Why You Need to Train Your Fat Burn System
You may think that fat burn all boils down to calories or how much you work out. But what if I told you it's possible to be bad at burning fat? Think of a Hybrid Car (an analogy my professor liked to use): the battery is fat for fuel, the gas is carbs. If you’re bad at burning fat, your body switches from using the battery as its primary source of fuel to gas at much lower speeds than someone who is an efficient fat burner. In practice, that means basic movement feels harder for you, sooner, than it does for someone who trains. Walking a few blocks, hiking, flights of stairs - many things you experience in the day-to-day are affected as the majority of our day (basically every minute outside of the gym) is low to moderate intensity. If you train your fat burn system, through zone 2 cardio (which we’ll explain below), you increase the bucket of activities your body finds easy. You increase the speed at which your Prius says “oh wait, now we need the gas.” The threshold at which your body says “this is getting hard” goes up, and up, and up.
So do hard workouts actually burn fat?
The short answer is, not really. Not during the workout at least. The idea that workouts burn fat stems from research behind excess post-exercise oxygen consumption (EPOC): the physiological fact that if we work really hard during a workout, our body will increase fat burn after the workout. This part is true, after a HIIT class or hard lift, fat burn will be higher. However, most fail to explain that and also avoid discussing potential downsides: the impact on appetite and time to recovery between high-intensity workouts. That’s not to say HIIT workouts don’t have value, they do. But, they are just one exercise tool to incorporate 1-3x per week—certainly not a definitive solution for weight loss for everyone.
So, when do we burn fat?
All day really, but before we dive into the physiology of fat burn let’s differentiate two key terms: fat burn and fat loss. Fat burn is the process of your body using fat as fuel for movement or other organ functions. It occurs all day but is upregulated during certain intensities of exercise. Fat loss is the actual loss of body fat, subcutaneous or visceral, derived from a net negative energy balance or calorie deficit.
A calorie deficit is a requirement for fat loss, but not a requirement for fat burn. It’s like a square rectangle. You can burn fat without having fat loss: by consuming net even calories or more than what you burn. In this case, any fat burned will be replaced by fat stored from the calories consumed. But, you can’t have fat loss without fat burn, and if you are in a negative energy balance the fat you burn will stay off. To accomplish a calorie deficit, a nutrition plan is key - not a fat-burn workout. But, we’ll touch on how burning fat for fuel during exercise can assist your calorie deficit.
In terms of exercise, fat burn (oxidation), is a metabolic function that provides fuel to muscles at low to moderate intensities. Why not high intensities? At higher intensities, oxygen is more limited (hence the wheezing). The conversion of fat into usable energy is a slow process that also requires oxygen (hence the term oxidation), something we can’t provide in significant quantities at higher intensities. That doesn’t mean you shouldn’t work out at higher intensities. There are plenty of other benefits of working at higher intensities, including higher calorie burn/hour, but you certainly will not be burning significant amounts of fat during the workout.
So what does our body burn as intensity increases?
Carbohydrates mostly. Carbohydrates can be burned both aerobically (with oxygen) and anaerobically (without). The process of using carbohydrates aerobically is a bit faster than fat oxidation, so we can use that at slightly higher intensities than we can fat - but even after a certain intensity, we can’t provide enough oxygen to rely solely on aerobic carbohydrate usage. That’s when we would rely more on anaerobic carbohydrate use for our main source of energy —this is typically when you start to feel the ‘burn’ most intensely.
How does it relate to fat loss if a calorie deficit is the key for fat loss?
Two reasons. First, higher-intensity exercise is still a great way to add calories burned in a small amount of time, but it does require more recovery. Low to moderate intensity exercise can be done daily and in higher quantities without as much demand for recovery, making it easy to add a lot of calories burned. Second, our body has an immense amount of fat stored for fuel but only a small amount of carbohydrates. When we burn carbohydrates, they need to be replenished whereas fat burned doesn't.
