'Teenagers more likely to have severe asthma and eczema if they eat fast food more than three times a week,' The Guardian reports. The claim is based on wide-ranging research looking at the dietary patterns of children across the world…
“Fast food and takeaways linked to surge in child asthma and allergies,” reports The Guardian.
Along with many other papers, it reports on a study attempting to shed light on one of the enduring medical mysteries of recent times – what explains the sharp rise in allergic conditions that has occurred during the past few decades?
Researchers wanted to investigate the theory that changes to traditional diets in the developed world since World War Two may be partially responsible.
This was an international survey looking at the links between diet and three allergy-related conditions in adolescents and children:
- rhinoconjunctivitis (stuffy or runny nose and watery eyes)
The researchers found frequent fast-food consumption (three or more times a week) was significantly linked with an increased risk of what the researchers defined as severe asthma, severe rhinoconjunctivitis or severe eczema. In contrast, consumption of fruit at least three times per week was significantly associated with a decreased risk of severe asthma. However, these associations are not proof of direct cause and effect.
It is possible that there are other underlying factors associated with both diet and risk of these allergic conditions, which could explain the associations seen, such as socioeconomic status.
Whatever the evidence, encouraging your child to eat fresh fruit and vegetables regularly (at least five portions a day) is a good idea.
Where did the story come from?
The study was carried out by an international team of researchers. It was funded by a number of organisations, including the BUPA Foundation and Glaxo Wellcome International Medical Affairs, as well as by a number of New Zealand-based funding bodies.
The study was published in the peer-reviewed journal Thorax.
The results of this study were widely and accurately reported in the media, although reports didn’t make it clear that cause and effect cannot be implied from this study.
What kind of research was this?
The International Study of Asthma and Allergies in Childhood (ISAAC) study is a multicentre, international, cross-sectional study.
In this part of the study, the authors aimed to determine whether there was an association between food intake over the past year and current and severe symptoms of the following allergic conditions:
The study involved a group of children aged six and seven years old, and a separate group of adolescents aged 13 and 14 years old.
Cross-sectional studies are good for determining prevalence: in this case, the results of the study can provide us with information on the number of children with symptoms of asthma, rhinoconjunctivitis and eczema, and on the diet of the children.
However, this study design has some limitations (see conclusions) and to determine a link, a cohort study would be needed. However, even a cohort study could not show whether fast food was actually causing the development of asthma, as fast-food intake could be a marker of many other factors, such as socioeconomic status, which could be the real cause of any association seen.
A randomised controlled trial would be required to prove causation, though such a trial would have both practical and ethical issues (obviously few parents would be happy to find their child had been placed in the ‘kebab and cheeseburger’ group).
What did the research involve?
This international study included 319,196 adolescents aged 13 to 14 years, and 181,631 children aged four to seven years. Information on the diet and clinical symptoms of asthma, rhinoconjunctivitis and eczema were collected using questionnaires. Questionnaires were self-completed by the adolescents and were completed by the parents of the children.
Clinical symptoms included:
- Asthma: Wheezing or whistling in the chest in the past 12 months. If wheezing affected speech, occurred more than four times in the past 12 months, or caused sleep disturbance more than one night per week, then it was defined as severe symptoms.
- Rhinoconjunctivitis: Sneezing or runny or blocked nose when not suffering from a cold/flu, and itchy watery eyes in the past 12 months. Symptoms were defined as severe if the nose problems were accompanied by itchy watery eyes and were reported as interfering with daily activities ‘a lot’.
- Eczema: An itchy rash in the past 12 months. Symptoms were defined as severe if sleep disturbance was reported at least once per week.
The diet questionnaire asked questions on the average weekly consumption over the previous year (never/occasionally, once or twice per week, at least three times per week) of the following foods:
- meat (presumably home-cooked meat dishes as opposed to fast food)
- vegetables (green and root)
- pulses (peas, beans, lentils)
- pasta (including bread)
- fast food, such as burgers
The researchers then looked to see if there was an association between the food consumed and the clinical symptoms reported.
The researchers adjusted for a number of factors that they had also collected information on, as it was thought they could partially explain any association seen (confounders).
