I have had a longstanding issue with cardio equipment. Not the treadmill itself. Not the elliptical. Not even the stair climber, although that thing and I have exchanged some words over the years. It’s the buttons. Fat Burn. Cardio. Weight Loss. Performance. Heart Rate. Interval.
For years I have watched people stand in front of a treadmill trying to decide which magical button will finally convince their body to burn some fat. And for years I have wanted to walk over, hit QUICK START and say, “There. I fixed it.”
Here’s the thing: Your body burns calories in all of those zones. Your body can use fat for energy in all of those zones. There is not some magical heart rate where your body suddenly opens the doors to the fat storage facility and starts hauling boxes out.
So you can imagine my delight when the internet discovered another zone. Zone 2. If you have spent any amount of time in the health and wellness world lately, you have probably heard about Zone 2 cardio. It is being talked about like exercise physiologists recently discovered a secret metabolic portal that has been hiding between walking the dog and running a 5K. We didn’t. Zone 2 training has been around forever. We just gave something humans have been doing for a very long time a number, attached it to a smartwatch and made it sound considerably more complicated.
Technically, Zone 2 is generally considered lower intensity aerobic exercise performed just below your first lactate or ventilatory threshold. In normal human language, you are working. Your heart rate is elevated. Your breathing has picked up. But you can sustain the effort for quite a while without questioning every decision that led you to this moment.
Depending on your fitness level, that could be a brisk walk, an easy jog, cycling, swimming, rowing, hiking or using an elliptical. The activity itself does not determine whether you are in Zone 2. The intensity does. One person’s brisk walk might put them squarely in Zone 2 while another person could walk at that same pace and barely leave Zone 1.
And this is where those old “fat burning zone” charts caused so much confusion. At lower and moderate exercise intensities, a greater percentage of the energy your body is using can come from fat. As intensity increases, your body begins relying more heavily on carbohydrates because they can provide energy quickly enough to keep up with the increased demand. Somehow, “a greater percentage of your energy is coming from fat” turned into “this is the zone where you burn body fat.” Those are not the same thing.
You are still using energy at higher intensities. You are still burning calories. You are still using a combination of fuels. And spending your entire workout staring at the heart rate on the treadmill because you are terrified that going three beats higher will switch off your fat burning abilities is completely unnecessary. Your metabolism does not operate like a light switch.
Now, before anyone thinks I am coming for Zone 2, I actually love Zone 2 training. I just hate what we have done to it. Zone 2 is sustainable. It allows you to accumulate a meaningful amount of aerobic exercise without beating the absolute hell out of your body. Regular aerobic training improves cardiorespiratory fitness and produces adaptations within the cardiovascular system and skeletal muscle, including those famous little cellular power plants everyone suddenly cares deeply about, your mitochondria. It is especially useful if you have spent years believing cardio only counts when you are drenched in sweat, gasping for air and contemplating whether someone should call your emergency contact. You do not have to destroy yourself for exercise to work.
But you also don’t have to spend the rest of your life in Zone 2. In fact, one of the interesting things about the research surrounding the recent Zone 2 obsession is that scientists are pushing back on the idea that this particular intensity is somehow the best or only way to improve mitochondrial function, fat oxidation or cardiometabolic health. Higher intensity exercise creates valuable adaptations too. Your body doesn’t have one magical zone. It has an incredible ability to adapt to the different things you ask it to do.
Some days that might mean a long walk or easy bike ride. Other days it might mean intervals that send your heart rate climbing. Sometimes it means lifting something heavy. Sometimes it means getting off the couch and doing literally anything.
Current physical activity guidelines reflect that. Adults can accumulate their aerobic exercise through moderate intensity activity, vigorous activity or a combination of the two, while also strength training at least twice a week. Notice nobody said you must live exclusively in Zone 2.
So if you love Zone 2, fantastic. Do Zone 2. Build your aerobic base. Take the long walk. Ride your bike. Get on the elliptical. Enjoy the fact that you can finish a workout without needing to lie on the floor afterward. But occasionally, your heart needs to remember it has Zones 3, 4 and 5.
And the next time you climb onto a treadmill and it asks whether you would like Fat Burn, Cardio, Weight Loss, Performance, Hill Climb or whatever other option has been programmed into that tiny judgmental screen, remember that your body is significantly smarter than the buttons in front of you. We didn’t discover a secret metabolic zone. We put a sexy name on reasonably challenging aerobic exercise, bought a $500 watch to tell us we were doing it and started talking about mitochondria at dinner parties.
Sometimes you really can just hit QUICK START.
