Diets high in fats, sugars and processed foods are lowering toddlers' IQs, a new study has suggested.

It found eating habits among three-year-olds shape 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.

Food packed with vitamins and nutrients notably did the opposite, helping 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.

The Avon Longitudinal Study of Parents and Children (ALSPAC) is tracking the long-term health and well-being of around 14,000 children.

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 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.