Considerations for conducting and interpreting long-term follow-up of intervention studies: avoiding spoiled milk

Multiple early life exposures have been shown to influence the development of allergic disease in infancy and childhood. The timing of exposure to food allergens has been of particular interest. Two landmark trials, Learning Early About Peanut and the Enquiring About Tolerance studies, found early introduction of peanuts reduced peanut allergy in children,1 2 a fundamental difference from previously accepted beliefs. The impact from cow’s milk is less clear, with the most recent trial suggesting avoidance of early cow’s milk supplementation decreased the risk of childhood asthma,3 while others have found an increased risk or no effect on childhood allergy.4 5 In the current issue of Thorax , Hand and colleagues have further added to the ongoing cow’s milk debate by studying the effect of early avoidance on adult allergy.6 Investigating if early life exposures influence adult disease requires extended follow-up of participants. Trials are usually designed to investigate immediate or short-term outcomes, but it is possible to proficiently plan and execute long-term follow-up of an intervention during infancy, such as in the Promotion of Breastfeeding Intervention Trial trial, where participants were followed up until adolescence.7 However, due to the nature of extended longitudinal follow-up, such trials are prone to considerable limitations that must be carefully considered in the design, analysis and interpretation. In the current study, Hand et …