With its ability to get us fantastic health benefits with a smaller time commitment than other types of exercise, it’s no surprise that HIIT has surged in popularity over the last 10 to 15 years. Compared to traditional endurance training like moderate-intensity cycling or running, research shows we can get similar adaptations in lung and heart health, metabolism, and performance with as little as a quarter of the time commitment.

What’s more, regular HIIT helps increase insulin sensitivity (which means we get better at managing sugar, which is imperative for metabolic health), improves our cardiorespiratory fitness (which is associated with living longer), and can be great for losing fat (which can be beneficial for healthspan).

However, this doesn’t mean you should always be looking to add more and more HIIT to your workout routine—it turns out that there is a sweet spot to be found for each individual when it comes to the ideal volume of intense training.

The size of your high-intensity dose matters in the long term

Most people starting out will be sufficiently challenged by a 15- to 30-minute dose of HIIT three times per week, and as you get more used to the stimulus, you can stretch that to four or five sessions pretty comfortably, as long as your workouts stay short and sweet.

Some experts recommend getting a baseline of around 75 minutes per week of intense exercise to maintain optimal heart health, with a strong caution to not push beyond 4 to 5 hours of high intensity stuff in a week. Too much HIIT can cause overuse injuries and disruptions to important things like sleep patterns. This shouldn’t be a surprise—our bodies need the opportunity to recover.

In a really important recent study conducted at the Swedish School of Sport and Health Sciences, researchers found that participants doing 36 to 90 minutes of total high intensity exercise across a week (that’s roughly equivalent to 2 to 5 twenty-minute HIIT sessions a week) saw significant improvements in performance and cardiovascular adaptations.

However, once participants were exposed to excessive amounts of intense exercise (152 minutes of intense exercise across a week—roughly 8 to 9 twenty-minute HIIT sessions in a week, or 3.5 forty five-minute HIIT sessions), performance didn’t just stagnate: mitochondrial respiration decreased by 40% compared to the ‘sweet spot’ range (really not a good thing—it’s a sign of too much stress on the system), and the body’s ability to manage glucose was significantly negatively disrupted.

As researchers from this Swedish study wrote, “beyond this limit, negative effects on metabolic health and adaptation of physical performance, seemingly caused by a mitochondrial partial shutdown… start to manifest.” It really is possible to have too much of a good thing.

While you might get accelerated fat loss with an extreme exercise routine in the short term, it’s generally unsustainable due to the overwhelming load on your whole body—energy systems, metabolism, heart, muscles, joints, mental health, mood and central nervous system. Instead, it’s better to focus on building an exercise habit that you can healthily sustain, indefinitely.

As another group of researchers elegantly put it, “a lifelong habit of incorporating physical activity into one’s daily routine is essential to a lifestyle ideal for conferring well-being and longevity”.

The Case for Easy Exercise

So if the science says that it’s important to not get too greedy or impatient with the benefits of intense exercise, what can we do with the extra motivation we have to ‘get more done’?

Walk (or, cycle, swim, paddleboard, dance, play, etc, at an easy pace). Let’s explore why.

Besides being good for brain health and cognitive performance, the habit of consistent walking is associated with keeping a healthy blood pressure, retaining less body fat, and protecting against cardiovascular disease. Interestingly, in a meta-analysis of 173,146 total participants, researchers found that people who simply actively commute to work (cycling or walking) had an 11% reduction in cardiovascular disease risk. We’re talking 20 to 30 minutes of walking per day here, nothing strenuous at all.

Just as important as any of those physical health benefits is the positive correlation between long-distance walking and improved mental health. Oh, and low-intensity activity also happens to be fantastic for active recovery and boosting immune function.

Metabolic flexibility: the magic that happens when we mix easy exercise with hard exercise

We don’t need a biology lesson here, but for this next part, it’s important to talk about mitochondria for a moment (bear with me). Your mitochondria are the powerhouse of your cells—broadly, they help take the energy we get from food and turn it into energy we can use.

While HIIT can be really good for increasing mitochondrial health, content and size, consistent lower intensity activity like walking helps with mitochondrial efficiency. This is super important, as mitochondrial dysfunction is associated with obesity and type 2 diabetes, as well as ageing-related diseases, amongst other issues.

During easy exercise, your body primarily uses fat to fuel that activity, which is a hugely important mitochondrial function that you need to ‘exercise’ for long-term health. During intense exercise, because of the rate at which you need to expend energy, your body actually has to rely mostly on stored carbs to fuel that activity (don’t worry, you still use fat later on during your afterburn, as your body recovers over the many hours post-session).

