Tuesday February 8 2011
Healthy diet was linked to higher IQ but the effect was small
“Diet does boost your intelligence,” according to The Daily Telegraph.
The news story is based on a dietary study that following over 7,000 children. The study compiled information on how often the children ate different food groups at the ages of three, four, seven and eight-and-a-half years old. The researchers also assessed the children’s IQ at the final assessment, when they were eight-and-a-half, to see whether there was any association between diet and intelligence.
The researchers found that eating a diet high in sugar, fats and processed food at the age of three was associated with a lower IQ at eight-and-a-half years old. There was also an association between eating a healthy diet (including salads, vegetables, fish, pasta and rice) at eight-and-a-half years old and having a higher IQ at the same age. However, the latter association should be interpreted cautiously as it cannot be demonstrated that this diet caused the higher IQ. The researchers also point out that both these effects were very modest, calling them a “weak association”.
While this study does not prove that diet has any effect on IQ, a healthy balanced diet for children has many known benefits, regardless of any effect on intelligence.
Where did the story come from?
The study was carried out by researchers from Bristol University and was funded by The UK Medical Research Council, The Wellcome Trust and the University of Bristol. The study was published in the peer-reviewed Journal of Epidemiology and Community Health.
The research was reported by a number of news sources. Some of them suggested that a healthy diet could “boost IQ” while others implied that a poor diet could “damage intelligence”. These sources generally overemphasised the effect of diet on intelligence, which the researchers themselves described as “a weak association”. While this research was well conducted, other factors, such as the social make-up of the study population, raise questions about the validity of the effect observed.
What kind of research was this?
This cohort study looked at the effect of diet on children’s intelligence. The researchers said that several studies have looked at the association between breastfeeding and subsequent intelligence as the child ages, but few studies have assessed whether there is an association between diet (solid food) in early childhood and intelligence.
A cohort study can be used to examine possible cause-and-effect associations between diet and intelligence over time. However, the study also produced some results that are cross sectional in nature, namely when assessments of diet and intelligence were both carried out at eight-and-a-half years of age. As these assessments were performed at the same time, the results cannot show a cause-and-effect relationship between diet and intelligence.
What did the research involve?
The study used data from children who were participating in the ongoing Avon Longitudinal Study of Parents and Children (ALSPAC), also known as the Children of the 90s study. This overall cohort study was designed to investigate factors that influence development, health and disease during childhood and beyond. Pregnant women living in the Avon area of Southwest England who had an expected delivery date between April 1 1991 and December 31 1992 were eligible to participate in the study.
For this particular dietary study, information was collected through self-completed questionnaires given to the children’s main caregivers. Dietary information was collected with food-frequency questionnaires that were completed when the children were three, four, seven and eight-and-a-half years of age. The main caregiver was asked how often their child presently consumed a variety of foods. Consumption of the foods was described as:
- never or rarely
- once in 2 weeks
- 1-3 times a week
- 4-7 times a week
- more than once a day
The caregiver was also asked to record the number of cups of tea and coffee, glasses of cola and slices of bread consumed each day. They were also asked about what type of bread (white or other) and milk (full fat or other) was usually consumed. The questionnaires were amended slightly over the years to alter categorisation of foods or to allow for extra foods that the child may have been eating by that age.
The researchers were interested in dietary patterns rather than individual foods. They analysed consumption of clusters of food groups that were commonly eaten together. These were classed as:
- processed - food with high fat and sugar content and processed and convenience foods
- traditional - meat, poultry, potatoes and vegetables
- health conscious - salads, fruits, vegetables, fish, pasta and rice
- snack - a range of snack foods such as fruit, biscuits and cakes
When the children were aged seven, they were invited to attend an annual research clinic where physical and psychological tests were performed. When the children were on average eight-and-a-half years old, an IQ test was performed. Out of 13,988 children, a total of 7,044 attended the research clinic and had IQ data available.
The researchers considered that many confounding factors could influence IQ, besides diet. The researchers asked about these potential confounders using questionnaires. They adjusted their data to account for the influence of gender, child's age at IQ assessment, the person who conducted the IQ test, the number of stressful life events experienced by the child, breastfeeding duration, estimated energy intake at each time point (in calories consumed), score in a recognised measurement of parenting at 18 months of age (HOME score), maternal education level, housing tenure, social class and maternal age at the birth of the child. They also looked at maternal consumption of oily fish during pregnancy.
What were the basic results?
The researchers compared the characteristics of families that attended the research clinic and had IQ data available. They found that the children who attended the clinic were more likely to be girls, to have been breastfed, to have mothers with higher levels of education, to be of a higher social class, to be older, to live in a house owned by their caregiver, to have experienced fewer stressful life events and to have mothers who consumed oily fish during pregnancy. The children with IQ data available also had a lower birth weight on average than the rest of the cohort.
They found that eating a processed diet at the age of three was associated with a lower IQ at eight-and-a-half years old. Patterns of snack consumption at three years were associated with an increased IQ at eight-and-a-half years. The only other association they found was that a health-conscious dietary pattern at eight-and-a-half years was associated with greater IQ at the same age.
The researchers used the different food-frequency categories (never or rarely, once in 2 weeks etc.) within the diet clusters to give a score of the extent to which a child’s food intake followed each diet type. They used this estimate to see how an increase in food frequency level in each diet type would affect IQ. They found that for the processed diet at three years, each increase in food frequency was associated with a 1.67 drop in IQ at eight-and-a-half years of age (95% confidence interval [CI] -2.34 to -1.00). Each increase in level of snack consumption at three years of age was associated with a 0.9-point IQ increase (95% CI 0.39 to 1.42).
Increasing consumption of a health-conscious diet at eight-and-a-half years was associated with a 1.2-point increase in IQ (95% CI 0.52 to 1.88).
How did the researchers interpret the results?
The researchers said that they had shown “weak but novel associations between dietary patterns in early childhood, and current diet, with general intelligence assessed at 8.5 years of age”. They said that, “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.”
They also said that dietary patterns between the ages of 3 and 7 years were not predictive of IQ, and that further research is required to help determine the true effects of early diet on intelligence.
This study showed that eating “processed foods” at three years of age and following a health-conscious diet at the age of eight-and-a-half years old had a modest effect on child IQ at eight-and-a-half years of age.
Though there was also an association between eating a health-conscious diet, including salads, vegetables, fish, pasta and rice, and a higher IQ at the age of eight-and-a-half years, caution should be used when interpreting this association. Assessments of diet and IQ were both conducted around the same age, which means they cannot demonstrate a cause-and-effect relationship.
This study has strengths in that it included a large sample and took repeat measures of diet. It also adjusted the data for a large number of factors that may have affected the outcome. However, the researchers admitted that other factors that they had not adjusted for could have affected the outcome.
There are some other points to consider when interpreting this study:
- Although the original study population was very large and may have been representative of the general population, the researchers found that the children who attended the voluntary clinic session (and could be included in the study) were more likely to come from a more affluent background, where their mothers had a stronger educational background, than the children who did not. This means that this data may not apply to the population as a whole.
- As with any study looking at diet, participants may not have accurately recalled what they ate. Also, one-off questionnaires may not capture accurately the typical pattern of diet over the last year.
- This study analysed the data by looking at clusters of food. Although this has the advantage of representing diet more realistically than examining each food separately, the way that particular food groups are decided on may be open to debate. For example, the snacking pattern included consumption of both fruit and cake.
Overall, this study showed a modest association between diet and intelligence. Although the benefits of a balanced diet are well known for general health, further research is needed to assess the impact of diet on child brain development and intelligence.