Fitness influencers are giving up high intensity exercise. But why?

Anyone who survived 2010s diet and fitness culture will be familiar with high intensity interval training, better known as HIIT. In recent years, HIIT has remained popular, but there’s been a noticeable shift towards more gentle ways of moving, like walking and Pilates.

On social media, some influencers (including those who focus on women’s health) even argue that high intensity exercise is not only unnecessary, but is bad for us. The basic premise of these claims is that too much cardio and HIIT puts the body under stress, raising cortisol levels and making it harder to lose fat.

Influencers on TikTok claim high intensity exercise spikes cortisol levels and can impede fat loss.Credit: iStock

The relationship between stress and exercise

“Exercise is a stress”, explains Dr Stacy Sims, an exercise physiologist and nutrition scientist. An increase in cortisol increases our respiratory rate, helping to break down fuel and contract muscles. A transient increase in cortisol is normal and expected during exercise.

Emmanuel Stamatakis, professor of physical activity, lifestyle and population health at the University of Sydney’s Charles Perkins Centre, says what’s often forgotten is the distinction between acute (short-term) and chronic (long-term) stress from exercise.

Like Sims, he says stress is a natural and necessary outcome of exercise.

“It is true that right after exercise, we see an increase in cortisol levels. But when this is repeated, time and time again, we’re going to see chronic reductions. How this manifests itself is people have less stress.”

Stamatakis suspects some of cortisol’s bad rap comes from confusing psychological stress with physiological stress, even though exercise, a physical stress, can alleviate emotional stress.

Is HIIT actually that bad?

Sims says the argument that high intensity forms of exercise lead to increased cortisol are fundamentally misguided, and that most influencers are actually talking about moderate intensity exercise. “If you’re taking an Orange Theory or an F45 or even a CrossFit class, it’s 45 to 60 minutes. There’s no way you can maintain that true high intensity for that long.”

HIIT is typically defined as training intervals performed at 80 to 95 per cent effort, of one to four minutes, with varying breaks in between.

“I can’t count how many women [I know] who feel like they have to hammer themselves in every workout.” And without adequate rest and recovery, she says stress levels can remain elevated in the body, putting it into survival mode, and leading to inflammation and fat conservation.

While high intensity exercise elicits a “significant decrease in cortisol within two hours,” resulting in lower cortisol in the long term, Sims says cortisol levels can stay elevated for up to 24 hours following a session of moderate intensity exercise.

The obsession with cortisol

While TikTok may be obsessed with cortisol – high levels of which influencers say can make it difficult to lose weight, among other things – Sims says in most cases, it’s not the culprit.

“Some cortisol is good and I think that’s the other thing that’s missing. We need some cortisol just to function and a lot of inability to lose weight isn’t from cortisol – it’s from not eating enough or poorly timed eating.”

She says it can be tempting to assume that someone’s hormones are out of whack, but the answer is often a lot simpler. “Don’t leap for cortisol. Let’s look at sleep, let’s look at your nutrition. How many processed foods are you eating? What’s your alcohol intake? How’s your sleep? It’s a whole lifestyle thing.”

What is the ‘right’ kind of exercise?

While there is no one-size-fits-all approach to exercise, Sims, who works primarily with women, says polarised training – a programme that involves exercise of opposite intensities, with strength training throughout, is best.

Moderate exercise is fine, she says, but should not be the primary focus. An emphasis on recovery, where something like mobility work or yoga is incorporated, is also important.

Stamatakis thinks we have a tendency to get caught up in the “unnecessary details” of exercise, and says we should focus on moving in whatever way we can.

“The problem is we can focus too much on what is the best and most effective exercise, when the problem is that people don’t do [any] exercise.”

“We have hugely underestimated the value of incidental activity,” he adds, pointing to research he conducted on vigorous incidental life activity (VILPA). The study found that as little as three to four minutes of VILPA per day could reduce the risk of death by 40 per cent.

In other words? Worry less about the details, and just get moving – in whatever form that may take.

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