Dear Osa friends,

We’re continuing our conversation about exercise this week with a look at fat-burning (fat oxidation in science-speak) and how we might strategize to optimize this with the type of training we choose.

If you’ve ever exercised on any kind of aerobic equipment, you are probably familiar with the phrase “fat-burning zone,” because this is often depicted in some kind of catchy graphic right on the display panel of an elliptical machine, stationary bike, or other equipment. This zone may be more accurately described as the lower end of “zone 2“, which is defined as exercise at roughly 60% or so of your maximum heart rate. This so-called “fat-burning zone” typically describes low-to-moderate intensity work–the kind which increases your breathing rate but still allows you to carry a conversation. It sounds pretty good that this type of exercise would actually be the best for utilizing stored fat as energy, because stored fat is both a cosmetic and metabolic concern for many, many people, and this type of exercise feels quite doable. But also remember the strong evidence that higher VO₂ max conveys protection from death of any cause, which we discussed in the last two newsletters. Reaching and increasing your VO₂ max requires getting your heart rate closer to its maximum at least some of the time, so keeping it down at 60% all of the time is unlikely to have much of an impact on VO₂ max. How can we reconcile this with the “fat-burning zone”? Isn’t it a good thing to burn excess fat, and shouldn’t that impact longevity?

Well, it turns out there’s a catch to that “fat-burning zone”: even though the body does indeed utilize fat for a greater percentage of energy to fuel low-to-moderate zone 2 exercise, the net utilization of fat is actually more with a higher intensity workout of comparable or shorter duration due to the greater total energy demands of higher intensity exercise and the mitochondrial adaptations that occur in the recovery period. In summary, continuous low-to-moderate aerobic exercise will not burn as much fat as higher intensity exercise of shorter duration. The silver lining here is that you can achieve better results in less time by incorporating higher intensity training*.

Let’s take a look at a ​2020 study ​involving postmenopausal women that illustrates this point. Here is the short version: among 3 groups of women who exercised 3 times per week for 12 weeks, those who did 20 minutes of high-intensity interval training (HIIT) lost abdominal fat, whereas those who did 40 minutes of moderate-intensity continuous training did not reduce their abdominal fat. A third group who did 40 minutes HIIT plus 20 minutes weight training lost the most fat and also increased muscle mass.

Here is a more detailed description of the study: 27 overweight postmenopausal women were divided into 3 groups: a moderate-intensity continuous training (MICT) group who exercised for 40 minutes at 55-60% of peak power, a high intensity interval training (HIIT-only) group who performed repeated cycles of high exertion/near-sprint for 8 seconds followed by 12 seconds of slow pedal recovery for a total of 20 minutes, and a HIIT + resistance training (RT) group who did the same exercise at the HIIT-only group but also did a 20-minute weight routine consisting of upper and lower body exercises, with one warm up set and one set of 3-5 repetitions at 60-80% of perceived maximum exertion. All groups performed their aerobic exercise on bikes, 3 days per week for 12 weeks.

While all exercise did reduce body weight and had some impact on fat mass, only HIIT-only and HIIT + RT reduced abdominal and visceral fat (that’s the dangerous fat stored around organs). The HIIT + RT group also increased their percentage of muscle mass, whereas the other two groups did not. Having more muscle mass can impact fat loss over time because muscle increases the resting metabolic rate, which is how much energy is needed to sustain the body at rest. Indeed, even over the 12-week study period, the group who also incorporated weight training lost an addition 1.3 kg of fat compared to the HIIT-only group. However, even without the addition of weight training, and with exercising just half the time of the MICT group, the HIIT-only group experienced a significant loss of total abdominal fat, whereas the MICT group experienced no observable change.

A major takeaway for me from this is that it is beneficial to push our edges, and we don’t need to do this all the time to experience benefits. Also, when it comes to pushing edges, we need to be mindful that we don’t push ourselves over the edge, which can be counter-productive to progress. High intensity can still be in zone 2, meaning it is still aerobic work, but it will be at the upper end of it (roughly 70% of maximum heart rate) and not at the lower end. It is best to gauge by perceived exertion. Learn where your edge is first before you start to push its boundaries. And always include a warm-up and cool-down period before and after intense efforts.

One last tip I have for increasing the intensity of your exercise is to focus on your form first. If you cannot sustain good form at the pace you are attempting to meet, whether running or on the bike or in the pool, then you are likely going beyond what is appropriate and safe for your body. Form is the foundation of your movement–build that first.

Wishing you a beautiful and healthful week,

*Please know that this newsletter does not constitute clinical advice and is for educational purposes only. Please consult your healthcare provider about any exercise, nutritional, or supplemental changes to ensure your safety.