The researchers noted that all three conditions are rising in prevalence worldwide.
"If the associations between fast foods and the symptom prevalence of asthma, rhinoconjunctivitis and eczema is causal, then the findings have major public health significance owing to the rising consumption of fast foods globally," the researchers wrote.
The researchers gave food and medical questionnaires to the parents of more than 181,000 children between the ages of six and seven at 64 centers in 31 countries, plus 319,000 children between the ages of 13 and 14 at 107 centers in 51 countries. All the children were taking part in the International Study of Asthma and Allergies in Childhood (ISAAC).
Parents answered questions about how severe the children's symptoms of various allergy-related health problems had been over the preceding 12 months, including how often symptoms had been experienced and how often they had interfered with daily life and with sleep patterns. The health conditions involved in the study were eczema, asthma (defined by wheezing) and rhinoconjunctivitis (characterized by itchy and watery eyes along with a blocked or runny nose).
The parents were also asked about their children's intake of foods that have previously been linked to either increased or decreased risk of certain health conditions. Foods included on the questionnaire included meat, fish, milk, butter, margarine, eggs, nuts, fruits and vegetables, cereals, bread and pasta, rice, pulses, potatoes, fast food and hamburgers. Parents characterized their children's consumption of each food category as never, occasionally, once or twice a week, or three or more times a week.
Fast food increases both risk and severity
After adjusting for potential interference from risk factors, only consumption of fast food was associated with an increased risk of symptoms in both younger and older children. Eating three or more servings per week led to a 39 percent higher risk of severe asthma in teenagers and a 27 percent higher risk in younger children. It was also associated with more severe asthma and rhinoconjunctivitis symptoms.
In teenagers, fast food consumption was correlated with a higher risk of current and severe symptoms of all three conditions. In younger children, fast food consumption was associated with both asthma and rhinoconjunctivitis.
"Such consistency adds some weight to the possible causality of the relationship," the researchers wrote.
Notably, consumption of fruit three or more times per week significantly decreased the risk of all three conditions in younger children, and reduced the risk of both current and severe asthma and rhinoconjunctivitis in teenagers.
Although the study was not set up to explain why fast food might increase risk, the researchers speculated that it might have to do with those foods' high levels of trans and saturated fats, which have been known to affect the immune system. In contrast, fruit is known to be high in antioxidants that can benefit the immune system.
