OK, this is going to sound a little hypocritical, as I have hard-recommended every activity and pursuit, every wellness wheeze and rejuvenation exercise the modern world has dreamed up. Try hot yoga: it plugs you back into your inner child. How about a morning rave? All the cardio of a regular rave, none of the ecstasy: what’s not to like? Botox? Fine, it’s plastic surgery-lite, but also it makes you look much more friendly. Pilates, cycling, running, high-intensity interval training, Tough Mudders, barre, aerial silks, horse-riding: at some time or another, I have insisted to anyone who will listen that it’s only their failure to incorporate, say, a horse into their weekly schedule that is standing between them and their best self.

But there is a dark side to wellness, which I always, for shorthand, thought of as political: getting fit makes you more rightwing. The mechanism is incredibly simple: you embark on this voyage of self-improvement, and more or less immediately see results. You feel stronger and more energetic, probably your mood lifts, and pretty soon you think you are master of your own destiny. You’re still not, by the way: destiny does not care about your step count. But until that fact catches up with you, which it may never, there you are, high on self-righteousness. You can tell this has happened to you when you start inhaling performatively, like the hero of an Ayn Rand novel.

Inescapably, you start to situate other people’s problems within their failure to be as fit as you. This is particularly true if you don’t know them and they’re just a bunch of numbers. All those statistics – depressed people, obese people, people with IBS – imagine how much better they would be if only they took responsibility for their health, the way that you have.

And yet that harsh, judgmental inner voice will never be satisfied just shouting at numbers, so sooner or later you’ll turn it back on yourself. Fitness has a capitalist logic – I guess because there’s so much money in it? – so nothing is ever enough. As soon as you can run 5km, you want to run 10. Before you know it, you’re swapping Strava stats with people you used to think were tossers but now, miraculously, you find you have a lot in common with. Always competing, always striving for growth, even if by “grow” you mean “shrink”. You have internalised the market, unfortunately. Also, you’re getting on everyone’s nerves.

So now you’re almost your best self, except you could always be better, and this is when you start eating protein the whole time. What even is protein powder? I don’t mean: “What’s it made of?” – I know my way around whey. I don’t mean: “What does it taste like?”, as, funnily enough, I quite like it, but that’s only because it’s the taste of pure virtue. What does it do to your soul, that it knows what virtue tastes like and is preening on it? And that’s before you’ve hit lunch, carrying around a box of chicken thighs like it’s a handbag.

The only reason I can make all these insulting, highly personal remarks is that they are directed at myself. However obnoxious you’ve been, cycling through a red light, high on very low levels of endorphins because you weren’t going that fast, I’ve been worse. However much you have spent on a pair of leggings, convinced that you’re a yoga bunny now, a completely fresh person, calm and self-actualised, I’ve spent more, and given up faster. However long you’ve spent droning on, trying to make a philosophical case for a climbing wall, I’ve definitely done that for longer – which is to say, five minutes, which must have felt like five years.

In the fullness of time, I realise it’s not really a question of an unwitting slide into fascism, hastened by a treadmill. It’s more that there is a fixed amount of excellence in any self, and the more you spend on your biceps, the less you have for your personality. Wellness could turn you into a bit of a jerk, is what I’m saying.

• Zoe Williams is a Guardian columnist