If you’ve been anywhere near the weight loss and exercise forums over the last few years, you’ll have heard of the ‘fat burning zone’. The theory runs that maintaining your heart rate at 70-80% of its maximum for a certain period of time will push you into a metabolic ‘zone’ in which your body will burn more fat than would otherwise be the case.
It sounds like plausible science and, with modern heart-rate monitors and other wearable fitness tech, reaching and maintaining the fat burning zone should be easier than ever. But is this metabolic hack too good to be true? Let’s delve a little deeper.
What’s the link between heart rate and fat burning?
As a basic rule, your heart beats faster when your muscles need (or are likely to need) more fuel. Muscles are fuelled by glucose (metabolised sugars). Glucose comes from calories stored as either glycogen or fat.
Oxygen is needed to break down glycogen or fat into glucose, which is why your heart beats faster (and why you breath harder) when your muscles need a power-up: your cardiovascular system is busily speeding oxygen to glycogen and fat so that you’ll get the muscular boost you need.
So, the ‘fat burning zone’ makes sure that the body only burns fat, not glycogen?
Glycogen is way, way quicker and easier to turn into glucose than fat is. According to the fat burning zone theory, glycogen is therefore used for ‘emergencies’. So, seriously high intensity workouts would be more likely to draw from glycogen first, as muscle power is needed as a priority. The theory continues that exercising at lower intensities is less ‘urgent’ for the metabolism, meaning that the body will draw from its fat banks rather than its ‘emergency power source’, glycogen.
It sounds good, and plenty of us would love for it to be true. Who doesn’t want to give up the tough, high intensity stuff for a spot of hillwalking or a stroll round the block at lunchtime?
I sense a ‘but’, coming.
You’re right to do so. As with pretty much everything to do with weight loss, the truth is a bit more complicated.
While your body does indeed use up glycogen as an ‘emergency’, when looked at minute for minute, lower intensity workouts don’t actually burn more fat than high intensity ones.
Glycogen gets burned through pretty quickly. While your body will reach for the glycogen first during a high intensity workout, it doesn’t store much glycogen, so it gets through its stock fast. Then, it turns to its fat reserves. Because of how hard you’re working your muscles and how fast your heart is beating, you’ll probably get through more fat during a high intensity workout than you would on a ‘fat burning zone’ workout lasting the same amount of time.
But I hate high intensity workouts! Is my weight loss doomed?
Not at all! The good news is that there are plenty of ways to lose weight without putting yourself in physical pain. While the fat burning zone is a bit of a myth when compared, minute-for-minute with higher intensity workouts, if you’ve got hours to spare on lower intensity exercise then you’ll start seeing results quickly. Spending a few hours hiking, or cycling, or even strolling around a local park will all have an impact. Higher intensity workouts are just a way to get the same kinds of results more quickly.
If you want to combine the fat-burning power of a high intensity workout with the ease and enjoyment of a lower intensity one, try adding intervals to your exercise regime. If you’re a walker, try sprinting for a minute or so every now and again. If you do aerobics, up the ante for short periods of time before returning to a more comfortable pace. This will raise your heartrate and boost your metabolism without exhausting you.
One problem with high intensity workouts is that people come to dread them or find that they can’t complete them. This means that they ultimately give up on their exercise regime. Any weight loss regime needs to be achievable. If low intensity workouts (with or without intervals) are what will keep you exercising, then keep at it. Honestly, whatever you do, so long as you are exercising somehow and eating healthily, you will see results!