The arteries of overweight children can be in as poor condition as those of middle-aged smokers, finds research.
Unhealthy foods are fuelling an obesity epidemic
This could make them up to five times more likely than those of normal weight to have a heart attack or stroke before age 65.
But the team, from the Chinese University of Hong Kong, also found diet and regular exercise can reverse the damage.
The research is published in the journal Circulation.
Researcher Professor Kam Woo said: "We were surprised that the children had developed vascular abnormalities at such a young age - and by how readily these could be reversed with simple lifestyle measures."
The researchers studied 54 boys and 28 girls, with an average age 9.9 years. Based on body mass index (BMI), 28 were overweight and 54 were obese.
The study did not include children with a family history of early heart disease, but the youngsters already showed signs of early thickening of the arteries, which increases the risk of heart problems.
Using ultrasound, the researchers measured the ability of the brachial artery in the arm to expand in response to increased blood flow - known technically as endothelium-dependent dilation.
The failure of this artery to respond to increased blood flow is an early sign that it is thickening up.
The researchers also used ultrasound to measure the thickness of inner layers in the wall of the carotid arteries, which are in the neck and supply blood to the brain.
The tests showed that the children's arteries were in a similar condition to those of a 45-year-old adult who had been smoking for more than 10 years.
The researchers calculated that they were three to five times more likely to suffer a heart attack or a stroke before age 65 than children of normal weight.
Reversing the damage
All the children in the study were then put on a low calorie diet for six weeks, and half took part in a 75-minute exercise programme twice a week.
After six weeks all the children had lost weight, and increased their endothelium-dependent dilation.
However, those who took exercise as well showed a bigger improvement than those who only went on a diet.
Follow-up tests after a year, showed that the children who kept up regular exercise had significantly less thickening of the carotid wall.
Professor Woo said: "This highlights the importance of regular exercise in preventing obesity-related vascular dysfunction in children.
"Adopting a healthy lifestyle in childhood is the most cost-effective and practical way to prevent heart disease in adults."
Belinda Linden, of the British Heart Foundation, said previous research had also shown that thickening of the arteries can start young.
She said: "The suggestion that this damage can be reversed with increased activity and better diets is encouraging, but it is easier to prevent obesity and its associated problems than reverse it.
"It is now up to government, schools, food manufacturers, health services, sports facilities, the media, parents and children to work together to tackle obesity and help protect future generations from unnecessary premature deaths from coronary heart disease."