In a recent paper published in Thorax Canoy and co-workers1 concluded that babies with lower birth weight and poor infant growth may be at a higher risk of developing impaired adult pulmonary function. In contradiction to these findings, no association was found between birth weight and adult lung function or between birth weight and asthma symptoms in a Nordic-Baltic population studied by Laerum et al.2 Studies of birth characteristics and respiratory outcomes give contradictory results as methods used in different studies vary. Although Canoy et al1 showed some interesting findings in a large cohort study, it raises some methodological questions and thereby interpretation of the findings.

The authors state that data on potential confounders and mediating factors operating throughout the life course were prospectively collected. Many of the known early life variables and adult variables were taken into consideration in logistic regression analyses. Canoy et al have focused on the nutritional status of the mother and birth weight. However, I wonder whether some relevant factors known to influence weight during the first year of life were missing. For example, it is a known fact that children of diabetic mothers often have heavier babies (large for gestational age) at birth compared with other term babies. This has not been discussed in the paper. Furthermore, have the authors taken into account factors that could influence the development of weight during the first year? What about the nutritional status of the children or conditions that may lead to nutritional disorders during the first year? Did the authors consider other chronic childhood disorders that may impair growth? Only two measurements of weight (at birth and around 12 months of age) during the first year will hardly reflect the natural growth of the child over time.

The authors mention that weight gain during the first year was positively associated with lung function later in life, which remained significant after adjustments for various potential confounders across the life course. What about children with respiratory symptoms early in life? Studies have shown that there is an association between initial airway symptoms and later lung function and respiratory illnesses,3 and that subjects with asthma have persistent reduced lung function.4 Finally, did the authors account for family history of asthma and/or atopy or allergic sensitisation? In my opinion, many confounders and factors that may have an effect on the outcome were not addressed adequately in this paper, so the conclusions drawn may not be relevant.