Obese seven-year-olds are at greater risk of suffering emotional problems, such as anxiety and low mood, when they reach 11, a large UK study suggests.
The Liverpool researchers found obesity and mental health were closely linked, and gradually increased throughout childhood.
Girls tended to have higher BMIs and more emotional problems than boys.
Although the study didn't look at causes, it said poverty was likely to increase the risk of both problems.
The findings, to be presented at the European Congress on Obesity (ECO) in Glasgow, strengthen the case for early prevention in overweight children, the researchers said.
The researchers analysed information on more than 17,000 children born in the UK between 2000 and 2002, using statistical modelling to measure the link between obesity and emotional problems.
They had information on children's height and weight (BMI) as well as reports on their emotional problems, provided by their parents, at ages three, five, seven, 11 and 14 years old.
From the age of seven, the study found obesity and emotional problems were closely linked.
But the link wasn't apparent in younger children.
'Not as simple as eating less'
Dr Charlotte Hardman, senior psychology lecturer at the University of Liverpool, said the findings showed obesity and emotional problems were likely to develop hand-in-hand in childhood.
She said that was important for those who treat children with obesity.
"People think it's as simple as eating less and exercising more - but it's much more complex than that.
"Obesity and emotional problems are intertwined."
She said it was already known that obesity and mental health problems were interlinked in adulthood, and the same could be true in childhood.
"From the age of seven, mental health and obesity appear to be entwined and exacerbate each other."
Dr Hardman said that meant children "being stuck in vicious cycles".
"As both rates of obesity and emotional problems in childhood are increasing, understanding their co-occurrence is an important public health concern, as both are linked with poor health in adulthood," she said.
The study is published in JAMA Psychiatry.