Skinny people could be just as unhealthy as those who are overweight

TOFIs – Thin Outside, Fat Inside – are people whose bodies show little outward sign that they are packed with a particularly nasty form of fat - visceral fat, which can hide between organs, fooling people into thinking they are healthier than they are.

People who carry an extra 11cm around their waist have a 13 per cent higher risk of cancer, according to a study of 43,000 people that confirms the link between ill-health and waist measurement. It waShow more

You've been on a diet for weeks, trying to get rid of that pot belly without obvious success. Meanwhile, your friends look as trim as ever as they wolf down far more cake than you. How do they get away with it?

Here’s the thing: maybe they don’t. Maybe, deep down, they’re more obese than you are.

After years of warning us about our weight, health scientists are becoming increasingly concerned about a global epidemic of what they call sarcopenic obesity.

Triggered by a combination of sedentary lifestyles and nutrient-poor diets, it’s been linked to all the worst manifestations of conventional obesity, including diabetes, heart disease and cancer.

But here’s the twist: its victims may have no idea just what bad shape they’re in. That’s because they are TOFIs – “Thin Outside, Fat Inside” – whose bodies show little outward sign that they are packed with a particularly nasty form of fat.

We mostly think of fat as a single inert type of stuff, which while unsightly, does provide us with an energy store plus a bit of cushioning and insulation.

That, in turn, creates the impression there’s just one form of obesity – the one involving excess fat in layers under the skin, which afflicts one in three men and half of all women in the UAE.

But researchers now suspect that this so-called subcutaneous adipose tissue is not the real threat. That comes from visceral adipose tissue, or visceral fat for short.

As its name suggests, this coats our organs and fills the space in between them. But it doesn’t simply sit there. Visceral fat is like an organ in itself, seething with biochemical activity, churning out a host of molecules and hormones.

And these have been implicated in a host of chronic health threats.

Evidence of the importance of this “enemy within” comes from a perplexing discovery made by scientists looking at the link between levels of obesity and premature death.

Since the 1970s, the standard means of quantifying obesity has been via the so-called Body Mass Index, calculated by taking body-mass in kilograms and dividing by the square of height in metres. People with a BMI over 25 are regarded as overweight, while those exceeding 30 are deemed clinically obese.

Scientists have found BMI to be a fairly good pointer to increased risk of diabetes, heart disease and cancer. Fairly good – but not perfect.

One obvious flaw with BMI is that it doesn’t care what makes up body mass. It regards 10kg of honed muscle as just as “unhealthy” as 10kg of blubber.

But it’s now becoming clear that the biggest failing of BMI is that it doesn’t care where body mass is distributed.

Over the last decade or so, researchers have found increasing evidence that fat around the abdomen significantly increases the risk of premature death.

Last year, an international study of more than 41,000 people in Australasia and Europe showed that it was possible to have a healthy BMI in the range 20 to 25 and still face a 60 per cent higher risk of early death from cardiovascular disease.

Many health experts now think measures of BMI should be complemented, if not replaced, by waist measurement.

Current research suggests that if you’re a man and your waist circumference is at least 102cm, then you are seriously unhealthy; similarly for women with waists of at least 88cm.

Last month, the British Journal of Cancer published the results of a study of 43,000 people confirming the link between ill-health and waist measurement. Those carrying an extra 11cm around their waist were found to have a 13 per cent higher risk of cancer.

As for what makes waist measurement so important, scientists believe the answer lies in visceral fat.

Because it tends to accumulate around internal organs, it will be most obvious where most of our organs are: in our abdomen.

But as so often in obesity research, this simple “rule of tum” is now looking too simplistic.

While having a pot belly or muffin top may be a reliable indicator of increased risk, not having one is not necessarily a good sign.

That’s because visceral fat is quite capable of hiding within the spaces between our internal organs – leaving no visible trace of its lurking presence.

So how we tell if we’re playing host to the stuff? The only reliable non-invasive means is to undergo a medical scan using either X-rays or MRI. Scientists are trying to find cheaper, simpler methods but chances are it will never be a matter of plugging some do-it-yourself measurements into a formula.

That hasn’t deterred would-be health gurus from promoting exercise and diet plans said to target visceral fat, wherever it exists.

The good news is that while visceral fat is the first to accumulate, it is also the quickest to disappear.

Getting it to disappear is another matter, however.

Research suggests that exercises such as sit-ups alone are likely to prove useless: they don’t burn enough calories to make serious in-roads into abdominal fat reserves. They can, however, create the illusion of improvement, by reducing waist measurement by strengthening the abdominal muscles, which just do a better job of concealing the fat.

Some diets are said to target visceral fat – perhaps the best-known being the so-called 5:2 diet, which consists of five days of normal eating, with two non-consecutive days of 600 calories for men and 500 for women.

Pharmaceutical companies are also claiming that some drugs, such as orlistat, can seek out and destroy the enemy within.

But a new review of the evidence, just published in the International Journal of Obesity, suggests that nothing – not even bariatric surgery – really does target visceral fat.

The best way to get rid of it is by trying to lose weight in all its forms, by the age-old combination of a good diet plus exercise.

Cold comfort there for veterans of the war against the pot belly. But at least now they know they’re not fooling themselves about their health – unlike those who have always thought they can eat anything and get away with it.

Robert Matthews is visiting professor of science at Aston University, Birmingham, UK