BACKGROUND: In several longitudinal studies changes in body mass and in forced expiratory volume in one second (FEV1) have been found to be negatively correlated. This paper tests the hypothesis that failure to allow for the association can lead to error in the interpretation of longitudinal measurements of ventilatory capacity. METHODS: Male shipyard workers (n = 1005) were assessed on two occasions with an average interval between measurements of 6.9 years. A respiratory symptoms questionnaire, detailed anthropometric measurements, and dynamic spirometric tests were undertaken. Multiple regression analysis was used to identify variables which contributed to the changes in lung function. RESULTS: After allowing for age and growth in stature, a change in body mass of 1 kg was, on average, associated with a mean (SE) converse change in FEV1 of 17.6 (2.0) ml, and in forced vital capacity (FVC) of 21.1 (2.5) ml. Neglect of changes in body mass (which in this context reflected changes in body fat) led to underestimation of the longitudinal decline in FEV1 with age and failure to detect significant improvements in FEV1, both in smokers following discontinuation of smoking and in shipyard welders and caulker/burners as a consequence of leaving their employment. The estimated peak ages and associated peak levels of the indices were found to differ, depending on whether or not they were expressed at constant body mass. CONCLUSIONS: Neglect of changes in body mass can lead to erroneous conclusions being drawn from longitudinal measurements of FEV1.
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