The Fugue

My Swimming Book makes the following claim: if you work out at lower (but still aerobic) intensity you burn more fat per calorie than if you work out at higher intensities. Note that it says per calorie, which means you have to work out longer. A rough guideline is to go the same distance as you would at the higher intensity.

Unfortunately he doesn't give any references and no numbers or graphs. One is left wanting more information. But if you try to google that topic you will find two camps yelling at each other from across a great chasm.

Group 1 says keep your heart rate down in the fat-burning zone. Group 2 says group 1 is a bunch of fools and that although the ratio may be higher you burn more fat by working at a higher intensity because you burn more calories. The amusing thing is that neither side ever cites any references whatsoever, nor gives any concrete numbers. Neither side gives due diligence to pointing out that you can burn the same amount of fat either by working easy-but-long or hard-but-fast. Calorie burn rate (time) and intensity are a tradeoff, but to the two groups there is no compromise.

Finally I found a white paper with solid logic and numbers and graphs. (Still not enough references, but at least there are some.) Check out the graph on page 8 which confirms what I suspected: there is a point of diminishing returns. Choosing Zone 2 over Zone 1 is a no brainer. Choosing Zone 3 over Zone 2 looks marginally smart. Choosing Zone 4 over Zone 3 (when fat loss is the only consideration) is silly—you use more calories to burn the same amount of fat. Of course there are lots of reasons to go into Zones 3 and 4, like training for races, cardiovascular health, building more muscle, and burning off the extra helping you had at breakfast. But if you're looking to burn fat efficiently, It looks like roughly 70% of maximum heart rate (220 minus your age, as an estimate) is the point of diminishing returns, and a good aim.

Now, that said, there is a lot of variance in the fat burning range. In fact, the whole point of that white paper is to teach you how to increase that anaerobic threshhold so that you can burn a higher fat percentage at higher intensities, effectively raising that point of diminishing returns and allowing you to burn more fat per minute. This is another thing the TI book said you could do but didn't give enough details for my taste.

Of course, let it not be forgotten that the thing that makes the most difference is actually getting up off your duff and into the pool.