Children who snack all day instead of eating just three meals a day are more likely to have tooth decay -- as brushing their teeth can't ward off the effects of sugary treats.
A study based on a sample of nearly 4,000 pre-school children shows snacking habits are most strongly associated with decay.
Researchers found children who snack all day -- compared with just eating meals -- are far more likely to have dental decay.
The study also found that tooth brushing only partly protects against the effects of sugary snacks on children's teeth.
Researchers say the study shows relying on tooth brushing alone to ward off dental decay in children under five is not enough.
Scientists from the University of Edinburgh and the University of Glasgow used statistical models and survey data to predict dental decay by age five.
They used data collected on diet and oral hygiene from repeated observation of children from ages two to five.
Snacking was the factor most strongly associated with decay, with children who snack all day without eating meals having twice the chance of decay.
This was compared with those who did not snack at all.
Lead researcher Dr Valeria Skafida, of the University of Edinburgh's School of Social and Political and Sciences, says restricting sugar intake is desirable for both nutritional reasons and for children's dental health.
Dr Skafida said: "Even with targeted policies that specifically aim to reduce inequalities in children's dental decay it remains an ongoing challenge to reduce social patterning in dental health outcomes."
Parental socioeconomic factors, such as a mother's education level, explain more of the difference in children's dental decay than diet or oral hygiene.
Experts say that even though primary teeth are temporary, good oral hygiene habits are set in childhood, and this relates both to diet and tooth brushing.
Children who brushed less than once a day or not at all at two-years-old had twice the chance of having dental decay when they are five, compared with children who brushed their teeth twice a day or more.
Study co-author, Dr Stephanie Chambers, of the Social and Public Health Sciences Unit at the University of Glasgow said: "Among children eating sweets or chocolate once a day or more, tooth brushing more often - once or twice a day or more - reduced the likelihood of decay compared with less frequent brushing."
The study is published in the Journal of Public Health.
The researchers used data from the Growing Up in Scotland study - a social survey which follows the lives of children from infancy through to their teens.