When were you last really honest with a girlfriend? I don’t mean “I prefer the red on you” honest; I mean bluntly, no-nonsense, cut-to-the-chase honest along the lines of, say, “You’re too fat”. Well, here’s a tip: if you haven’t already told your friend she needs to drop a stone, then you should probably resist. According to a survey in conjunction with National Obesity Week, women don’t like their friends telling them they are fat (you don’t say) and one in 20 has ended a friendship after being told they were overweight.

We’ve all done it. I don’t mean telling our friends they are fat; I mean falling into the trap of believing that friendships depend on ruthless honesty. When you’re young, you pride yourself on being upfront with those closest to you, until the day you blurt out some unpalatable truth (“Your haircut is a disaster.” “Your boyfriend tried to get off with me.”) and, lo, you discover that honesty is the best policy — until it hurts. Then you get older, and you think that everyone is grown-up enough to handle it, and what is more, you owe it to each other. That’s an even bigger myth. If “You’re fat” is liable to get you into trouble, then try “Your toddler is fat” and see how well that goes down. The fact is, girl friendships across the years rely on honesty, but only the constructive variety. We like the truth delivered gently, in code, cushioned with lashings of love. And sometimes we don’t want it at all.

Constructive honesty goes like this. “I much prefer your new hair colour” — meaning I hated the old one and, for God’s sake, don’t go there again. Or “You look great in the blue: wear that”, meaning step away from the dress you are about to try on, because it makes you look like a sausage. If asked a direct question, such as “Is my boyfriend a bit of an arse?”, the rule is, always follow the questioner’s lead (“Now that you mention it, he might be a bit”). Never be tempted to let rip with your own list of his inadequacies. The girlfriend’s role is to confirm suspicions, like Deep Throat, not to point out a whole lot of separate horrors. And sometimes it’s flatly to deny the truth, because to rub it in would be cruel and achieve nothing. “Do you think she’s the only one he slept with?” is a question that always requires the answer “Yes”, even if you know different. “Do you think I’m fat?” requires the answer “No”. As if it could be any other way.

So I’m struggling to imagine when, exactly, you might feel the urge to tell a girlfriend that she’s unacceptably big. Over lunch? In the changing room? As she’s getting dressed for the party? The whole point of constructive honesty is that it makes you feel like someone is on your side, as opposed to judging you and finding you wanting. You might ­appreciate being told your cleavage is spilling alarmingly or your lipstick is on your teeth, but if someone criticises your size, that’s like criticising who you are. It’s like saying you’re a bit loud, or you could do with doing a bit more reading. Some people do have girlfriends like this, but we have come to know them as “frenemies”, and if you’ve got one, you should ditch her today.

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