‘Slow cardio’ claims to maximise fat loss and endurance: does it stack up?

High Intensity Interval Training is being challenged by the new trend of ‘Zone 2’ lower intensity training. Adherents claim it will make you burn more fat and even improve longevity – but is true?

Personal trainers the world over are now espousing ‘slow cardio’. Picture: istock

For at least a decade, we’ve been told smashing out short bursts of high intensity activity followed by (way too short) rests is the modern panacea for fitness gains. High Intensity Interval Training forms the basis of most circuit-style gyms that have popped up in virtually every suburb. And there’s no doubt about it, doing a session of HIIT leaves you feeling absolutely tanked.

But if you follow fitness trends closely, you may have noticed that personal trainers the world over are now espousing ‘slow cardio’. They claim you’re better off going long and slow in your aerobic training than fast and hard. It’s known as ‘Zone 2 Training’ and adherents claim it will not only improve endurance and speed in runners, but that it’s more effective for fat loss for everyone as well as improving insulin sensitivity, increasing the percentage of blood that gets pumped with each heartbeat, improving the body’s oxygen capacity and even increasing longevity.

Anyone who has worked out on a treadmill might remember the little graphs on the machine mapping the heart rate zones and specifying that the 60-70 per cent bracket was the “fat burning zone”. Zone 2 Training is built on pretty much this concept. It’s a bit like back to the future.

“This is certainly the new concept on the block,” says Phil Bellinger, exercise scientist and Senior Lecturer at Griffith University. “It is pretty much the concept of the fat-burning zone repackaged.”

In order to evaluate the fitness claims of Zone 2 Training, you need to consider the science of how the body utilises energy during exercise.

Cycling is an effective type of ‘Zone 2’ training. Picture: istock

In lower-intensity exercise such as brisk walking, steady-paced swimming or cycling, or slow jogging, fat oxidisation is maximised. That is, the body predominantly uses fat as a fuel source for the muscles. Because the process is more energetically efficient, it’s possible to continue to exercise at a slower, steady pace for much longer, potentially hours (good luck with that). When the heart rate increases into Zone 3 and higher, that is, above 70 per cent of maximum heart rate, the body draws on glycogen, which is stored glucose in the cells, as an energy source.

“When you start exercising, your body’s demand for energy is met through either the burning of fat and or the burning of carbohydrate,” Dr Bellinger says. “The first port of call is fat utilisation, because the body’s got quite a large store of that. However, the downside is that it’s limited in terms of the rate of energy that you can derive from fat. So that’s why we eventually turn to carbohydrate, because we can break it down more quickly.

“Essentially when exercising at an intensity in Zone 2, you do maximise the rate of fat oxidisation. If exercise intensity is low enough, then fat will be the primary fuel source.”

So if you’re burning mostly fat for energy in Zone 2 Training and it’s a hell of a lot easier, why wouldn’t you merrily cruise during cardio, stick on podcast and have a nice long walk instead of smashing yourself in a HIIT workout? Losing maximum fat is what most of us want, am I right?

Let’s start with one baseline concept: all exercise is good exercise. If you generally hate significant exertion, just walking, or any form of movement that raises your heart rate moderately, is awesome. You will experience some fat loss, increases in cardiovascular function, and even lower your risk of Alzheimer’s disease and some cancers.

But the idea that HIIT doesn’t burn potentially just as much fat as slow cardio is a myth, according to scientists.

“The key point that gets lost is it’s not as if when you push into say Zone 3, that you stop burning fat, it’s just that you also burn carbohydrate,” says Dr Bellinger. “It’s never the case of one energy source switching on and the other switching off, they’re always working synonymously.

“Some people think if you stay in Zone 2, that’s going to make you burn the most fat, but really, if you’re doing interval training, then you’re also going to burn fat, it’s just that you will also burn carbohydrates.

“So your overall energy expenditure, including both carbohydrate and fat will be greater than Zone 2. To do 30 minutes in Zone 3 or 45 minutes in Zone 2, I don’t think you could say one session is better than the other. It’s just that they have different physiological demands. And the key message in a well rounded training program is you really need both types of stimuli.

During steady-paced swimming, fat oxidisation is maximised. Picture: istock

“But the idea that there’s this magical fat burning zone, that’s definitely a myth. I think it’s like anything in the fitness industry: you need a catchy headline and this is the latest trend.”

The take-home message according to Dr Bellinger is your training should incorporate a mix of different types of exercise and intensity. Go for your life in Zone 2, but you shouldn’t abandon short blasts of higher intensity exercise, which can be as little as 30 seconds of maximum effort in rounds in a session lasting only five or six minutes. Include strength training at least twice a week, and rest days of just walking or gentle yoga will allow recovery which is also essential to building strength and endurance.

Not only will you get the benefits of all types of training, but such a routine means you’re far more likely to stick with exercise as a long-term habit. “A program which maximises long-term adherence has different elements of all types of sessions,” Dr Bellinger says.

We can probably conclude, then, that F45 won’t go completely broke quite yet.