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How much is too much body fat?

Michael Mosley investigates the evidence behind some of the claims we hear about in the media. Are some things really ‘all bad’ or ‘all good’?

This time, Michael talks to two leading obesity experts who have different opinions on how much is too much when it comes to body fat.

What’s the debate?

There is little disagreement that extreme obesity is a bad thing. But there is a debate around how obesity is defined, and whether we should look simply at how much fat someone has – or whether we should look more closely at where that fat is located in the body.

Firstly – how obesity is measured. The current UK guidelines on body fat focus around BMI (body mass index) – a ratio of weight to height. They state that a BMI of 18.5 to 24.9 is healthy. Higher than 25 is overweight, and 30 or above is obese. However, as we have previously discussed on Trust Me, I’m a Doctor, BMI is not a good measure of someone’s body fat.

Secondly, though, there is a growing body of evidence to suggest that ‘not all fat should be treated equally’ - and that there is such a thing as ‘good fat’ which should be taken into consideration along with the ‘bad fat’.

It is generally accepted that the belly is the most dangerous place to carry excess body fat, so waist circumference has recently become a popular extra measure to determine whether or not you have a healthy level of body fat. However, this measure in itself may not be enough. Now, there are arguments that there could be ‘good fat’ on the bum and thighs, which can counteract ‘bad fat’ on the belly, and therefore that a waist-to-hip ratio may be the best measure. This, though, is controversial.

The interviews

Professor Fredrik Karpe is Professor of Metabolic Medicine at the Oxford Centre for Diabetes, Endocrinology & Metabolism.

He believes we need to look at the ‘quality’ not just ‘quantity’ of body fat when deciding whether someone needs to lose weight. BMI is hugely flawed and waist circumference does not tell us the whole picture. He says we need to look at lower body fat as well as upper body fat because lower body fat is protective. If you have a lot of fat below the waist, then in his view, weight loss may not be a good thing.

His advice is to measure your waist (where your belly button is, not where your trouser band is) and your hips (the widest point around your bum), and if you are a man, your hips should be about the same size as your waist, or larger. If you are a woman, your hips should be larger than your waist. If this is the case, then he believes that – regardless of your BMI – you are likely to have a healthy amount of fat.

Professor Nicholas Finer is Consultant in Endocrinology and Bariatric Medicine at University College Hospital, London.

He believes that total fat loss is generally a good thing if you are classed as ‘overweight’ or ‘obese’. He believes that the evidence shows that if your BMI is above 25 and you reduce your weight then you reduce your disease risk. In his view, excess fat should be judged by BMI, and possibly waist circumference if you are around the borderline between healthy and unhealthy. He believes that weight loss makes little difference to the waist-to-hip ratio as fat tends to be lost evenly between the two.

His advice, then, is to measure your BMI, and possibly your waist, and that if your BMI is over 25 and your waist is over 35 inches/88c, if you’re a woman, or 40 inches/102cm if you’re a man, then you should lose weight.