Dragon's Den star Sara Davies is fast becoming one of our favourite Instagram health experts, thanks to the sensible advice and exercise hacks she shares on social media. And if you've ever felt exhausted after a run, the entrepeneur has some news for you! She recently shared on Instagram that she's found a way to stay in a fat-burning mode while running – and happily, it involves slowing right down.

'Recently I’ve completely re-thought my whole exercise strategy after learning this one thing about running and fat burning - and it’s had SUCH an impact,' she posted.

'If you're running to burn fat and tone up (like me) then your heart rate needs to stay in the 60-70% zone. Go above 70% and you're not burning fat anymore - you're burning glucose, straight from your energy reserves.'

Sara even shared the formula to work out your perfect workout zone. 'Take 220 minus your age - that's your max heart rate. Multiply by 0.7 to get your 70% cap. Apparently if you can hold a full conversation while running, that's about right.

I checked on my Apple watch - to stay under 70% I was having to run so slowly... so slowly in fact I might as well have been walking... so that's exactly what I started doing, just walking. I switched to energetic walking then went a stage further and got myself a weighted vest.'

So what do the experts think? Should we all slow down to get fitter? 'Sara is definitely on to something,' says Tom Brown, celebrity trainer and founder of @1stclassbootcamp luxury fitness retreats. What Sara's describing is a hot topic in the fitness world - Zone 2 training - and, says Tom, it's a method he uses both personally and for his clients, alongside strength and higher-intensity work.

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'In simple terms, Zone 2 is steady aerobic exercise at a relatively comfortable, sustainable intensity. Your heart rate and breathing are elevated, but you’re not working flat out. A useful practical guide is the talk test - you should generally still be able to hold a conversation while exercising. I think one of the biggest misconceptions in fitness is that every workout needs to leave you exhausted to be effective. It doesn’t.

'I work with a number of high-profile and very busy clients, and one of the most important parts of programming their training is managing the balance between doing enough to create an adaptation and actually being able to recover from it. More intensity isn’t automatically better.

'That’s where lower-intensity aerobic work can be really useful. It allows you to build your aerobic fitness and accumulate cardiovascular exercise without making every session extremely demanding.

'For someone new to it, I wouldn’t overcomplicate things. Brisk walking, incline treadmill walking, cycling, jogging or a cross trainer can all be used. Start with something like 30–45 minutes at an intensity you can comfortably sustain and build from there.

'At lower-to-moderate exercise intensities, fat can provide a greater proportion of the energy being used compared with harder exercise. But that doesn’t mean doing Zone 2 automatically results in greater body-fat loss. Fat loss is influenced by the bigger picture - particularly overall energy balance, alongside nutrition, physical activity and training.

'For runners, including those in midlife, not every run needs to be hard. Easier aerobic sessions can form part of a well-balanced programme alongside harder sessions and strength training.

'Ultimately, consistency beats constantly trying to destroy yourself in training. A session doesn’t have to feel brutal to be doing something very positive for your fitness.'