This time last year, I could not get out of bed. Just days into January, a New Year resolution to get back into the gym had been rendered laughable by a flu unlike anything I’d ever known. Emergency childcare was marshalled from under the duvet; when it arrived, I had to crawl to open the front door. Friends took one look at the pitiful figure on her knees, clinging to the hallway radiator, and gently suggested I might need to start taking better care of myself.

They weren’t wrong. While I wouldn’t say I had lived a wildly unhealthy life, I’d never been what one would call health-conscious either. Partly this is because I am by nature quite greedy and lazy, and partly because until my 40s my health had appeared to take care of itself. As long as I pottered along to the gym now and then, and kept an eye on my weight, I’d managed to muddle through with no need for spirulina smoothies or a Holland & Barrett loyalty card.

It hadn’t occurred to me that this policy might one day stop working. But after undergoing pretty brutal treatment for cancer in 2015, I found myself overweight and horribly out of shape, with an immune system no longer worthy of the name. The flu was the final straw. Clearly, it was time for drastic measures. I had to get help.

I found a small company called Detox-Fit. It’s a kind of one-stop shop for fitness, and provides personal trainers and nutritional support – only not any old nutritional support. Detox-Fit is militantly, evangelically vegan.

The truth, of which I am not proud, is that until then I’d not given animal welfare more than a passing thought in my life. I loved meat. In as much as I’d ever paid attention to veganism, in my mind it was a bit precious and a massive palaver. The obvious question would therefore be: couldn’t I just eat healthily without becoming a vegan? Why not simply listen to my body, and eat what it asks for? I can see that for lots of people – maybe most – this would be the sensible solution. In my case, however, it is a terrible idea, the message I consistently get from my body is that a great breakfast menu plan would be two Mars bars and a Cadbury’s finger of fudge.

Encouraged by the fact that the couple who run Detox-Fit look like cartoons of physical perfection, I signed up to a personal trainer and a vegan diet for what I thought would be a three-month experiment. To eliminate the possibility of willpower failure, I took the precautionary measure of posing for a “before” photograph for Women’s Health magazine. I’ve always found the fitness magazine’s “body challenge” series hypnotically compelling, and there is nothing like the prospect of an “after” photoshoot to keep you away from the fridge.

So in late January last year I began training three or four times a week with a trainer called Rory Lynn, who used to be a professional rugby player and confounded my prejudices about PTs. Having always suspected they were little more than a lifestyle status symbol, and never fancied the idea of paying someone to shout at me in the gym, I had been doing the same workout routine by myself for nearly a quarter of a century. It was pretty much what you see half the people in any gym doing – some weight machines, a spot of cardio on a treadmill, plus some Jane Fonda-ish wiggling of legs in the air. The possibility that this had been an almost total waste of time had never crossed my mind.

Not one of the things I used to do in the gym featured in Rory’s workouts. I was introduced instead to an unfamiliar new world of bear crawls and burpees, Turkish get-ups and Russian twists, single leg glute bridges and crab walks. A lot of his exercises were quite like moves one might make in real life: lots of stepping sideways up on to a box, slamming medicine balls down on to mats and walking up and down carrying heavy weights like suitcases. These always looked either easy or even quite fun when demonstrated by Rory. Minutes later, I would be flat on my back, gasping for breath. When were we going to move on to the weights machines, I asked plaintively. We weren’t.

The really big surprise, however, was the unexpected joy of surrender. It was infinitely easier to train to the point of nausea with Rory than it had ever been to amble around the gym by myself. Being temperamentally indisposed to relinquish control, it came as quite a revelation to discover how much simpler everything becomes when you do. Half the battle with the gym is simply getting yourself there; and once inside, the temptation to slope off after 20 minutes makes the whole business an endless internal battle. But with Rory in charge, I could stop thinking about it. You show up when he says, do what he tells you to, and, er, that’s it. There is no willpower required.

Weirdly, I found myself adopting other uncharacteristically healthy habits, almost without noticing. I began setting my alarm for 5am, and beginning the day with a 15-minute cold bath on the advice of a friend who had also been through chemotherapy and swore by them, the thinking being that they boost the immune system. The first time I tried one I screamed the house down. The trick, I soon learned, is to get into the bath when it’s still empty and let the water level rise over you. I wouldn’t go so far as to claim it’s a pleasurable experience, but the sensation when you get out is not unlike taking class A drugs – and on a good day the buzz can last until lunchtime. Dry brushing is also remarkably effective at making your body feel alive. It helps with lymphatic drainage and the excretion of toxins, is very simple, and works exactly as it sounds. You brush yourself all over with a dry brush for about 10 minutes, and within a few days begin to visibly glow.

