VALIDITY OF METHODS USED TO ASSESS VITAMIN AND MINERAL SUPPLEMENT USE

Assessing vitamin and mineral supplement use is important because supp lement use per se is an exposure of interest for the risk of several c hronic diseases and because supplements contribute a large proportion of total (diet plus supplement) micronutrient intake, another importan t exposure in epidemiologic research. Unfortunately, little is known a bout methods for obtaining valid information about supplement use. The authors conducted a validation study in 1996 comparing supplement dat a collected in a telephone interview and from a self-administered ques tionnaire with data derived from a detailed, in-person interview and t ranscription of the labels of supplement bottles (i.e., a gold standar d) among adult supplement users in Washington State (n = 104). Spearma n correlation coefficients comparing average daily supplemental vitami n and mineral intake from the interview or questionnaire with the gold standard ranged from 0.76 (95% confidence interval 0.66-0.83) for Vit amin C to 0.08 (95% confidence interval -0.14 to 0.29) for iron, with a mean of about 0.5. The principal sources of error were inaccurate as sumptions about the micronutrient composition of multiple vitamins and respondent confusion regarding the distinction between multiple vitam ins and single supplements. These results suggest that commonly used e pidemiologic methods of assessing supplement use may incorporate signi ficant amounts of error in estimates of some nutrients.