Most swimmers have put in countless hours of long and slow swimming. While there is proven anecdotal evidence behind long and slow swimming including:

  • Ability to increase biomechanics efficiency without excessive neuromuscular fatigue (or in Layman’s terms, helps a swimmer learn to “hold” more water).
  • Increased mitochondrial/capillary density.
  • Burns fat.

Obviously, because this is anecdotal evidence, it can be argued that swimmers can achieve the above through high intensity work. But today, I’m going to be talking about staying in the “fat burning” zone. The fat burning zone most people are aware of, states that if you go above 60-70 % of your maximal heart rate (HR), then you will stop burning fat, and switch to glucose for fuel. However, this may not be true. 

Exercise Target Zone


Assuming that an individual is healthy, the main energy source that is utilized during exercise are mainly carbs and fats. The ratio of the utilization varies by the training intensity and duration. This ratio is called the Respiratory Exchange Ratio (RER) or Respiratory Quotient (RQ). At a lower intensity, fat is the main source to produce energy. As training intensity goes up the duration goes down, and the contribution of carbohydrates increases. RER is calculated by oxygen intake and CO2 production during exercise via gas analyzer.
The chart below is my own personal VO2 max test. During the test, intensity incrementally increases on a treadmill. I pulled data from two different intensities during the test, to prove my point. At 69% of my HR max you can see that I burn 15.5 kcal from fat/min, while at 82% of my HR max I burn 18.9 kcal from fat/min. Therefore, I burn more fat at a higher intensity. Additionally, short high intensity work may burn more total calories than long high intensity work when matched in duration (Tucker et al., 2015).  The myth where you HAVE to exercise at a low intensity to burn fat comes from relative numbers and not absolute numbers. At 69% of HRmax, my RELATIVE fat burn is much higher due to the low carbohydrate contribution. At 82% of HRmax my RELATIVE fat burn drops due to the increase carbohydrate contribution. Now, although everybody’s kcals from fat at each HR will differ, most swimmers will probably burn fat at a higher intensity, due to their above average fitness levels. These number may also look different from when I working out on land vs. if I did the same test with a Douglas Bag in the water.  Therefore, if you have some work to do this summer before the season starts back up again and fat burning is the goal, some form of high intensity work should be implemented into your workout regimen, whether it be interval training, sprints, or resistance training. High intensity training has also been proven to have health benefits including: improved endothelial function and cardiometabolic health, so why not start adding some to your training? (Gillen et al. 2016; Ramos et al. 2015; Sawyer et al. 2016)

Take Home Points

  • Low intensity exercise has a higher RELATIVE fat burn, but high intensity exercise has a higher ABSOLUTE fat burn.
  • If the goal is fat burn, you want a higher Absolute number.


Gillen, J. B., Martin, B. J., Macinnis, M. J., Skelly, L. E., Tarnopolsky, M. A., & Gibala, M. J. (2016). Twelve Weeks of Sprint Interval Training Improves Indices of Cardiometabolic Health Similar to Traditional Endurance Training despite a Five-Fold Lower Exercise Volume and Time Commitment. Plos One, 11(4). doi:10.1371/journal.pone.0154075

Ramos, J. S., Dalleck, L. C., Tjonna, A. E., Beetham, K. S., & Coombes, J. S. (2015).The Impact of High-Intensity Interval Training Versus Moderate-Intensity Continuous Training on Vascular Function: a Systematic Review and Meta-Analysis. Sports Medicine, 45(5), 679-692. doi:10.1007/s40279-015-0321-z

Sawyer, B. J., Tucker, W. J., Bhammar, D. M., Ryder, J. R., Sweazea, K. L., & Gaesser, G. A. (2016). Effects of high-intensity interval training and moderate-intensity continuous training on endothelial function and cardiometabolic risk markers in obese adults. Journal of Applied Physiology, 121(1), 279-288. doi:10.1152/japplphysiol.00024.2016

Tucker, W. J., Sawyer, B. J., Jarrett, C. L., Bhammar, D. M., & Gaesser, G. A. (2015). Physiological Responses to High-Intensity Interval Exercise Differing in Interval Duration. Journal of Strength and Conditioning Research, 29(12), 3326-3335. doi:10.1519/jsc.0000000000001000