By David Derbyshire, Consumer Affairs Editor
Last Updated: 2:14am GMT 12/12/2006
It used to be said that inside every fat person was a thin person trying to get out. Now it seems it could be other way around.
A scanning technique pioneered by British doctors has discovered that many outwardly slim people are storing up dangerous levels of fat in their bodies.
Images from MRI (magnetic resonance imaging) scanners suggest that up to four out of 10 people could be a "Tofi" — thin outside, fat inside.
Prof Jimmy Bell, head of the molecular imaging group at the Medical Research Council's centre at Imperial College, London, said this hidden fat could trigger heart conditions and diabetes.
"The important message is people shouldn't be happy just because they look thin; it's not about looking fit or looking thin, it's about being healthy.
"You can look healthy, but have a lot of fat internally, which can have a detrimental effect on your health."
Even apparently slim people can have internal fat collected inside or around the liver, gut, heart and pancreas, or streaked through under-used muscles, he said.
Prof Bell and his team began using the MRI scanner to seek out internal fat while researching type 2 diabetes, the version of the disease that develops later in life and which is normally associated with obesity. His suspicions arose when several people in the study who were slender had the medical markers for type 2 diabetes.
The findings are raising questions about the BMI (Body Mass Index), the indicator of obesity used by doctors and public health campaigners. The BMI is a relatively crude measure which takes into account a person's height and weight. To calculate a BMI, a person's weight in kilograms is divided by their height in metres squared.
Some doctors believe the BMI is flawed because it pays no attention to the nature of the weight.
A rugby player, for instance, with heavier than usual muscles will come out with a high BMI and could be classified as overweight, even though they have low levels of internal fat.
In Japan, sumo wrestlers have been put through MRI scanners to look at their fat composition. Even though they have a BMI of 56 and are eating up to 5,000 calories a day, they have very little internal fat. 'They have low cholesterol, they have low insulin resistance and a low level of triglycerides [fatty acids],' said Bell. 'Their fat is all stored under the skin, on the outside.’
"When you exercise you tend to burn the internal fat," said Prof Bell. "Lifestyle changes have to include physical activity. It is not just enough to diet."
Imperial College researchers have discovered that a typical woman has around three litres of internal, or visceral fat, while a man has around five litres. However, overall, women have 37 litres of fat, compared with 35 litres in men.
Ben Schwartz, 28, who was scanned at Hammersmith Hospital last week, discovered his internal fat levels are too high even though he is outwardly healthy.
The MRI showed that he had 3.75 litres of internal fat out of a total of 20.75 litres in his body.
Mr Schwartz, who works for a television company, said he spent half his day at a desk, and half the day outside, but did not have the time to go to the gym.
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