Obese children as young as three years old show signs of future heart disease, say US researchers.
A study of 16,000 children and teenagers showed the most obese had signs of an inflammatory marker which can predict future heart disease.
In all, 40% of obese three-to-five-year olds had raised levels of C-reactive protein compared with 17% of healthy weight children, Pediatrics reported.
But more work is needed to prove the link with heart disease in later life.
The study, carried out by a team at the University of North Carolina (UNC), looked at children aged one to 17.
Overall, nearly 70% were a healthy weight, 15% were overweight, 11% were obese and 3.5% were very obese.
In the older age groups, the proportion of those in the very obese category with high levels of C-reactive protein (CRP) increased even further.
By age 15-17, 83% of the very obese had increased CRP compared with 18% of the healthy weight children.
CRP is found in the blood, and high levels are a sign of inflammation in the body.
Because the damage seen in heart disease is caused by inflammation in artery walls, CRP can be used as a general marker for the risk of heart disease.
In adults, studies have linked high levels with a future risk of heart attacks.
The researchers also looked at two other markers of inflammation in obese children and found levels were higher in one from the age of six and the other from the age of nine.
Study leader Dr Asheley Cockrell Skinner, an assistant professor of paediatrics at the UNC School of Medicine, said the findings were a surprise.
"We're seeing a relationship between weight status and elevated inflammatory markers much earlier than we expected."
Co-author Dr Eliana Perrin added: "In this study we were unable to tease apart whether the inflammation or the obesity came first, but one theory is that obesity leads to inflammation which then leads to heart and vessel disease later on.
"A lot more work needs to be done before we figure out the full implication of these findings.
"But this study tells us that very young, obese children already have more inflammation than children who are not obese, and that's very concerning."
Judy O'Sullivan, a cardiac nurse at the British Heart Foundation, said it was an interesting finding but whether inflammatory markers in children were related to an increased risk of heart disease later in life needed further research.
"Nevertheless, this study reinforces the importance of ensuring children maintain a healthy weight right from the start, to keep them healthy throughout their lives.
"Children should be encouraged to adopt a healthy lifestyle and as part of this, regular physical activity and a balanced diet should be viewed as vital components."