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There is a scene in Curtis Sittenfeld's last novel, Eligible (a modern update of Austen's Pride and Prejudice), in which our heroine Lizzy Bennet comes face to face with the insidious fitness trend that has come to dominate our age.

"Lydia pointed toward the roll on Liz's plate. 'Don't carbs make you feel sluggish?' 'Everything in moderation,' Liz said. There were many reasons she found her sisters' enthusiasm for CrossFit and the Paleo Diet irritating, including that Liz herself had been familiar with both long before they had, having written an article about CrossFit back in 2007. Another source of irritation was that her sisters looked fantastic; they had always been attractive, but since taking up CrossFit, they were practically glowing with energy and strength."

In this, as in most things, I am with Lizzy Bennet – a clever young woman with an appetite for dinner rolls and sensible distaste for cultish body fads. But the CrossFit/Paleo paradigm and its lean army of evangelical proponents is inescapable these days. As Lizzy points out, it's been around for a while now but, in recent times, we have hit Peak Paleo.

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Just a quick primer for the uninitiated: Paleo/CrossFit is a diet and exercise regime that broadly subscribes to the notion that, in order for humans to achieve optimum fitness, we must live the way our ancient hunter-gatherer ancestors did. This means no processed foods, legumes, dairy or grains. Activity must come in short, fast bursts, as if we were running from a sabre-tooth tiger.

If it sounds a bit crazy and extreme, that's because it is. And yet, it's everywhere you look. Many restaurants now offer "Paleo salads" featuring great bowls of spinach and hazelnuts and chicken dressed astringently with oil and lemon. There are Paleo cocktails (a fancy term for straight vodka) and Paleo burgers (beef patties wrapped in iceberg lettuce, hold the cheese and ketchup).

At the gym near my house, where I've started reluctantly dragging myself a couple of times a week, there's a whole new system in place. Instead of having a monthly membership, you pay per half-hour-long class. But here's the catch: Most involve high-speed circuits of vomit-inducing tuck jumps, burpees and jumping jacks that make you think murderous thoughts at 6 a.m. Gone are the days of lady-like Swiss ball crunches or prancing on a treadmill for 30 minutes and chalking it up as a workout. These days, it's all about high-intensity interval training. Which is basically the same idea as CrossFit, without the brand name. When it comes to exercise, hard and fast and painful is in; long and slow and pleasant is out.

Unlike most fitness crazes, there is good science to back this one up. The Paleo diet is essentially an ultralow-carb diet, which prompts the body to burn fat by putting it in a state of ketosis. During ketosis, the body runs out of sugar so it starts to burn fat. You might think of it as the first phase of long-term starvation.

This is why Paleo enthusiasts are generally very thin and bad tempered – in essence, they are semi-permanently starving.

No surprise there.

More counter-intuitively, there is a mounting pile of research to suggest that high-intensity interval training can get you much fitter faster than the sort of prolonged gentle exercise one might actually do for pleasure, such as jogging, swimming or cycling.

The One-Minute Workout, a bestselling book by McMaster kinesiology professor Martin Gibala, outlines how he and his team, over a period of 10 years of research, were able to prove definitively that a person could perform an interval workout containing only 2.5 minutes of high-intensity exercise three times a week and reap roughly the same fitness benefits – or more – than the same person working out at moderate intensity for 4.5 hours a week. This staggering result has been replicated widely in studies all over the world and, in essence, it's the reason why the system at my local gym has changed. You may not enjoy high-intensity interval workouts, but the fact is, they work.

I know this because I've been doing them now for a few miserable weeks and the results, as they say in late-night diet commercials, are astonishing. After a second pregnancy and birth that left me weak, tired and well over my ideal fighting weight, I've been able to lose fat, gain muscle and feel mostly human again just by doing two or three hard, fast workouts a week. I stopped having a cheese and mayo baguette every single day for lunch, which also may have helped.

I can't say I'll ever actually enjoy doing 15 fast burpees to thumping hip hop, but the efficiency is enormously appealing. Where I used to work out for hours every week and not see any change in my strength or appearance (I ran a marathon and gained weight), now I can suffer briefly and reap ample rewards.

Paleo/CrossFit is the most pervasive fitness trend I've seen in a lifetime of fitness-trend watching. The reason? Unlike most diet/exercise regimes, it actually works. But it's also joyless, miserable way to live, which is why its long-term, obsessive proponents are as tedious (albeit fabulously fit) as Lizzy Bennet's younger sisters.

You'd be boring, too, if you'd learned to enjoy eating a diet of chicken and seeds. Or if you liked doing tuck jumps.

But no one could actually like tuck jumps. That is a scientific fact.

Registered dietitian Leslie Beck explains why foods containing fibre, protein and water can help you stay fuller longer

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