Fortunately, our bodies are pretty cool; the carbohydrates can be replenished by breaking down stored fat and converting it to carbs in our liver through a process called gluconeogenesis. However, that process is fairly slow and the main method we replenish is by, you guessed it, eating. Some people are lucky and can work out super hard without getting hungry after. But, if you’re in my group, this higher-intensity workout often comes at the cost of an insatiable pit in our stomach. So, the benefit of burning calories from fat is both direct and indirect: you do get to burn the fat you want to lose directly assuming your diet is in line, but more importantly you burn more fuel from a source that’s less likely to impact your appetite.
How do we train fat burn?
There are 5 cardio zones we typically define. Zone 1 being the easiest effort, and 5 being the hardest. Fat is the dominant source in Zones 1 and 2, drops significantly in Zone 3, and doesn’t do much for Zones 4 and 5. Zone 1 is anything above rest, but below training zone—you burn a high percentage of fat but you don’t actually make cardiovascular and metabolic improvements. Zone 1 is your step count, your random activity throughout the day, and your walk to lunch. We want as much of it as possible but the focus here is Zone 2.
Most wearables have generic heart rate zones programmed in, where Zone 2 is 50-65% of the predicted heart rate max. However, these averages can be really off if you’re on either end of the spectrum: untrained in Zone 2 or a pro endurance athlete. I happened to fit in the first group. I was a lifter and football player in high school, and never really spent time doing long duration aerobics. The textbooks would tell me my heart rate should be between 120-140, but whenever I tried that it was miserable. I could fight through it if I had to, but purely unenjoyable. After doing direct testing with our Tier X Program, we found my target heart rate was way lower with my lack of training. I had to work at 90-99 bpm before I could work my way up to the averages of 120-140. On the flip side, a pro runner could be 140-160 for Zone 2.
- So, while most people will fit in the 50-65% zone, there are accurate alternatives to judge what your right intensity is:
- Rating of Perceived Exertion (RPE): How difficult it feels from 1-10. You want to be in the 4-6 range
- Talk Testing: full sentences should be easy, but if someone was on the phone with you they could tell you were probably moving
- Physical discomfort: there should be none, only some boredom
- Theoretical duration: it should feel like a pace you could hold for quite literally 4-6 hours if you had to (please don’t actually go that long)
- Easier as you go: if you go for an hour, minutes 55-60 should only feel easier than minutes 5-10 or 15-20.
Any of these methods are shockingly accurate for finding your Zone 2. If you like using a heart rate target, wear a heart rate monitor and use these tools to find your zone. Then match your heart rate to that zone, and hold that as a target for at least a few weeks. Keep in mind that Zone training means time spent in Zone 2, not looking at the average HR after a workout. You want to sustain that target zone for the duration of the workout.
How frequent?
150 minutes per week is a good target, and this is about duration, not intensity. If you’re just starting, aim with 3 x 30 min per week. After a month or two, increase to 3 x 45 and eventually 3 x 60 min. It should truly only feel better every time you increase duration, so sometimes starting at 2 x 45 or 2 x 60 before adding a third day feels better.
What device?
Any and all cardio devices work for Zone 2. The only considerations are you’d have to train at a lower target heart rate (~10 bpm lower) for a few devices: bike, stairs, rower, and airbike. Treadmill, walking, hiking, running, and elliptical should be true to the targets.
How long before I see improvements?
The more frequent and longer you go, the faster your improvements will be. While it does take time to truly build your aerobic foundation, you’ll likely notice some acute effects in your energy levels throughout the week and your ability to recover from harder exercise early on. I wouldn’t rush to increase your Zone 2 speed, the benefits come from doing it more than getting to a certain mile split while in Zone 2.
Final Thoughts
I don’t love using anecdata, especially in this instance as there’s plenty of research available, but I’m a prime example of someone who was visually “fit” but was horrendous at using fat for fuel. After testing in 2018, my body switched from fat to carbs at only a 2.5 mph flat walk - a stroll if you will. Now, since I’ve improved it, it’s hard to describe but also simple: everything is easier. It sounds silly, but I feel light on my feet, I jog to the bathroom during sessions without sweating, and have finally found running (moderately) enjoyable after years of hating it. I used to drag my feet, even after a red-eye from Starbucks, and never understood where my ironman coworkers’ energy came from. Getting stronger was a lot of fun, but building an aerobic foundation after having none was life-changing.