These included exercise, television watching, maternal education, maternal smoking in the first year of life and current maternal smoking. Gender, region of the world, language and per capita gross national income were also adjusted for.
What were the basic results?
For adolescents; milk, fruit, and vegetables were found to be ‘protective’ foods.
Fruit intake at least once or twice per week or at least three times per week was associated with a reduced the risk of current wheeze, severe asthma, rhinoconjunctivitis and severe rhinoconjunctivitis.
Milk was associated with a reduced risk of current wheeze when consumed once or twice per week, and severe asthma when consumed at least three times per week. Milk consumption once or twice per week was associated with a reduced risk of rhinoconjunctivitis and severe rhinoconjunctivitis. Drinking milk once or twice per week or at least three times per week was associated with a reduced risk of eczema, and when consumed once or twice a week, with severe eczema.
Eating vegetables was associated with a reduced the risk of current wheeze when eaten at least three times per week, and of severe rhinoconjunctivitis and severe eczema when eaten once or twice per week.
Butter, fast food, margarine, pasta, potato, pulses, rice, seafood and nuts were associated with an increased risk of one or more conditions when eaten once or twice per week or at least three times per week, without being associated with a reduced risk of any conditions.
An increased risk of all three conditions (current and severe) was associated with eating butter, fast food, margarine and pasta at least three times per week. In some cases, an association was seen if the foods were eaten once or twice per week.
The greatest increase in risk was associated with eating fast food at least three times per week. Eating fast food at least three times per week was associated with an increased risk of current wheeze, severe asthma, current rhinoconjunctivitis, severe rhinoconjunctivitis, current eczema and severe eczema.
For children; eggs, fruit, cereals, meat, milk, nuts, pasta, potato, pulses, rice, seafood, and vegetables were found to be ‘protective’ foods, and were associated with a reduced risk of at least one condition without being associated with an increased risk of any condition.
Eating eggs, fruit, meat and milk at least three times per week was associated with reduced risk of all three conditions (current and severe).
Fast food was found to be a ‘risk factor’ food. Eating fast foods once or twice per week or at least three times per week was associated with an increased risk of current wheeze and severe asthma. Eating fast foods at least three times per week was associated with an increased risk of current rhinoconjunctivitis, severe rhinoconjunctivitis, and severe eczema.
How did the researchers interpret the results?
The researchers conclude that their results “suggest that fast-food consumption may be contributing to the increased prevalence of asthma, rhinoconjunctivitis and eczema in adolescents and children. For other foods, the picture for adolescents and children is less clear. However, in concordance with international dietary recommendations, diets that have a regular consumption of fruit and vegetables are likely to protect against asthma, allergic disease and other non-communicable disease”. The researchers also suggest that further exploration of this association is needed.
This cross-sectional study found that for both adolescents and children, fast-food consumption was associated with an increased risk of what the researchers defined as severe asthma, rhinoconjunctivitis and eczema. Consumption of fruit at least three times per week, by contrast, was associated with a decreased risk of severe asthma.
Although eating a balanced diet including the recommended daily intake of fruit and vegetables has many health benefits, there are limitations to this study, some of which were noted by the authors.
These limitations include:
- The cross-sectional study design meant that the children were not followed over time, so we don’t know the order of events. For example, we can’t say whether consumption of fast food occurred before the development of asthma or whether children/adolescents with asthma ate more fast food.
- Diet and symptoms over the whole year were either self-reported or reported by the parents of the children. This means that they may be subject to recall bias.
- Not all confounders could be adjusted for and it is possible that there are other underlying factors associated with both dietary habits and risk of these allergic conditions. For example, the researchers only adjusted for socioeconomic status at the country level, rather than at the individual level.
- Diagnoses of asthma, rhinoconjunctivitis and eczema – in particular what the researchers defined as ‘severe’ cases of these conditions – were not confirmed by medical records, so we don’t know whether they were entirely accurate.
Despite these limitations, this was an impressive and wide-ranging study (involving more than half a million children from across the world) whose results suggest that the possible association between diet and allergy deserves further investigation.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter.