By JAYA NARAIN Last updated at 22:00pm on 10th December 2006

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It Is commonly assumed that people who are slim must be much healthier than those who are obviously overweight.

But scientists have discovered that many outwardly slim people are storing up dangerous layers of fat inside their bodies.

And while they may look thin on the outside their health is often far more at risk than those people who are obviously fat or obese.

Scientists have branded these people Tofi - Thin on the Outside, Fat on the Inside - but say the physiological phenomenon could have serious health implications for milllions of people.

Thanks to advancements in hi-tech magnetic resonance imaging (MRI) doctors can look inside the human body in far more detail.

What they have found is that many people who look slim carry large amounts of fat located around important organs such as their heart and liver.

It is this fat, rather than the subcutaneous fat found under the skin, which can eventually lead to serious health problems.

So a person who appears overweight can often be far more healthy than an individual who looks thin but whose internal organs are shrouded in fat.

Professor Jimmy Bell, at the Medical Research Council's centre at Imperial College, London, said: "This is particulary true of men who have a slim build but who do little or no exercise."

The research comes after a survey published last week which showed four out of five adults in the UK were storing up health probelms by failing to exercise enough.

The Sport England survey, covering 364,000 people in all 354 local authority regions, revealed fascinating geographic patterns of high and low activity.

Richmond upon Thames emerged as the sportiest area, with nearly a third of people taking regular exercise - a target defined as three half-hour sessions a week of moderate intensity sport or active recreation while in Boston, Lincolnshire, six out of seven failed to hit the target.

Scientists say this lack of exercise, combined with a genetic pre-disposition, can lead to fat being stored around vital organs, streaked through underused muscles and wrapped around the heart.

"Our work so far has shown that you can take two men of the same age, with the same BMI (body mass index) and find one with five litres of fat within him and another with two litres.

"What we don't yet possess is enough information about how different genetic groups store fat. But we do know that you can manipulate the way the body stores it by changing the diet."

But judging one's fat by BMI alone can be misleading because it does not take into account how much fat is stores around organs.

So a rugby player or similar large athlete sporting heavy muscles could have a high BMI but very low levels of fat around the organs.

Prof Bell said that what really counts is where the fat, which is the body's energy supply, is stored rather than just the amount.

The Imperial College team has found that the average male has 5.4 litres of fat around organs compared to just 3.08 for women.

But women carry more fat overall, mostly on the thighs and bottom, and the total amount of fat in the average woman is 37 litres and 30 litres for men.

He said: "Over the past five years, we've demonised fat and become obsessed with obesity, which is mostly talked about in terms of weight loss. But what matters is where it is distributed.

"As you lose weight, it tends to go from the bottom and bottom of your body first, so it can become concentrated in the abdomen.

"That is the most dangerous zone of all and it's possible that going on a constant series of diets actually encourages the storage of fat in this region."

The National Institute for Health and Clinical Excellent (NICE) is unveiling new guidelines on obesity in another attempt to encourage the nation to lose fat.

Official figures suggest the UK will see a 14 per cent rise in obesity by 2010, resulting in 27 million people being termed obese.

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