In our previous post we explored the relative energy contribution from fats and carbohydrates – particularly as it relates to exercise intensity. Figure 1.0 below adds context to this discussion as it reflects the anticipated caloric distribution for an average 130lb. female over the course of a 30 minute cardio workout at 2 distinct exercise intensities.

Figure 1.0: Relative Caloric Expenditure + Source Substrate

Note the relative input from fat was 10% lower during the high intensity workout, yet total caloric and absolute fat expenditure was notably higher. While this supports the existence of the “fat burning zone” it clearly demonstrates its limitations in practice.

Bear in mind, these figures are limited to calorie and fat utilization within the workout itself. Of course, we have since learned that the real fat burning magic of high intensity exercise occurs following the workout. Known in the scientific community as EPOC (excess post-exercise oxygen consumption), the “afterburn” effect is markedly greater following high intensity exercise.(1) Of course we already knew that. So, what does it mean? And how does it impact our training decisions?

Consider the outcomes of a recent study published in The Journal of Applied Research.(2) Authors of the paper investigated the effects of a short duration HIIT workout on the afterburn effect. Following a mere 6 mins. of high intensity, interval work consisting of “squats, jumping in place, lunges, and split jumps”, participants’ metabolic rate was measured at various stages in the recovery process. Here are a few of their findings:

• Average caloric expenditure was 63.2 kcals. - almost half that of the 30 mins. “fat burning zone” workout above!

• At the 3, 6 and 24 hr. post exercise mark, metabolism remained elevated by 26, 20, and 15% respectively.

• Metabolism remained elevated for up to 36 hrs. following the workout!

Perhaps the most important finding however… Authors noted that carbohydrates predominated in exercise while fats were the primary contributor during the recovery period – WHEN THE VAST MAJORITY OF CALORIES WERE BURNED!!! In fact, post exercise calorie expenditure over the 24 hrs. recovery period was 5 times higher than during the workout itself. Furthermore, if you recall from our previous post, fat utilization increases beyond our normal resting level following high intensity work.

Allow me to reiterate these findings are following a SIX MINUTE WORKOUT that included a 1 min warm-up!!! Consider the outcomes following a 20 mins. HIIT workout!?! Hmmm… Jane Fonda would be SO proud! These results effectively explain why I feel like I am in the best shape of my life despite working out less putting in a ton of work and family time. And in case you believe these findings are anomalous, they are well supported by other researchers.(3, 4, 5) But beyond these seemingly un tenable GAINZ (lol), the body actually adapts to HIIT in other ways that further enhances its fat burning/body sculpting effects… But that’s a topic for another day! Stay tuned.