effectively target body fat, it’s important that we have a basic understanding of how our bodies utilize different fuel sources relative to varying heart rate intensities. 

In a laboratory setting, we can calculate fat utilization through metabolic assessments by measuring the ratio of carbon dioxide exhaled to oxygen inhaled per breath. This measurement is known as the respiratory exchange ratio (RER). An RER of 1.0 indicates that 100% of fuel being consumed is from carbohydrates, a ratio of 0.7 is all fat, and 0.85 is an even mix of the two. This means that as exercise intensity increases, we gradually shift from burning fat to carbs for energy (RER goes up). 

So, how do we use this information to build a smart, personalized cardio program?

Our plan of attack here is pretty simple. We want to spend 1-2 hours each week doing cardio with ~80% of our time at low/moderate intensities (RER 0.85) and the remaining ~20% of our total weekly time at high intensities (RER 1.0). Moderate intensities should be performed at a 4-5/10. High intensities should be performed at an 8-9/10. Use a heart rate monitor to closely track changes and adaptations over time. 

By spending time at both ends of the intensity spectrum, we’re able to maximize our rate of fat oxidation and increase our aerobic capacity thanks to quite a few pretty cool physiological adaptations. Low intensity work burns fat directly and high intensity work improves our aerobic base, allowing us to delay the transition from fat to carbs in higher intensity exercise settings. If we can delay this fuel crossover, we can burn more fat for longer. 

To read more about fat loss cardio, check out Fitness & Nutrition Programming for Beginners