By making a habit of taking long walks each week, you train your body’s capacity to use fat directly as a fuel source. Unsurprisingly, building this capacity to use fat directly for fuel makes it easier to stay lean in the long term, especially if you’re already doing a few HIIT or strength sessions each week.

This metabolic flexibility, where your body gets really good at switching between using carbs and fat for fuel throughout life, is really impactful to our health over the long term. While metabolic flexibility tends to decline with age, we can strongly influence it with the right types of exercise: a mix of intense and easy activity.

In fact, lower intensity exercise is even good for improving our ability to use lactate as a fuel source, which is hugely beneficial in our ability to smash high intensity sessions (yes, lactate is actually an important fuel source, and not the thing that causes the burn in muscles as per the ‘lactic acid’ myth that was popularised 20 years ago—you can read about the history of this mistake in this short New York Times interview with one of the leading researchers in the field).

How much easy exercise should we do?

Human beings evolved to walk often, and from what we know about modern hunter-gatherer societies like the Hadza people in Tanzania, humans are probably designed to walk around 5 to 9 miles a day, just to live life. That works out to about 1.5 to 3 hours of walking spread across a day, or roughly 10,000 - 20,000 steps.

In a study of 4,840 participants in the US with an average age of 56.8 years, researchers followed participants for a 10-year period to study all-cause mortality. The data were broken down into four groups:

• Group 1: Those who walked less than 4,000 steps per day
• Group 2: Those who walked 4,000 to 7,999 steps per day
• Group 3: Those who walked 8,000 to 11,999 steps per day
• Group 4: Those who walked 12,000 or more steps per day

Across the 10-year period:

• The death rate in group 1 was 64% (419 of 655 individuals)
• The death rate in group 2 was 28% (488 of 1727 individuals)
• The death rate in group 3 was 11% (176 of 1593 individuals)
• The death rate in group 4 was 9% (82 of 919 individuals)

This was an observational study, so we cannot conclude that walking more lowers death rates (for example, a confounding variable is that people who walk more tend to have healthier habits, as well as lower rates of obesity and metabolic dysfunction, etc). However, it does make a very good case that consistent walking is probably a very good habit to have, and that each week, we could aim for, say, a daily average of 5,000 to 12,000 steps (or, 10,000 to 20,000 steps if you want to mimic hunter-gatherer humans who have excellent heart and metabolic health).

How hard should I push in easy exercise?

In order to stay in the sweet spot of utilising mostly fat to fuel the exercise, there’s a really easy trick: maintain a pace where you can continue to have a conversation with someone. It can be more challenging to keep up the conversation than at rest, but it should feel doable and sustainable. Once you start to get out of breath and can’t finish your sentences, you start to use more glucose for fuel (and less fat), so slow it down a little.

Some longevity experts recommend going for the ‘red zone’ of this fat burning zone, where you maintain the maximal rate of fat oxidation throughout, which is essentially right at that limit of “I can still maintain this conversation” (for the geeks: this is sometimes called ‘zone 2 exercise’, and is measured in a lab as maintaining a lactate level of below 2.0 mmol/L).

Other experts recommend staying within this zone of exercise for 150 to 300 minutes per week, but it is debatable, especially when you consider the inclusion of exercise that’s done at higher intensity ranges. There’s also research indicating that interval walking (for example, walking fast for 3 minutes, then slow for 3 minutes, repeatedly) might be more beneficial than steady-state walking. While we don’t have precise clarity yet, the important takeaway is clear: exercising at a lower intensity, often, is a good idea.

Practical recommendations:

• Build the habit first: start with two short walks a week, and build from there over time
• You can accumulate your 5,000 to 12,000 steps a day across the day, not in one shot: find ways to include small bits of easy activity throughout the day, like choosing to walk short distances instead of driving or taking the bus
• Pair the habit of walking with other well-established habits in your day: stacking your walk immediately before or after meal time is an easy one
• Do it with other people: since being able to maintain a conversation is how you stay in the right ‘zone’, it makes sense to use this time to connect with family, or even do work calls
• Bonus points if you do it outside: walking in nature has been shown to lower stress hormone levels and blood pressure, as well as elevate mood.

Easy exercise improves multiple health markers like blood pressure and cardiovascular health, helps us recover well from intense exercise, is good for our brain function, helps us get really efficient at using fat for fuel, enables us to be more metabolically flexible, improves lactate usage that will help us get more intensity out of our high intensity sessions, promotes immune health, and is good for our mental wellbeing in the long term.

It is, indeed, the perfect complement to HIIT. It’s not either/or; it’s both.