Diets high in fats, sugars and processed foods have been linked with slightly lower IQ in young children, a study has found.

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, compared to children who eat a diet rich in fresh fruit and vegetables, according to a Bristol-based Avon Longitudinal Study of Parents and Children (ALSPAC), which is tracking the long-term health and well-being of around 14,000 children.

The scientists who carried out the study, published in the Journal of Epidemiology and Community Health, emphasised however that they could not say for certain that processed foods are the cause of lower IQs as other factors such as social class and educational upbringing may be implicated.

Parents were asked to detail 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. A diet rich in processed food at aged three was linked with an IQ that was 1.67 points lower on the IQ scale, where a score of 100 is average intelligence. "This suggests that any cognitive/ behavioural effects relating to eating habits in early childhood may persist into later childhood, despite any subsequent changes to dietary intake," the scientists said.