Gyms can seem uncomfortable places at the best of times: the neon lighting, pumping music, the warm, moist changing rooms in which you can almost smell the viruses adrift on the fan-generated breeze. And while they can potentially be vile for anyone, gyms are often a particularly hellish prospect if you're fat. The reasons for this are pretty obvious, but a similarly fleshy friend once boiled it down nicely. "How could I ever feel welcome somewhere where every single person is intent on ensuring that they don't look like me?" she asked.

She was right. Being a fat person at the gym can feel like being a meaty sausage at a vegan retreat. I have been to gyms as a fat person, and, while there are exceptions, I have seen the disdain pour out of fellow exercisers, and especially the instructors, at the same rate as their sweat. You go in feeling good about yourself. You come out feeling like a pig.

So it won't surprise anyone to hear that I have yet to join a gym. Instead, in my bid to start moving, I turned first to that refuge of the exercise-shy: the fitness DVD. I had one old favourite already, a kickboxing number, which I have used intermittently for years, and which I prize for the instructor's gung-ho, irony-free attitude. Trouble is that I first used this when I was younger, poorer, and, as a result of living outside London, still able to afford a flat with a sitting room bigger than a postage stamp. I now live in London. This means that every time I kick out in my tiny front room, I have visions of skewering the television with my foot, severing a major artery, and being found bloody and lifeless in the middle of the floor, clad in my ageing gym kit.

I have therefore been looking online for a similarly inspiring DVD that involves slightly less kicking. This has proved difficult. It's not primarily because a large proportion of the bestselling DVDs are presented by soap stars - I'm perfectly willing to accept that Natalie Cassidy, who once played trumpet-tootling Sonia in EastEnders, has become a tip-top fitness instructor. What pisses me off is how many are illustrated with before-and-after shots.

Such images have long been a staple of the diet industry, of course, which means we're all aware of the form. One picture shows a glum woman with no makeup, dressed in an ill-fitting outfit beneath lights that accentuate every spare ripple of flesh. The second shows a smiling woman with tons of makeup, in an expertly styled outfit, beneath studio lights primed to lengthen and flatter.

Scrolling through these DVDs, it occurred to me, once again, why these images proliferate. The diet industry relies on recidivism to keep going - if a diet worked, the industry would collapse. It therefore makes sense for it to instil a massive dose of self-loathing in anyone setting off on their "weight loss journey", and these before and after pictures give one firm message: being fat is ugly, depressing and awful.

While this message might drive people to start a diet or exercise programme, it is also an obvious precursor to failure. If you begin a project having been convinced by the people advising you that you are a hideous, slovenly beast, your resolve to look after yourself, to make strong decisions for the good of your physique and your psyche is greatly reduced. Start with this outlook, and there is no self-confidence to protect you when the idea of bingeing crops up, as it very possibly will, halfway through the second week.

As I have said before, I like myself. I don't think of fat people as lazy, stupid, or inherently greedy, and, as far as I'm concerned, it would be hugely unhealthy - both in my attitude to myself, but also in my social attitudes - if I bought into anything that suggested as much. I therefore plan to steer well clear of products and places that define me and other fat people implicitly, or in many cases, quite explicitly, as pigs. When it comes to exercise, I guess I'll walk.

· Kira's column will appear fortnightly.