If you’ve glanced at a newspaper in the last few months you’ll have noticed that obesity is public health enemy number one. The Guardian has a special section devoted to it – seriously. Its mantra is: 'Britain is in the grip of an obesity crisis.' In 2007 the National Obesity Forum issued a report which predicted that half of the population will be obese by 2050; now they are saying that was an underestimate.
That’s all well and good, but it’s not much use knowing that we’re in the midst of an epidemic if the tools we use to diagnose obesity aren’t up to scratch. Everyone knows about the BMI (body mass index) measurement but, as my colleague Christopher Snowden points out, BMI is a pretty crude concept: invented in the 1840s, it's calculated by dividing a person’s height by their weight.
BMI doesn't tell you anything about a measurement that really matters – your waistline. To quote the National Obesity Forum:
The measurement of waist circumference provides information about the distribution of body fat and is a measure of risk for conditions such as coronary heart disease (CHD). It is now well known that people who carry their excess fat centrally (within the abdominal cavity) are more likely to suffer the consequences of being overweight.
When men struggle to fit into their trousers, they think 'I've put on a few pounds'. True, but perhaps they should really be thinking: 'My abdominal cavity is full of fat.' And do something about it.
You can be clinically overweight judged by your BMI and very healthy – in his prime, the rugby player Jonny Wilkinson was 'overweight' according to his body mass index, but the extra weight was muscle. And even if your extra weight is made up of fat, that's not necessarily a problem – depending on where the fat is sitting.
Waist circumference tells us more than BMI. Your GPs should look at your middle to see if you're at risk of developing heart disease. An unhealthy waistline can cause metabolic syndrome, a particularly worrying cocktail of high blood pressure, diabetes and obesity. It's also associated with strokes and microalbuminuria (albumin in the urine), which is a warning sign of chronic kidney disease.
The problem for middle-aged people is that after 40 they begin to suffer from the effects of what academics call the ‘regional distribution of that fat within the body’. Fat gravitates towards the middle in both men and women –sometimes catching them by surprise, because they're used to looking in the mirror and checking whether their face looks fat. Warning: your face can stay thin while those extra inches creep on.
This is particularly bad news for people from the Indian subcontinent, where health risks related to obesity crop up at a much lower threshold. As a report in the Journal of Cardiovascular Disease Research explains, South Asians are prone to carry fat in dangerous places:
• Excess body fat: South Asians have higher percentage of body fat for a given BMI as compared to Caucasians and Blacks.
• Abdominal obesity: Is seen with increased prevalence in South Asians even amongst those who have a BMI less than 25 kg/m2.
• Truncal subcutaneous fat: It is a particular distinctive feature and an important correlate of insulin resistance in this ethnic group.
No one disputes this, so far as I know, but biological differences between people of different ethnic backgrounds are a sensitive subject, and as a result it’s under-reported. BMI is especially misleading if you’re South Asian, and more people from that part of the world should know that.
And for all of us, surely the message is: if you're worried about your weight, don't rely on the scales. Reach for the tape measure.