A new scanner can reveal whether you are a 'TOFI' – thin on the outside, fat on the inside.

How fat are you? The bathroom scales can't tell you and neither can a tape measure – not in a meaningful way. The fat you can see and feel is subcutaneous. Unbecoming and unwanted perhaps, but not necessarily harmful.

The damaging stuff is on the inside. Called visceral or intra-abdominal fat, it is deposited around your vital organs and from there, wreaks havoc on internal systems. Visceral fat has remained out of sight and conveniently out of mind for most people. But soon you will be able to look at it, thanks to a new portable imaging system.

I slid under the ViScan with some trepidation. Though a modest 81/2 stone, and as a cyclist, pretty muscly, there were things not in my favour. I'm approaching the age where women's bodies start, like men's, to store unhealthy fat around the middle, I have a penchant for full-fat dairy foods and several members of my family in previous generations have died of heart disease in middle age. All increase your visceral fat stores.

Using a belt placed on the abdomen, the scanner reads the total visceral fat around the organs: up to 13 is OK; after that you need to rethink your lifestyle. A figure of 2.5 gave me the all-clear, while my husband was put on the alert with one of 15.

Visceral fat is bad news because it is much more bioactive than subcutaneous fat. It changes the biological environment, raising your risk of heart disease, diabetes, hypertension and high cholesterol.

So in the absence of a scanner – which will begin to appear in doctors' surgeries later this year – how can you know how fat you are inside? Look at your midriff. The more belly fat you have, the more visceral fat you are likely to be concealing. "Although people think of it as the invisible fat, in a way you can see visceral fat because there is such a strong correlation between that and abdominal fat – so the bigger your belly the more likely you are to have fat around your vital organs," says Dr Susan Jebb, head of research at the Dunn Nutrition Centre.

It's time to stop worrying about your flabby thighs or bingo wings, she says: "It is the location of your fat that matters more than the amount and if it is not around your middle, and therefore your vital organs, it is probably not a great health hazard."

So if the hazardous fat isn't the stuff you can see, who has it? Putting aside those witha muffin-top – a sure sign of intra-abdominal fat – it's hard to tell. Around 40 per cent of people have visceral fat and many are in the 25-30 BMI range where their size doesn't stand out much these days.

Then there is a class of people who hide it well and whom obesity specialists call the "tofis": thin on the outside, fat on the inside. According to Dr Louise Thomas, an obesity researcher at Hammersmith Hospital where the ViScan was tested, these are typically women who have controlled their weight by dieting erratically and who may smoke and drink. "Thin people with an unhealthy diet and lifestyle can have the same amount of visceral fat as fatter people who exercise and eat healthy foods, if too many of them," she warns. A study found that 17 per cent of lean men and 13 per cent of lean wo-men had too much visceral fat.

Willa Franks is a case in point. A svelte size 8, at 5ft 8in she has a body mass index of just 21: at the bottom end of the healthy BMI range, it would suggest all was well. But her scan revealed a visceral fat score of 10. "It was alarming because I'm never ill, never put on weight and have a pretty average diet." She is now paying more attention to her food – more fruit, fewer saturated and trans-fats – and exercising more. "The dogs get walked more and I walk to the shops."

Modest though that is, daily exercise is the key to reducing visceral fat load and it doesn't have to be radical. A team from Exeter and Bedfordshire universities compared fat people, fit or unfit, with thin people of both types. The fit people accumulated less visceral fat, regardless of their size. The fit fatties also had lower blood fats than would be expected from their BMI. Those with excessive visceral fat usually have high cholesterol, as well as high blood pressure and high blood sugar.

It is not known how exercise fights visceral fat build-up, but exercise reduces insulin levels and too much insulin leads to fat storage. Exercise also combats stress, which adds to visceral fat build-up and cuts the size of visceral fat cells.

The bad side-effects of visceral fat go beyond the physical. Visceral fat is so metabolically active, that the substances it produces can affect the mind. A new study at Rush University Medical Center in the US has shown that fat around the organs (but not elsewhere) is linked to depression. It explains what medics have long known: depressives have a high risk of cardiovascular disease. It seems the many substances released by visceral fat include cortisol and other stress hormones implicated in depression. "So many things secreted by visceral fat have an impact on health. It's not surprising it can affect your mood," says Dr Thomas.

The accumulation of visceral fat is that it is now starting early in life. A recent study at the University of South Australia has shown that over the past 50 years children's middles have become relatively fatter than their limbs. Add to that the facts that fatty livers are now more common among children and children with fat around the organs are more likely to become insulin resistant in their teens, and you have the first visceral fat generation.

"It's worrying because we know fatness tracks into adulthood and the longer you have visceral fat, the higher the risk," says Dr Jebb.

But there is good news. Because visceral deposits are metabolically responsive, when you lose weight the visceral fat starts to go first. You may have joined the ranks of Fotis (fat on the outside, thin on the inside). Fashion aside, that can only be a good thing. "There is a lot we don't know," says Dr Jebb. "But we know fat is not an aesthetic issue."