The British Medical Research Council says that you may be slim and still have dangerously high levels of fat within you.
Using MRI body scanners doctors demonstrated that even super-slim people could have high levels of internal fat collecting around the heart, liver, kidneys and pancreas. These people, dubbed "skinny-fats", could be seriously putting their health at risk.
"The fat we can see on overweight people is subcutaneous fat," The Daily Telegraph quoted Dr Ron McCoy, Melbourne-based spokesperson for the Royal College of Australian GPs, as saying. However, what could be more dangerous is visceral fat or the fat we can't see but which surrounds vital organs.
Visceral fat is also believed to produces more hormones and proteins than subcutaneous fat, affecting glucose levels and leading to the onset of type 2 diabetes and other health problems like cardiovascular disease.
But the question is if one is thin then how does fat accumulate inside the body.
Lack of physical exercise is the biggest reason.
Sam Mower, an exercise physiologist, said: "If your body isn't moving, it doesn't metabolise the fat that's building up - either outside or inside."
Diet is another factor.
"If you're eating foods high in saturated fat, such as butter, cheese, cakes and biscuits, it's nearly all stored as visceral fat," Mower said.
The menopause and sugar intake by way of alcohol consumption are among other causes.