Kids who eat a diet high in fats, sugars, and processed foods in early childhood appear to grow up to have a lower IQ, while those who eat a diet rich in vitamins and nutrients grow to have higher intelligence, new research suggests.

The findings were made by researchers who looked at data from the Avon Longitudinal Study of Parents and Children. The study is tracking the long-term health and well-being of around 14,000 children born in 1991 and 1992.

Parents complete questionnaires detailing the types and frequency of the food and drink their children consume at the ages of three, four, seven and 8.5 years old. IQ is measured when they were 8.5 years old.

This study focused on 3,966 children for whom complete data were available. The researchers broke up the kids into three dietary patterns:

• "processed" high in fats and sugar intake
• "traditional" high in meat and vegetable intake
• "health conscious" high in salad, fruit and vegetables, rice and pasta.

After the researchers adjusted their findings for several factors, including breastfeeding duration and the amount of oily fish their mothers consumed while pregnant, they found that a highly processed food diet at the age of three was linked with a lower IQ at the age of 8.5. Every 1 point increase in dietary pattern score was associated with a 1.67 fall in IQ.

On the other hand, a healthy diet was associated with a higher IQ at the age of 8.5, with every 1 point increase in dietary pattern linked to a 1.2 increase in IQ.

They also found that it didn't matter if the children's diets improved or worsened at older ages. Dietary patterns between the ages of 4 and 7 had no impact on IQ.

The authors note that the differences in IQ were modest. Nevertheless, they say the findings are in line with previous research from the same study group which found a link between early childhood diet and later behaviour and school performance.

"This suggests that any cognitive/behavioural effects relating to eating habits in early childhood may well persist into later childhood, despite any subsequent changes (including improvements) to dietary intake," say the researchers, whose study appears in the Journal of Epidemiology and Community Health.

The authors note that the brain grows at its fastest rate during the first three years of life. Other research has found that head growth at this time is linked to intellectual ability.

"It is possible that good nutrition during this period may encourage optimal brain growth," they suggest.

The researchers say more research is needed to understand the extent of the effect early diet has on intelligence.