Children who consume too much sugar are more likely to be violent, drink alcohol and smoke, research suggests.
A review of existing studies has found eating and drinking a lot of sugar increases 11 to 15-year-olds' chances of taking part in risky behaviours.
Children were nearly three-and-a-half times as likely to be a bully if they had a lot of sweets and energy drinks, one study found.
Another revealed high levels of sugar make children more than twice as likely to get into fights, and 95 per cent more likely to get drunk.
And the scientists behind the study say energy drinks worsen children's behaviour more than chocolate and sweets because they contain caffeine as well.
The research by Bar Ilan University in Israel studied 137,284 children aged either 11, 13 or 15 and living in 25 European countries or Canada.
Scientists compared how much sugar the children consumed with how often they got into fights, bullied other children, smoked cigarettes, drank alcohol and got drunk.
They found 'strong and consistent relationships' between the two.
In the UK, children who consume a lot of sweets, chocolate and energy drinks are 89 per cent more likely to smoke and 72 per cent more likely to drink alcohol.
They are also 2.13 times more likely to get into fights and have a 69 per cent higher chance of being a bully.
The study is thought to be the first to look at the effects of sugar on violence and substance abuse in children.
Data was used from a study called Health Behaviour in School-Aged Children, done by the University of St Andrews and the World Health Organization in 2013-14.
Information was collected on children from Austria, Belgium, Canada, Czech Republic, Denmark, Estonia, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Portugal, Romania, Slovakia, Slovenia, Sweden, Switzerland and the UK.
Swedish children are worst affected by sugar, the results suggested, and were more than twice as likely to be violent or drink or smoke.
Children were grouped according to whether they had a high sugar consumption or not, based on how often they ate chocolate or sweets, or drank energy drinks.
The study did not make clear how much sugar was considered too much, but the NHS has clear guidelines.
Children aged over 11 should not consume more than 30 grams of added sugar – there is more than an entire day's maximum in a single can of Coca Cola or a Mars bar, which contain 35g and 33g respectively.
Younger children should have even less – four to six-year-olds should have no more than 19g per day.
The study authors wrote: 'The relationship between sugary drinks and involvement in substance use and peer violence is stronger than that with sweets and chocolate.
'Since sugary drinks also often contain many additives, including caffeine, it is possible that sugar in combination with some additives contained in soft drinks makes soft drinks a more powerful or consistent predictor.'
They said consuming a lot of sugar could be a 'red flag' for teachers or youth workers.
Children's eating habits could give an idea of how likely they are to cause trouble or have emotional problems when they're teenagers, the authors said.
In the majority of cases, the link between children's sugar intake and behaviour was not affected by how wealthy their family was, according to the study.
The research was published in the journal Social Science & Medicine.