No gyms, no weighing scales – the ‘Levi’s jeans test’ Jerry Hall swears keeps her svelte at 70, and why it works

It doesn’t sound super scientific. But still. When Jerry Hall speaks, we listen. Especially when her advice sounds temptingly low-effort. ‘I don’t diet, but I do wear Levi’s jeans, and if the buttons are tight, then I won’t eat pasta for three days,’ the Celebrity Traitors star recently told The Telegraph.

At 70, the supermodel’s Levi’s have become her own informal, DIY health check: if they feel tighter, she changes what she eats for a few days. Simple. But is it too good to true?

It turns out, there is, in fact, some sense in it. A growing body of research suggests that BMI – long the standard measure of whether your body weight is healthy for your height – is not a flawless diagnostic tool. Where we carry fat matters, as well as how much of it we have. And your waist really is a critical zone.

The problem with BMI

BMI – body mass index – compares your weight with your height. And while that can be genuinely useful, it has its limits. It can’t, for instance, show whether your body fat is concentrated around the middle, or distinguish fat from muscle. And this matters.

A 2017 analysis of 42,702 adults in England and Scotland looked at BMI alongside waist-to-hip ratio, a measure of so called ‘central obesity’ (fat around your middle). It found that people with a normal BMI but central obesity had a higher risk of death than those with a normal BMI and no central obesity. In other words, having a BMI in the ‘healthy’ range does not always give you a complete picture of your health.

A new Rutgers Health study adds to this picture. Researchers compared different ways of identifying excess body fat in 1,900 US adults aged 20 to 59, checking the results against body-fat measurements from DEXA scans. They found that waist circumference alone was better than BMI alone at identifying people who met the study’s body-fat threshold, suggesting that measuring the waist can offer information that BMI alone may miss.

The string test

So is the tightness of your jeans a good measure of your visceral fat levels? Well, there might be better ones. Dr Clare Bailey Mosley, for instance, prefers another method. ‘The string test is a better measure of your metabolic health than BMI,’ she says in a recent Instagram post with her son, and fellow doctor, Jack Mosley.

‘One of the fats we’re most concerned about is your visceral fat,’ explains Jack. ‘This is the fat around your midriff, a fat that’s metabolically active, it clings to your organs and this is a fat that we’re more concerned about.’

Visceral fat releases harmful substances that trigger inflammation, disrupt hormones and damage arteries. So how can we keep an eye on our own levels? The Mosleys suggest this at-home method. Take a piece of string, use it to measure your height, then cut it to length. Once your string is the same length as you, cut it in half and wrap one half around your waist, just above your belly button. If the two ends fail to meet, says Clare: ‘It means you are more likely to have metabolic issues like heading towards diabetes and being overweight.’

The waist to height ratio test

The NHS recommends using this ‘waist-to-height’ ratio test, as well as BMI, to help assess your health risks. Put simply, you’re aiming for a waist measurement that’s less than half your height. And you don’t have to use a piece a string.

If you have a tape measure instead, measure your waist and height in the same units, then divide your waist measurement by your height. If the result is over 0.5, you might have a bigger risk of developing heart disease, high blood pressure or type 2 diabetes.

Your jeans can help you notice a change, but the fit depends on the cut, the brand and how the garment sits. A tape measure gives a clearer number – and the waist-to-height ratio is the measure NICE recommends alongside BMI for many adults.

What you do with the result is up to you. No need to skip pasta if you don’t fancy it.