title: Vitamin E and the risk of pneumonia: using the I 2 statistic to quantify heterogeneity within a controlled trial.
abstract: Analyses in nutritional epidemiology usually assume a uniform effect of a nutrient. Previously, four subgroups of the Alpha-Tocopherol, Beta-Carotene Cancer Prevention (ATBC) Study of Finnish male smokers aged 50-69 years were identified in which vitamin E supplementation either significantly increased or decreased the risk of pneumonia. The purpose of this present study was to quantify the level of true heterogeneity in the effect of vitamin E on pneumonia incidence using the I 2 statistic. The I 2 value estimates the percentage of total variation across studies that is explained by true differences in the treatment effect rather than by chance, with a range from 0 to 100 %. The I 2 statistic for the effect of vitamin E supplementation on pneumonia risk for five subgroups of the ATBC population was 89 % (95 % CI 78, 95 %), indicating that essentially all heterogeneity was true variation in vitamin E effect instead of chance variation. The I 2 statistic for heterogeneity in vitamin E effects on pneumonia risk was 92 % (95 % CI 80, 97 %) for three other ATBC subgroups defined by smoking level and leisure-time exercise level. Vitamin E decreased pneumonia risk by 69 % among participants who had the least exposure to smoking and exercised during leisure time (7·6 % of the ATBC participants), and vitamin E increased pneumonia risk by 68 % among those who had the highest exposure to smoking and did not exercise (22 % of the ATBC participants). These findings refute there being a uniform effect of vitamin E supplementation on the risk of pneumonia.
authors: Department of Public Health,University of Helsinki,Helsinki, POB 20, FIN-00014,Finland.
authors: Harri
authors: http://orcid.org/0000-0002-4710-307X
authors: H
authors: Hemilä
mesh_terms: D000368:Aged; D015331:Cohort Studies; D019587:Dietary Supplements / Q000009:adverse effects; D015987:Effect Modifier, Epidemiologic; D015444:Exercise; D005387:Finland / Q000453:epidemiology; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D011014:Pneumonia / Q000453:epidemiology / Q000209:etiology / Q000276:immunology / Q000517:prevention & control*; D015203:Reproducibility of Results; D012141:Respiratory Tract Infections / Q000453:epidemiology / Q000209:etiology / Q000276:immunology / Q000517:prevention & control; D012306:Risk; D057185:Sedentary Behavior; D012907:Smoking / Q000009:adverse effects*; D013223:Statistics as Topic; D014810:Vitamin E / Q000009:adverse effects / Q000627:therapeutic use*; D024502:alpha-Tocopherol / Q000008:administration & dosage; D019207:beta Carotene / Q000008:administration & dosage
chemical_list: D019207:beta Carotene; D014810:Vitamin E; D024502:alpha-Tocopherol
keywords:  AT DL-α-tocopheryl acetate;  ATBC Alpha-Tocopherol;  BC 
                    β-carotene;  Beta-Carotene Cancer Prevention; Antioxidants; Dietary supplements; Effect modifiers (epidemiology); Population characteristics; Respiratory tract infections