The party was winding down. It had been great to catch up with family and friends, share the gossip over a barbecued sausage and a glass of cider. I’d had fun.

Then a woman I'd only met a few times before took me by the arm and leaned in confidentially. I thought she was about to say goodbye. In fact what she said was this: "We think you're too thin."

I blinked. I floundered. The comment was delivered quite seriously – I had been assessed and found wanting – and particularly unsettling was that use of "we". Had they all been sitting round talking about me? The whole crowd of them? I’d thought I’d looked nice. Evidently not.

Of course it's not the first time in my life that I've been shamed for my weight. As a child I was tubby, and often attracted negative comments from my parents' friends and even from strangers. At a bus stop one wintry day, in full view of my schoolmates, a man pelted me with snowballs, yelling, "I like little fatties!" I wanted to die of shame.

What's become obvious to me as I've grown older is that, whatever shape you are, someone will find fault. More than that, they will feel it's their right – nay, duty – to point out to you that you don't measure up to their standards. Crucially, it doesn’t matter whether your BMI is in or around the healthy range. They have an opinion so it must be expressed.

And yet this kind of crass judgment can be really damaging. During some recent research I read again and again how, for some people, hearing just one thoughtless remark can spark a confidence crisis that spirals into an eating disorder. Unprovoked criticism cuts deep. Even a weight-related compliment can backfire. A male friend of mine once told a woman how "lovely and slim" she was looking, only to then discover she was in fact very ill and dreadfully self-conscious about her changed figure.

Well, I have an idea: why don't we start a polite revolution where we just stop pronouncing on other people's body mass index? Block our ears to the media's running commentary on celebrity dress sizes, who's 'showing off her curves' this week and who's 'worryingly thin'. Those of us who don't spend our lives on the red carpet have more pressing concerns than today's exact reading on the bathroom scales. The point is, while I'm healthy, how much I weigh is my own affair. It's not up for public debate.

In the meantime I've started carrying a small card in my purse. I'm sure you mean well, but my weight is none of your business, it says. Next time I'm at a barbecue and someone feels the urge to criticise, I'll be prepared. I won't gape and stutter. I'm going to whip that card out and wave it in their face. "You know what?" I'll say. "I'm happy being me-shaped. End of story. So pass me another sausage, and let's talk about something more interesting."

Kate's new book Something Only We Know is published by Simon & Schuster and is available from Amazon.