It's time to stop using an antiquated system to determine a person's ideal weight, says Sarah Vine

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Sometimes life can be so very confusing, especially when it comes to highly emotive issues such as size.

Last week for example, the Chief Medical Officer for England, Dame Sally Davies, warned that obesity was becoming "normalised".

"Larger mannequins are being introduced into clothes shops," she said, going on to explain that so-called size inflation was warping shoppers' perceptions of their true size. If we didn't shape up, she explained, the Government would have to start thinking about a sugar tax.

In practically the same breath, however, she also expressed concerns that “being underweight is often portrayed as the ideal, particularly in the fashion industry".

So, basically, it's either feast or famine, and neither will do. But the real question here, is what is "normal" in weight terms?

Human beings come in all shapes and sizes. Big heads, small heads, long limbs, short limbs, muscular, weedy. Some babies are big, others small. Some people can live on pizza and never put on a pound; others only have to look at a cake to put on weight.

And yet the way the medical profession determines what is "normal" in terms of weight is by a blanket term called BMI.

Ever since I can remember, I have been technically obese. Even when I got married 10 years ago and was the skinniest I have ever been, I was still technically obese. What can I say? I've always been a big girl. But on the few occasions when I have managed to reach my target "normal" weight of around 72 kgs I have had to quite seriously starve myself - not to mention exercise like a caged gerbil.

Recently a woman from London, one Anita Albrecht, was told that she was borderline obese. This despite being a personal trainer and a successful female body builder. She told a newspaper that she had been instructed to go on a 1,000 calorie a day diet which, when you see what she looks like, is absurd.

The problem almost certainly lies with the fact that the BMI measurement was developed in the first half of the 19th century. It seems astonishingly arcane that, decades on, we are still using it to determine a person's ideal weight.

No one is denying that Britain has a weight problem; but if Dame Sally wants us to keep our weight in check, it would help to have some realistic - and achievable - targets to aspire to.