Thursday August 14 2008
The researchers looked at the diet of three-year-olds
"Junk food diet 'makes children more likely to fail at school',” is the headline in The Daily Telegraph. It reports on a study that showed “even when other factors, such as low income or poor housing were removed, diet significantly affected the children's development". Children who "lived on sweets, crisps and chicken nuggets from an early age were 10% more likely to be failing between the ages of six and 10 than their classmates”, the newspaper reports.
Other newspapers report this study from a different angle. The headline in the Daily Mail says, “School meal junk food ban 'won't help pupils’.”
The differences in interpretation are because there were results from two parts of this study. The main result was that eating a lot of junk food at age three was associated with slower progress in primary school, but a poor diet at four and seven made little educational difference. An extra finding highlighted by the researchers was that eating either packed lunches or school meals made no difference to children’s educational attainment. This finding is less reliable because of the small number of children involved.
Where did the story come from?
Dr Leon Feinstein and colleagues from the Institute of Education at the University of London, the London School of Hygiene and Tropical Medicine and the University of Bristol carried out this research. Core support for the study was provided by the UK Medical Research Council, the Wellcome Trust and the University of Bristol. It was published in the peer-reviewed medical journal Journal of Epidemiology and Community Health.
What kind of scientific study was this?
This was an analysis of data from a cohort study. The data came from the Avon Longitudinal Study of Parents and Children (ALSPAC), an ongoing population-based study designed to investigate the effects of environmental, genetic and other influences on the health and development of children. Complex statistical methods were used to assess the relative importance of diet on school attainment at different ages.
The participants were pregnant women who lived in the former Avon health authority in south-west England and were expected to give birth between April 1 1991 and December 31 1992. Of all the potential mothers in this area, the researchers recruited a group of 14,541 pregnant women and their 13,988 children who were alive at 12 months old. The mothers completed questionnaires during pregnancy and when the children were various ages. Children’s educational attainment at 6–7 years and 10–11 years old was assessed by obtaining the Key Stage 1 (KS1) results for reading, writing and maths and Key Stage 2 (KS2) results for English, maths and science from the relevant education authorities. Key Stages are national standards for what children should be taught at particular ages, children’s levels of attainment of the set curricula are assessed at each age or Key Stage.
Information about the children’s diet was collected from mothers or main female carers using a food frequency questionnaire which asked about their child’s consumption of food and drink at about three years, about four years and about seven years old. The researchers identified three different patterns in the children’s diets: “junk food”, containing high-fat processed foods (sausages and burgers), snack foods high in fat and/or sugar (such as crisps, sweets, chocolate, ice lollies and ice creams), fizzy drinks and takeaway meals; ‘‘health conscious’’, comprising vegetarian foods, nuts, salad, rice, pasta, fruit, cheese, fish, cereal, water and fruit juice; and ‘‘traditional’’, typically meat and cooked vegetables.
In the questionnaire about the seven-year-old children, the mothers also reported whether their child ate meals served at school or packed lunches provided from home, and how often they did this. Frequency was recorded as: rarely, once in two weeks, once a week, two to four times a week or five days a week. Details of socioeconomic, demographic and lifestyle factors were also collected.
Of the 13,988 children in the original set of data, dietary information at all three ages was only available for 7,703 children and, of these children, only some had information on both KS2 and KS1 scores. The final study sample therefore consisted of 5,741 children with complete information on food frequencies and both school attainment scores results (41% of the original sample of 13,998 children).
What were the results of the study?
The data collected in the questionnaire about seven-year-old children showed that 29% ate school dinners five days a week and nearly half (46%) ate packed lunches five days a week.
The researchers found that higher ‘‘junk food’’ dietary pattern scores at three, four and seven were associated with lower average KS2 results. In contrast, a positive association was shown for the ‘‘health conscious’’ dietary pattern and KS2 results. The ‘‘traditional’’ dietary pattern showed no association with KS2 results. When potential confounding factors were taken into account, a weak association remained between “junk food” at age three and lower attainment.
There was no evidence that eating packed lunches or eating school meals affected children’s attainment, once the impact of junk food dietary pattern at age three was accounted for in the model.
What interpretations did the researchers draw from these results?
The researchers conclude that “early eating patterns have implications for educational attainment that appear to persist over time, regardless of subsequent changes in diet.”
What does the NHS Knowledge Service make of this study?
This study and the author’s conclusions should be interpreted with caution for a number of reasons:
- Complete data was only available for 41% of the original study cohort and the authors mention that there were fewer ethnic minorities and disadvantaged families in this group than in the original cohort. This sort of data loss and loss to follow-up may seriously affect the reliability of the results. Although the researchers have attempted to adjust and investigate the differences between those followed up and those who had missing data, it remains possible that the results of this study would not be the same in a similar study which had more complete data, or if all participants had been available for follow-up.
- The fact that there was no evidence that eating packed lunches or eating school meals affected children’s attainment (after the impact of the “junk food” dietary pattern at age three had been taken into account) does not necessarily mean that such an effect does not exist. It may be that there weren’t enough children assessed to allow a significant difference to be detected.
- There is an assumption that packed lunches are more likely to contain less nutritious food than school meals. However, as the nutritional content of each was not measured it is not possible to say how they differed.
This research does highlight the importance of diet before children go to school for later educational attainment and supports a call for concerted efforts to improve the nutritional intake of all children. The negative finding of the authors, reported by some newspapers, that whether children ate packed lunches or school meals did not affect their educational attainment will need confirmation in larger studies with more complete follow-up.
Sir Muir Gray adds...
The evidence is clear – eat natural. It's good for the individual and the environment.