Obese children as young as eight are showing signs of major heart disease, a new study has found.

US researchers carried out magnetic resonance imaging (MRI) tests on the hearts of 20 obese children and 20 children who were considered a normal weight. An MRI allows the function and dimensions of the heart to be measured.

The study found that children who were obese had 27% more muscle mass in the left ventricle of their hearts and 12% thicker heart muscles. Both of these are signs of heart disease.

Furthermore, 40% of these children were considered to be at ‘high risk' because of the thickened muscles in their heart. This is because thickened muscle can impair the heart's ability to pump blood properly.

However, none of the children showed any physical symptoms of heart disease.

"Parents should be highly motivated to help their children maintain a healthy weight. Ultimately, we hope that the effects we see in the hearts of these children are reversible, however, it is possible that there could be permanent damage. This should be further motivation for parents to help children lead a healthy lifestyle," the researchers said.

They noted that some of the 20 obese children who took part, seven of whom were teenagers, already had conditions associated with excess weight, including high blood pressure and depression.

Furthermore, the researchers pointed out that they had to exclude some children from taking part in the study because they were too large to fit into the MRI machine.

"As a result, this means the actual burden of heart disease in obese children may have been ¬under¬-estimated in our study because the largest kids who may have been the most severely affected could not be enrolled," they explained.

They pointed out that not all of the obese children in the study showed signs of heart disease, however some as young as eight years old were displaying signs already.

"This implies that obese children even younger than eight years old likely have signs of heart disease too. This was alarming to us. Understanding the long-term ramifications of this will be critical as we deal with the impact of the paediatric obesity epidemic," they added.

Details of these findings were presented at the American Heart Association's Scientific Sessions 2015 in Florida.