Being slim may not be always a sign of good health, says a survey that discovered that many outwardly thin people store up dangerous layers of fat inside their bodies.
It is commonly assumed that people who are slim must be much healthier than those who are obviously overweight.
But according to Professor Jimmy Bell at the Medical Research Council's centre at Imperial College, London: "Some people may look thin on the outside... but they are often far more at risk than those people who are obviously fat or obese."
This is particularly true of men who have a slim build but do little or no exercise, Bell was quoted as saying in the online edition of Daily Mail.
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 millions of people.
The new research comes after the Sport England survey covering 364,000 people in all 354 local authority regions last week showed four out of five adults in Britain were storing up health problems by failing to exercise enough.
They found that many people who look slim carry large amounts of fat 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.