Tuesday June 2 2009
A study has found that “teenagers who eat lots of take-aways are more likely to behave badly,” reported the Daily Express. It said that the finding confirms the belief that poor diets are linked to mental health problems. According to the newspaper, the researchers blamed junk food for problems such as depression, aggression and delinquency.
This study looked at diet and behaviour in nearly 1,600 Australian adolescents, aged 14. The study cannot prove that diet caused the behavioural characteristics seen, as both diet and behaviour were assessed over the same period of time. Also, other factors may be responsible for both poor diet and poor behaviour. The study did not look at whether the children had received a formal mental health diagnosis, such as for depression, and therefore cannot say whether diet affects the likelihood of such diagnoses. A balanced, healthy diet has many benefits and should be encouraged for people of all ages.
Where did the story come from?
The research was carried out by Dr Wendy H Oddy and colleagues from the University of Western Australia and the Curtin University of Technology in Perth, Australia. The study was funded by the Raine Medical Research Foundation, the National Health and Medical Research Council of Australia, the Telstra Foundation, the Western Australian Health Promotion Foundation, the Australian Rotary Health Research Fund, the Telethon Institute for Child Health Research and the Commonwealth Scientific and Industrial Research Organisation.
The study was published in the peer-reviewed journal Preventive Medicine.
What kind of scientific study was this?
This was a cross-sectional analysis of children taking part in a prospective cohort study called the Western Australian Pregnancy Cohort (Raine) Study. The cohort study recruited 2,900 pregnant women between 1989 and 1991 in Perth, and 2,868 (96%) of the babies born to these women were available at birth for future follow-up.
The current study looked at data collected when the children were 14 years old, at which point 1,860 children participated (65% of those available for follow-up). At age 14, the children’s behaviour was assessed using the Child Behaviour Checklist, a validated tool for assessing behaviour that is completed by the child’s primary caregiver. This checklist gives the children an overall behaviour score, with higher scores indicating more behavioural problems. The score was also broken down to specifically measure the children’s internalising behaviours (withdrawal, bodily [somatic] complaints or being anxious or depressed) and externalising behaviours (being delinquent or aggressive) over the previous six months.
The children’s dietary patterns were assessed using a food frequency questionnaire that was completed by the child and their main caregiver. This questionnaire assessed the frequency and consumption of 212 food and drink items over the previous 12 months. Based on the answers to this questionnaire, the children were scored on whether they had either “Western” or “healthy” patterns, and their intake of 38 different groups of food (such as yellow or red vegetables, leafy green vegetables, red meat or cakes and biscuits) were determined. Western diets included more take-away foods, sweets, red meat, refined grains, processed meats, potatoes (fried, not fried or crisps), soft drinks, cakes and biscuits, sauces and dressings, and full-fat dairy products). Healthy diets included more vegetables, fresh fruit, legumes, whole grains and steamed, grilled or tinned fish.
The researchers used statistical methods to assess whether there was a relationship (correlation) between a child’s dietary patterns and their behaviour. They also looked at whether the level of consumption of the main food groups comprising the Western and healthy patterns were associated with behaviour.
The researchers adjusted for (took into account) factors that might have affected the results (confounders) such as the adolescents’ overall energy intake, exercise habits, hours spent in front of a screen each day (television or video viewing and computer use), weight classification based on standard body mass index criteria for their age (whether they were underweight, normal weight, overweight or obese), and sociodemographic and family characteristics (maternal education, family structure and current family income).
What were the results of the study?
The researchers included data from 1,598 adolescents whose primary caregivers provided both behavioural and dietary information. They found that there was a relationship between dietary pattern and behaviour. There was an association between higher levels of behavioural problems, including both internalising and externalising behaviours, and eating a more Western diet. This association remained significant after taking into account potential confounders. Higher red meat and confectionary consumption was associated with higher behavioural problem scores.
Although lower levels of behavioural problems, in particular lower levels of externalising behaviours, were associated with having a more healthy pattern of eating, the association was not significant after adjusting for potential confounders. Higher leafy green vegetable and fresh fruit consumption was associated with lower behavioural problem scores.
What interpretations did the researchers draw from these results?
The researchers concluded that their findings “implicate a Western dietary pattern in poorer behavioural outcomes for adolescents” and that “better behavioural outcomes were associated with a higher intake of fresh fruit and leafy green vegetables”.
What does the NHS Knowledge Service make of this study?
There are a number of important points to note when interpreting this study:
- As the study was cross sectional, it cannot prove that the children’s diet caused their behaviour as it cannot show the behaviours developed after they started to eat their current diets. For example, it is possible that eating junk food against parental advice is one of the defiant behaviours exhibited by externalising teens.
- There could be confounding factors that affect both diet and behaviour and are responsible for this association. Although the researchers adjusted for a number of potential confounders, this may not have completely removed their effect and there may have been other unknown confounders.
- The food frequency questionnaire assessed the children’s diets over the previous 12 months. This may not have been representative of their diet before this point. There may also be some inaccuracies in the children’s and their parents’ recall of their typical diets over this period.
- The checklist used to assess the children’s behaviour in this study is not a diagnostic checklist. This mean that it does not diagnose whether children have conditions such as depression or anxiety, but rather measures their internalising and externalising behaviours. As such, the study cannot say whether there is an association between poor diet and specific mental health diagnoses.
- Just over half (56%) of the children were available for follow-up from birth. The findings may have differed if the children who dropped out were included.
- Associations were found between behaviour and consumption of different groups of foods that characterised the Western and healthy dietary patterns. However, these results should be interpreted cautiously as they involved conducting multiple statistical tests. This increases the likelihood of identifying significant results just by chance.
- These results were obtained in a population of Australian adolescents. The results may not be representative of what would be found for adolescents from other countries or cultures.
This study by itself cannot prove that a poor diet causes children’s behavioural problems. A prospective cohort study would be needed to evaluate whether there might be a causal relationship. It is important to note that this study defined a “Western” diet as one including higher levels of foods that are unhealthy in large quantities. However, a diet of Western food can also be healthy. A balanced, healthy diet has many benefits for people of all ages and should be encouraged.