<sec id="st1"> <title>SETTING</title> Increase in energy expenditure and/or decrease in nutritional intake leads to low body mass index (BMI). The balance between energy expenditure and nutritional intake has rarely been evaluated in a large population of patients with chronic obstructive pulmonary disease (COPD). </sec> <sec id="st2"> <title>OBJECTIVE</title> To evaluate BMI, nutritional intake and physical activity and the association of these factors with the severity of airflow obstruction in COPD patients. </sec> <sec id="st3"> <title>DESIGN</title> We analysed the Korean National Health and Nutrition Examination Survey (KNHANES) data set from 2012 to 2015. </sec> <sec id="st4"> <title>RESULTS</title> Among the 9682 individuals (1601 with COPD and 8081 without COPD) recruited, BMI was lower in COPD patients than in non-COPD participants (males, 23.86 ± 2.76 vs. 24.28 ± 2.80, P < 0.001; females, 23.63 ± 2.94 vs. 23.98 ± 3.10, P < 0.05). As the stage of COPD advanced, BMI, intake of nutrients (food, water and carbohydrates) and total energy levels declined in COPD patients. Total time spent walking in the preceding week decreased with advancing COPD stage in male patients with COPD. COPD severity was an important risk factor for the limitation of physical activity due to respiratory problems (OR 3.92, 95%CI 2.77∼5.34, P < 0.001). </sec> <sec id="st5"> <title>CONCLUSION</title> Patients with COPD had a low nutritional intake with little physical activity, which worsened with advancing COPD stage. In late-stage COPD, impaired nutritional intake outweighed the decrease in physical activity, resulting in weight loss. </sec>.