Measuring obesity

SEVEN out of 10 Irish men and more than half of Irish women are either overweight or obese.

When you consider the serious hazards associated with overweight and obesity (high blood pressure, stroke, heart disease and several common cancers), the public health implications of having such an overweight population come into sharper focus. However, it’s now becoming increasingly clear that not all people who are overweight or obese have the same health risk profile. The explanation for this may lie in the way that these people are classified in terms of their bodyweight, and indeed, this was highlighted in a presentation at the Nutrition Society’s Annual Summer Meeting at DIT a couple of weeks ago.

The figures quoted for the Irish population above have been derived using a measurement called BMI or “body mass index”, which in simple terms indicates the person’s weight for height. It’s calculated by squaring the person’s height in metres and dividing their weight in kilograms by this figure (i.e. weight (kg)/ (height (m)). Unfortunately, BMI gives no indication as to that individual’s body composition. So there’s an assumption being made that the person’s increased bodyweight is attributable to an excess of body fat; an assumption which may or may not be true. To give an example, some athletes (especially those involved in power disciplines like rugby, sprinting, weight-lifting and boxing) are heavy for their weight. In these cases, however, their increased weight for height which sees them classified into the “overweight” or “obese” BMI categories is attributable to muscle, a situation which actually reduces their risk of the serious disorders highlighted above.

To get around this problem of mis-classification, public health specialists have in recent years begun to emphasise the importance of waist measurement as a key indicator of obesity-related health risk.

That’s because it’s the fat that gathers around the tummy that’s most closely associated with health conditions like diabetes, high blood pressure, heart disease and cancer; especially if it collects around the organs inside the abdomen. So while an overweight person mightn’t be able to pinch much fat on their tummy, the simple fact that they’ve got a large waist circumference suggests that they’ve accumulated a lot of this hazardous fat inside the abdomen, a situation which dramatically increases their health risk.

From the early 1990s onwards, researchers worked hard to identify waist circumference thresholds above which the risk of disease and death increased.

After much analysis, they discovered that men whose waist measurement exceeded 37 inches had a greater risk of ill-health than those who remained under this threshold, with these risks increasing dramatically once the waist measurement went above 40 inches. For women, the equivalent figures were 32 inches and 35 inches, meaning that women encounter the same health risks at even lower waist measurements than their male counterparts.

Unfortunately, even the use of these waist measurements doesn’t tell the full story. For instance, while a waist circumference of 36 inches might be no problem at all for man who’s 6ft tall, this same waist might elicit considerable risk in a man who’s only five feet tall, even though he’d still be under the official 37in cut-off.

For these reasons, experts have recently proposed that people assess their bodyweight by working out their waist to height ratio. In other words, if you divide your waist measurement (e.g. 34in) by your height (e.g. 68in for someone who’s 5’8” tall), the answer should ideally be below 0.5.

If it’s above 0.5, that’s an indication that you should aim to lose some weight, with the added promise that if you do manage to get below (and stay below) this threshold, this should have considerable benefits for your long-term health.

* Dr Daniel McCartney, Lecturer in Human Nutrition & Dietetics at DIT.


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