Lab Notes: Is fast food making children stupid? Don't swallow the stories

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Junk food rots your brain, scientists say. Well, actually, I just made that up. What scientists have really shown is that feeding junk food to young children from the age of three "may be associated" with a slightly lower IQ score at the age of eight.

Words are as important in science as in a court of law. As any good epidemiologist will point out, given 0.5 the chance, association does not automatically mean causation. Because one event (junk food in early life) can be tied statistically to another (slightly lower IQ five years later), it doesn't necessarily follow that one causes the other.

The study in question, published last week in the Journal of Epidemiology and Community Health, was carried out as part of the Avon Longitudinal Study of Parents and Children (Alspac), which is tracking the long-term health and well-being of some 14,000 children born in 1991 and 1992.

Scientists at the University of Bristol wanted to test the hypothesis that a more "healthy" diet in childhood would be associated with an increase in intelligence. They wanted to see whether there was a difference in IQ scores between children fed on a diet rich in salads, pasta, rice, fish and fruit, and children given processed food with high fat and sugar content, such as crisps, fizzy drinks, chips and burgers.

The trouble with such studies is the influence of confounding factors. A mother (and it is usually the mother) who feeds her children a health-conscious diet may also be more likely to be a woman who reads to her kids at bedtime. So is it the diet, or the reading at bedtime that makes those children bright in later childhood? Or is it neither?

The scientists behind the study obviously tried to take such confounding factors into account by "controlling" for social class, maternal age at birth, type of housing, and maternal education. But even so, there is always the nagging possibility that a bad diet is just a marker for something else that really is affecting a child's intellectual development.

It may be, for instance, that a mother's intelligence, which has a heritable component, is linked with the likely diet of her children. Interestingly, this study did not control for maternal intelligence, though the authors of the study believe that controlling for maternal education should have eliminated this as a confounding factor.

Another potential source of error is the nature of the questionnaires used to assess the diet of the children. The mothers were asked to complete "food frequency" forms at the age of three, four, seven and eight-and-a-half years.

An intelligent mother may have filled in the form in a less-than-honest way, perhaps reflecting her health-conscious intentions rather than the actual reality – processed foods may be more convenient for a busy mum who is aware that it may not look too good if she is seen to be relying on them too much.

So it is perhaps easy to see why the scientists used carefully phrased words to conclude their scientific paper: "In this population of contemporary British children, a poor diet, associated with increased intake of processed foods, fat and sugar, in early childhood may be associated with lower IQ at the age of 8.5 years."

Interestingly, the title of the paper is even more circumspect: "Are dietary patterns of childhood associated with IQ at 8 years of age?"

This headline went through an intriguing evolution in the course of its development from press release ("Processed-food diet in early childhood may lower subsequent IQ") to splash headline in the Daily Mail: "Junk food diet hits a child's IQ"). But the one thing that this study has not yet shown is that junk food lowers a child's IQ.