The Mythical Fat Burning Zone
Look at any cardio machine at your gym and you'll see a little sticker that shows which heart rate ranges are better for... whatever reason you have to use their machine. There's one that's called "Fat Burning" that I'm sure you've seen and thought "hm... I wonder if I'll lose more weight if I try to stay in that zone?" It's usually between 50% & 60% of your maximum heart rate (the elliptical machines at my gym have it between 90 and 120 bpm).
In short, this zone is really only needed by ultra-endurance athletes, and unless 60% of your max heart rate is as intense as you can go, it's pretty terrible for weight loss. But before I go into that, it's important to look at what energy sources your body uses while exercising.
Your body require a specific molecule, ATP, in order to fuel muscle contraction. ATP is a very short term fuel source, and is only stored in tiny quantities (seconds worth at any given time), so it must be constantly created from other sources like fat, carbs, or protein. Protein does not contribute very much while exercising, so for the purposes of this post, I'm going to skip it.
At around 50% of your max heart rate, fat supplies most of the fuel for ATP creation, which is why you see the unfortunate "fat burning zone" label on the machines at the gym. Fat is a very efficient fuel source, but it takes a while to convert it to ATP. The good thing (or bad thing in the case of losing weight) is that you have a ton of it stored in your body. It's a slow burning fuel stored in a big tank. If fat were a non-renewable fossil fuel, it would be coal.
At around 60-70% of your max hear rate, you will start using more carbs as a fuel source than fat. That's not to say that you stop metabolizing fat (in fact, when your heart rate increases, you will metabolize fat at a faster rate), it's just that you will start burning a higher percentage of calories from carbs. Carbs are much more efficient to convert to ATP than fat. If fat is coal in our analogy, then carbs are jet fuel.
But why do we care about burning a bunch of calories from carbs when we could just burn fat? Sure, higher intensities might burn slightly more fat, but is it worth it for me to expend the extra effort?
The reason is pretty simple. You have a limited store of carbs that your body can use while exercising (about 1 to 2 hours worth depending on intensity). After you work out, and especially while you are sleeping, your body has to refill its store of carbs so that it can be ready for whatever else you throw at it. To refill the store, your body needs to convert from an existing energy source, your body fat. So while you are sleeping, your body is busy converting your fat onto carbs to replace the ones you used while you were working out.
So, the next time you're working out, decide what you'd rather do
- Pay heed to the "fat zone" label and burn maybe 150-300 calories in an hour, burning fat only while you are working out.
- Increase the intensity. Burn more carbs than fat as a fuel source, and two to three times as many total calories, and then burn fat later while you sleep.
So why did I mention the thing about super-endurance athletes? Well, you can literally workout for several hours (days even) provided that you have enough fat in your body. If you keep your heart rate at about 50%-60% you can keep going until your fat stores run out. There are ultra-marathon races that are 100 miles long and start and end with the sun down. In this scenario, wouldn't it be better to burn mostly fat?
Your carb stores on the other hand only last about 1 to 2 hours, after which, you must go back down to a lower intensity to burn fat (or, you can replenish carbs by eating/drinking while you're working out, but then you're burning calories directly from your food and not the calories stored in your body).
I don't know about you, but spending more than 2 hours at the gym doesn't sound very appealing. The pure and simple fact is that you will burn the same amount of fat regardless of the type of energy you use while exercising. Fat loss is only related to the amount of calories you burn, and you will burn way more calories by kicking up the intensity than by sticking in the so called "fat burning zone".
The processes above are specific to a certain type of exercise. They require the presence of oxygen, supplied by your blood, to convert carbs or fat into ATP. This process is called aerobic respiration. However, once your intensity increases to a certain level, your muscles will no longer be able to get oxygen fast enough to create the ATP to keep moving.
When this happens, the carbs stored in your muscles will be converted to ATP without the presence of oxygen, in a process called anaerobic respiration. Anaerobic respiration is highly inefficient compared to aerobic respiration. It only produces 1/20 the amount of ATP from the same amount of carbohydrates, and it also creates lactic acid as a byproduct. As lactic acid builds up, your muscles will start to burn, become fatigued, and eventually you will become physically unable to continue at such a high intensity.
There is a specific type of training that takes advantage of this process, called High Intensity Interval Training (HIIT). The premise behind HIIT is that, by exercising anaerobically, you will metabolize carbs more quickly than via normal aerobic exercise. It usually consists of a period of very high intensity exercise (90-100% max heart rate) lasting anywhere from 30-90 seconds. This is followed by a period of low intensity exercise for the same duration or until your heart rate drops below a certain threshold (70%-ish of max). You then repeat the intervals anywhere from 8-12 times.
The appeal of this type of exercise is that, through the inefficient process of anaerobic respiration, it allows you to burn through a large chunk of your carb store in as little as 20 minutes. Because of the lactic acid build up, it also shouldn't be performed without at least 48 hours to recover, so for those MWF people, who have limited time to spend at the gym, it can be a great way to burn a ton of calories with a minimal time investment.
HIIT isn't for everyone though. It's extremely physically demanding. You shouldn't even attempt it unless you have a good amount of endurance built up from a regular exercise plan. The goal is to go as hard as you possibly can for each of the high intensity intervals, until you literally can't continue (due to lactic acid buildup). It's a lot higher impact than normal aerobic exercise, and you have to be able to maintain a certain level of control to avoid injury (falling off the machine). If you do it right, by the time you finish your last interval, you'll be close to passing out or puking or both, and you'll barely be able to stand on legs that feel like rubber. So if that doesn't appeal to you, then it's probably not for you.
The other major reason not to attempt HIIT is if you aren't limited by time. You will be able to burn more calories through normal aerobic workouts, since you aren't limited by the 48 hour recovery period.
So hopefully I was able to shed some light on some of the processes that occur during exercise and how they relate to your weight loss goals. I'm sure I still have a few nuggets on the subject that you can expect in a future update.