Sweat addict Charlotte Sinclair ditches the lengthy exercise sessions and tries a daily 15 minute workout and a lot of eggs to find out if high intensity training is all it's cracked up to be...

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Working out can sometimes feel like one more demand pressed on already busy lives. Who even has time for exercise?  And who’s to say that all that running or Bikram is even the best way to exercise? Couldn’t there be a more efficient way? Enter The Library in London’s Notting Hill, a gym with a short-is-sweet attitude to fitness. Its owner, Zana Morris, puts clients through high intensity training sessions that span a blink-quick total of 15 minutes.

Surely nothing more significant than a speedy manicure can be done in 15 minutes? It sounds too good to be true. “We’ve got exercise all wrong,” says Zana, who is blonde and Irish and very friendly – managing to be reassuring even while using calipers to measure the fat on my stomach. (Ugh.)

The gym is styled like a private member’s club, with sofas and a billiards table. The only concession to fitness is a vintage pommel horse, a central set of weights machines, and a punch bag.  The building is a former synagogue – complete with a stained glass Star of David. A temple to fitness, then.

Okay, but 15 minutes? “It’s the difference between a sprint and a marathon,” says Zana. “Olympic athletes have always used high intensity training as the most effective way to train.” For the rest of us, working shorter – but harder – is a quick way to zap the fat and build lean muscle. “High intensity training has the advantage of raising your metabolism and increasing your muscle recovery time. When you over train, running for an hour four times a week, your body doesn’t have time to recover. It begins to burn muscle, not fat.” A short sharp shock of exercise can even make you look younger. “The human growth hormone decreases as you age,” Zana explains. “But studies have shown that high intensity exercise increases its production far more than endurance exercise.”

MORE GLOSS: Our chat with Freedom2Train about high intensity training

Of course there’s a catch. In order to maximise the effect, I must follow a high fat, no carbs, no sugar, no dairy diet for 12 days. This, Zana promises me, will nuke the saddlebags. She gives me a bag of prettily packaged – but no less scary – slimming and energising supplements that I’m to take in between my meals of eggs and avocado, salmon and salad, and handfuls of peanuts, macadamias and walnuts. I hate diets, but Zana is so persuasive of this regime’s powers to blitz the fat – which has been thoroughly weighed and measured by this point – that I agree.

For twelve days, I come to the gym and do a set resistance training routine, each day working on a different part of my body. There is to be as little respite between sets as possible – three sets of six lunges, three sets of six thigh curls, three sets of six ‘gorilla lifts’ from a weighted chair. The point is to be breathless. Which I am. But though I find it hard, often unable to complete a set of six lifts, I don’t sweat. And just as I’m getting into my stride, it’s finished. I can’t believe it’s doing me any good.

Then there’s the diet. At first it’s fine. Poached eggs on avocado is delicious for breakfast once. After the sixth day the novelty has palled. I begin to hate walnuts. And macadamias. And peanuts. I crave sugar, and milk in tea, and fruit and tomatoes. (Red veg is verboten. Too much sugar.) Of course I cheat. A few crisps. Three glasses of wine over a weekend. A piece of rye toast when I was getting desperate. I confess and Zana rolls her eyes, telling me to text her if I’m having a weak moment. (Yes, I think, but then I’d be texting you every hour.)

The final day of reckoning comes. I’ve had a glass of wine the night before – oops – and a packet of pop chips. In my defense, there’s only so many nuts and eggs one woman can consume before she loses her mind. My expectations are therefore pretty low. Yet, in spite of myself, I’ve lost an inch on my waist, half an inch on my legs, my fat is down by 3.5lbs and my muscle up by 1.5lbs, giving an overall body fat percentage drop of 2%. Not bad for twelve, not particularly well-behaved days.

As a sweat addict, this is a revelation. How can such little exercise produce such good results? I’m not stupid - the restricted diet was a major factor in my results. But there’s been a corollary change in my muscle tone and strength too. I can do higher weights when I squat; my thighs have begun to tighten. I don’t really understand how it works, but it does. Just, please, spare me the eggs.