Young adults who are obese are much more likely to be morbidly obese later in life, a new study has shown.

Scientists assessed data from a major nutrition survey that was carried out in the US between 1999 and 2010.

A person is considered obese if they have a body mass index (BMI) of 30 or more. A BMI of 30-35 is considered class one obesity, 35-40 is class two and greater than 40 is class three - known as severe or morbid obesity.

The study found that men who were obese at the age of 25 had a 23% probability of being in the class three obese category after the age of 35. Men who were a healthy weight at the age of 25 had a 1% probability of reaching class three obesity after the age of 35.

Among women, the results were even more startling. Those who were obese at the age of 25 had an almost 47% probability of being in the class three obese category after the age of 35, compared to 4% of women who were a healthy weight at the age of 25.

However, the study did show that current weight, as opposed to how long a person had been obese, was a better indicator of heart and metabolic risk. This means that weight loss at any stage can reduce health risks, irrespective of how long a person has been overweight.

"The current findings suggest that the biological risks of longer-term obesity are primarily due to the risk of more severe obesity later in life among those obese early in life, rather than the impact of long-term obesity per se.

"This is good news in some respects, as overweight and obese young adults who can prevent additional weight gain can expect their biological risk factors to be no worse than those who reach the same level of BMI later in life," the scientists said.

However, they did point out that duration of obesity can still have major implications for ‘mobility and musculoskeletal disease'.

"Prevention of weight gain at all ages should thus be a clinical and public health priority," they insisted.

Details of these findings are published in the American Journal of Preventive Medicine.