Diets high in fat, sugar and processed foods are lowering children's IQ, a new study suggests. The report says that eating habits among three year olds shapes brain performance as they get older.

A predominantly processed-food diet at the age of three is directly associated with a lower IQ at the age of eight and a half, according to a Bristol-based study of thousands of British children.

The Avon Longitudinal Study of Parents and Children is tracking the long-term health and wellbeing of around 14,000 children.

Food packed with vitamins and nutrients notably helped boost mental performance as youngsters got older, the Journal of Epidemiology and Community Health reports.

Researchers said toddlers' diets could change IQ levels later in childhood, even if eating habits improve with age.

"This suggests that any cognitive/behavioural effects relating to eating habits in early childhood may well persist into later childhood, despite any subsequent changes to dietary intake," the authors wrote.

During the study, parents completed questionnaires detailing the types and frequency of the food and drink their children consumed when they were three, four, seven and eight-and-a-half years old.

Every one-point increase in the study's dietary pattern score – a record of processed fat intake – was associated with a 1.67-point fall in IQ.

The brain grows at its fastest rate during the first three years of life.

"It is possible that good nutrition during this period may encourage optimal brain growth," the report added.

The School Food Trust's director of research, Michael Nelson, said: "Given that around 23% of children start school either overweight or obese, it's absolutely clear that healthy choices as part of their early development will stand children in good stead – not only for keeping a healthy weight as they grow up, but as this evidence suggests, improving their ability to do well at school.

"These findings also demonstrate the importance of helping everyone involved with children's early development to get the information and advice they need on good nutrition."