Anyone concerned about gaining extra weight will be confused by the latest research as scientists in the U.S. say being thin is no guarantee a person has a low body fat ratio.

Researchers at the Mayo Clinic are suggesting that thin people can still have high body fat percentages inside of their bodies which puts them too at risk of heart disease and the other diseases and conditions that affect overweight and obese people.

The researchers reached this conclusion after conducting a study involving more than 2,000 participants all of whom had a normal body mass index (BMI).

The team discovered that those with a higher internal body fat percentage relating to their body size were at the most risk of developing cardiovascular problems.

Researcher Dr. Francisco Lopez-Jimenez says normal weight obesity appears to be highly prevalent.

To test for body fat the scientists used a technique called bio-impedance analysis which records the resistance of electrical impulses throughout the body.

The level of resistance reveals the percentage of body water, lean body mass muscle and of body fat.

Dr. Lopez-Jimenez says more than half of the patients with a normal weight as defined by the BMI had normal weight obesity, while 806 had a normal body fat content.

The research also revealed that as many as 13% of those who met the normal weight obese threshold, also met the criteria for metabolic syndrome.

A BMI between 18.5 and 25 is widely considered to be within the healthy range but many experts believe body fat percentage is the most important measure of healthy weight.

They say to be healthy, women should have no more than 30% fat and men should have no more than 20% and the most dangerous fat was the "metabolically active" intra-abdominal or visceral fat which sits between organs and produces substances that increase the risk of heart disease and diabetes.

A healthy waist circumference is no more than 94 centimetres for European men, 90 for Asian men and 80 for women.

The researchers say the study should be taken very seriously as even thin people can be fat, and this means that the percentage of people who are overweight or obese is higher than estimated.

The research was presented at the American College of Cardiology's 57th Annual Scientific Session.