***

Another big surprise about my new regime was that, far from being complicated, veganism makes life amazingly simple. For the first few weeks, while I was panicking about what to cook, Detox-Fit delivered vegan ready meals to my door – which was very convenient, obviously, but quite unnecessary. If you are someone who needs rules in order to eat healthily, an omnivore diet becomes an endless negotiation between angels and devils, and everything you eat involves a decision. The joy of veganism is that you have to make only one decision: to eat no animal products. Once that’s done, you barely have to give food another thought. The ceaseless clamour of adverts and billboards urging you to eat things you shouldn’t is miraculously silenced. Junk food can shout at you all it likes; you can no longer hear. The world suddenly becomes blissfully calm.

If you eat only things that grow, the opportunity to eat anything very bad for you becomes so vanishingly small that you can pretty much stop worrying about it. Instead of trying to shoehorn into your diet the things everyone agrees you should eat more of – vegetables, seeds, legumes, fruit – you find there is room for legumes and linseed and alfalfa sprouts, without having to think about it. You can, of course, still binge on popcorn and fries if you like, but unlike, say, fried chicken or cheesecake, these foods cannot be chemically engineered to deceive your senses into failing to notice when you are full. As a consequence, there is a limit to how much damage they can do.

Eating out becomes trickier, though. The solution I discovered is a brilliant app called Happy Cow. Wherever you are in the world, you just enter your location and up pop all the nearby places serving vegan food. Even in the US Washington state outpost of Spokane, a Krispy Kreme doughnut kind of town, Happy Cow guided me to a local juice bar serving organic vegan rice bowls. In Melbourne it led me to a fabulous fast-food joint called Lord of the Fries, selling vegan “chicken schnitzel” and “bacon burgers”; and in London I found the answer to fast-food cravings at the Sanctuary restaurant, owned by Detox-Fit. Their tapas menu includes eggless frittata – how, I do not know – and the most extraordinary vegan fish and chips, which consists of a fillet of textured tofu that flakes just like cod, on top of a sheet of nori, deep-fried in batter.

Veganism was proving surprisingly easy – until a friend invited me to a dinner party. I had worried about how it would work in other people’s houses, but naively imagined a quick text message would solve the problem. “Do not cook anything especially for me,” I instructed firmly, thinking that should do the job. But it turns out that my horror of becoming the nuisance guest is as nothing compared with hosts’ panic at the idea of a guest filling up on bread and salad. Nobody wants that person at their table. I arrived to find an exquisite vegan meal-for-one on my plate – which was delicious, but embarrassing.

I always used to think: why don’t vegans just eat whatever they’re given and go back to being a vegan in the morning? But then, I’d also assumed they would be secretly grateful for an excuse to cheat. I certainly imagined I would be. As concern for animal welfare had nothing to do with my decision to go vegan, it seemed hardly likely to trouble me if I fell off the wagon.

But here was yet another surprise. When I look at meat now, what puts me off isn’t what harm it might be doing me but the thought of what happened to it before it reached my plate. The primitive reflex of disgust is phenomenally powerful, and in this case very helpful, as it erases all trace of temptation. As soon as you start to think about where meat comes from, putting it in your mouth no longer feels tenable. A bacon sandwich would, of course, still taste delicious. But then, keeping a slave in one’s house would also be very handy. It’s just not something any sane person is going to do.

***

When the three months were up, there was no going back. It felt as if Rory and I had only just got going, so we extended the body challenge and kept training until the end of the year. As the months passed, my only reservation about all this wholesome living was its tendency to make one insufferably smug. My old identity as someone cheerfully cavalier about health was beginning to unravel, and I wasn’t sure how I felt about that. Few things are more tedious than a health bore, but secretly I was beginning to enjoy the novelty of being someone who takes care of herself. Disingenuous affectations of my former indifference to health were becoming increasingly implausible.

By the end of the year I had lost 18kg, developed muscles I never knew existed, and for the first time in years felt physically strong again. The “after” photoshoot for Women’s Health magazine was considerably more fun than the “before” one. The most radical change of all, however, has come through making peace with my new identity. Having found it mildly embarrassing in the beginning to admit to being vegan, I’ve come to like it. I like no longer feeling implicated in the immoral horrors of the modern western diet. I like taking myself and the planet more seriously.

My only worry now is where all this could be heading. Friends from LA came to supper recently. They’ve always been a fanatically health-conscious couple, and over the years have taken a very dim view of my eating habits, so I emailed them in advance to say that dinner on this visit would be vegan and gluten-free, and did they have any other dietary requirements? In all honesty, I wasn’t really asking so much as showing off. Gluten free and vegan surely covered all bases; what other dietary requirements could anyone conceivably have?

“Nowadays we only eat foods permitted according to our blood type,” they emailed back. After I’d stopped laughing about the madness of such pseudoscientific narcissism, a terrible thought occurred. Is this what happens once you set off down this path? If I’m writing about eating according to my blood type next January, someone please order me a Big Mac.

• Decca Aitkenhead’s body challenge appears in the March issue of Women’s Health, out on 6 February.

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